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Proceedmgs of the 7th Iniernaiumal Wmkmg Conference all Stored-product Proteciuni - Volume 1<br />
Models of stored-product pests:<br />
Their relevance to biological control in traditional storage in<br />
developing countries<br />
C Stalk!' 3 , W van der Werf 2 , and A van HUlst<br />
Abstract<br />
Dunng the past decades, models that simulate the population<br />
dynamics of stored-product pests have been developed Some<br />
of these have been developed from an mterest m ecological<br />
and evolutionary processes that shape populations Others<br />
have been constructed with the ultimate aim of improvmg<br />
control of these pests We diSCUSSboth approaches m<br />
relation to biological control of stored-product pests m<br />
developmg countries<br />
Storage of food products in developmg countnes differs<br />
from that m mdustnahzed countries Food ISoften stored at<br />
a small scale, It ISmostly mtended for pnvate consumption,<br />
and msect mcrdence IS hrgh Brologrcal control of these<br />
msect pests IS considered an mterestmg alternative to<br />
traditional and chemical control methods The situation of<br />
control of bruchids m stored cowpea ISpresented as a case<br />
Existing "apphed ' models do not answer the type of<br />
questions that arise from the system of stored cowpea They<br />
often do not mclude natural enemies, they are only vahdated<br />
for relatively low msect densities and temperatures, they<br />
assume control practices winch are not appropnate to<br />
resource-poor farmers, and they often draw upon empmcal<br />
relationships We suggest that a model that predicts WhICh<br />
measures WIll Improve the biological control status of<br />
tropical stored-product pests should combme the "ecological<br />
reahsm ' of theoretical ecology WIth the accuracy of the<br />
'pest management' models<br />
Introduction<br />
Storage of food products m developmg countnes differs from<br />
that m mdustnalized countnes m many respects (Table 1)<br />
Cereals and pulses, stored m traditional grananes, are often<br />
1Laboratory for Entomology, Wagerungen Agncultural University, P<br />
o Box 8031 ,6700 EH Wagenmgen, The Netherlands<br />
2 Sub-department of Theoretical Production Ecology. POBox 430,<br />
6700 AK Wagemngen, The Netherlands<br />
3 Presenting author<br />
senously mfested by msect pests (Compton et al , 1993,<br />
Gahukar, 1994) Traditional methods of controllmg these<br />
pests, such as the use of sand or wood-ash as protectants,<br />
fall short m effectivity and efficiency for large quantities<br />
(see Compton et al , 1993) Chemical control IS too<br />
expensive and Imposes health risks, especially for illrterate<br />
subsistence farmers For such systems, biological control of<br />
stored-product pests ISan mterestmg option (Hames, 1984,<br />
Van Huis, 1991, Van Huis et al , 1991), especially because<br />
the tolerance for insect presence and cosmetic damage m<br />
stored products IShigh (Hames, 1984)<br />
A simulation model can be useful m selecting a natural<br />
enemy for biological control, because It can predict the<br />
effects of characteristics of mdividuals at the population<br />
level SImulation models can also be used to study the<br />
sensinvity of the system to changes m management or<br />
changes m one of ItS parameters ThIS can be helpful in<br />
determmmg<br />
system<br />
the best control strategy for a particular<br />
A large number of models deal WIth the biology of<br />
arthropod pests that occur m stored products (Throne,<br />
1995, Longstaff, 1991) We examme the existing hterature<br />
on these models for their relevance to biological control<br />
situations m traditional storage of small-holders m<br />
developmg countries The system of bruchids m stored<br />
cowpea m West Africa ISused as an example<br />
We start WIthan mtroducnon to bruchids m cowpea and to<br />
the research questions that have ansen from preVIOUSwork<br />
on this system Subsequently, we bnefly charactenze and<br />
dISCUSSexisting models and evaluate their Importance to<br />
stored-product protection m the trOPICS Fmally, we dISCUSS<br />
future prospects for models of the type we have m mind<br />
Biological Control of Bruchids in<br />
Stored Cowpea in West Africa<br />
West Africa, cowpea, and bruchids<br />
Many countrIes m West-Mnca, espeCIally m the Sahel<br />
region, have poor economies and a relatively low standard of<br />
hvmg (measured by such cntena as Gross National Product<br />
per capIta and hfe-expectancy at bIrth) (Barratt Brown,<br />
1995 151, Michel, 1996 A<strong>87</strong>) ThiS regIOn, where<br />
<strong>72</strong>
Proceedmgs of the 7th Internaiumol WOIklllg Conference on Stored-product Protection. - Volume 1<br />
agnculture IS one of the mam actrvrties (FAO, 1986 16,<br />
Ben Yahmed, 1993), IS a mam area of cowpea production<br />
(FAOSTAT Internet database, Ben Yahmed, 1993, Nwokolo<br />
& Ilechukwu, 1996) Cowpea (Vigna unquiculata Walp )<br />
IS an Important source of protem for low income famihes<br />
(Nwokolo , 1996, Van Alebeek, 1996) Cowpea, hke<br />
sorghum, millet and maize, ISstored dunng the dry season<br />
In rural areas, cowpea IS normally stored on-farm m<br />
traditional granaries, which are made of natural matenals<br />
such as wood, straw, and clay (Sagnia & Schutte, 1992,<br />
Kone, 1993) Cowpeas are often stored as whole-pods, and<br />
only when needed for consumption or for sale, they are<br />
threshed and stored as grams m smaller contamers (e g ,<br />
clay Jars) Typically, a subsistence farmer stores several<br />
hundred kilograms of cowpeas (Sagrna & Schute, 1992)<br />
Cowpeas are not only stored for home consumption but also<br />
for the market, as the pnce of cowpea mcreases towards the<br />
end of the storage season (Caswell, 1974, Sagma &<br />
Schutte, 1992)<br />
Unfortunately, stored cowpea ISoften heavily attacked by<br />
bruchids, of which Collosobruchus maculatus Fab<br />
(Coleoptera Bruchidae) ISthe most Important one (Jackal &<br />
Daoust, 1986, Singh et al ,1990) The bruchid mfestation<br />
starts m the field, where adult females lay smgle eggs on<br />
npenmg pods Larvae that emerge from these eggs<br />
penetrate the seeds, m which they develop and pupate The<br />
adult emerges through an emergence hole m the bean The<br />
total development time IS approximately three weeks at<br />
32°C Adult females live for about a week, dunng which<br />
they may lay some 100 eggs Populations of C maculaius<br />
can grow rapidly m stored cowpea After several months of<br />
storage, the stock may be totally destroyed due to the larval<br />
feedmg<br />
The nutritional value and the quality of stored beans<br />
declme as a result of bruchid mfestation (Modgil & Mehta,<br />
1994, 1996) Although It IS drfficul t to determme exact<br />
losses (Boxall , 1991), quantitative losses are also severe<br />
The loss to bruchids m northern Nlgena m terms of weight<br />
has been estimated at 3 5 - 4% per year (Caswell, 1981)<br />
This corresponds to approximately 20 - 30% of the beans<br />
bemg mfested Weight losses m stored beans m Ghana after<br />
five months of storage were as high as 20 or 45%,<br />
dependmg on whether the beans were stored unthreshed or<br />
shelled, respectively (Adams, 1977, Schulten, 1982)<br />
Control methods<br />
A large vanety of control methods agamst bruchlds m<br />
stored cowpea IS known (Van HUls, 1991, Sagma &<br />
Schutte, 1992, Llenard & Seck, 1994) They are aimed at<br />
either preventmg bruchld mfestatlOn, ehmmatmg or<br />
mmlmlzmg bruchld populations, or a combmatlon of these<br />
effects. The control methods can be roughly dlVlded mto<br />
three categones traditional methods, chemical methods and<br />
73<br />
alternative methods<br />
Traditional methods compnse among others the use of<br />
mert matenals such as sand and wood-ash, and the use of<br />
plant matenals with insecticidal or msect-repellant action<br />
(Javaid & Ramatlakapela, 1995, Apuuh & Villet, 1996,<br />
Ofuya, 1986) Although these methods are often effective,<br />
they are generally labonous and not suitable for large<br />
quantities (e g Taylor, 1975, Compton et al , 1993)<br />
Some methods require a lot of labour when labour demand IS<br />
already high because of the harvest In addition, some<br />
'natural msecticides ' may have chronic negative effects on<br />
human health (Schulten, 1991, Compton et al , 1993)<br />
Chemical control methods equally have several<br />
disadvantages Some pesticrdes do not kill all the larvae and<br />
pupae mside the seeds, wlule they do kill the natural<br />
enemies of the bruchids, which are often more sensitrve to<br />
msecncides (Caswell, 1973, Hames, 1984, Brower et al ,<br />
1996) In addition, pesticides are expensive for subsistence<br />
farmers, and not always available (Lienard & Seck, 1994)<br />
Moreover, farmers are generally not tramed m the use of<br />
pesticides, this results m a low efficacy of their use and an<br />
mcreased health nsk (Schul ten, 1991, Dmham, 1993,<br />
Compton et al , 1993, Schwab et al , 1995, Udoh, 1998)<br />
Alternative methods of control, such as storage of beans<br />
m hermetically closed plastic bags or 011 drums, have shown<br />
prormsmg results (Caswell, 1974, Lienard & Seck, 1994)<br />
Hermetically closed storage suffocates msects within a<br />
relatively short time However, to most Sahehan farmers,<br />
empty 011 drums or a sufficient number of strong plastic bags<br />
are as yet not available or too expensive (S Sagrua, and A<br />
Adandonon, personal commumcations, C Stolk, personal<br />
observation, Gahukar, 1994) Another disadvantage of<br />
plastic bags ISthat they have to be stored m such a way that<br />
they cannot be damaged by e g rodents, as even a small<br />
hole m the bag WIll nulhfy the suffocatmg effect ThIS IS<br />
probably hard to achieve Even the bruchids themselves may<br />
sometimes perforate plastic bags (Lienard & Seck, 1994) It<br />
may also prove dIffIcult to prevent re-mfestatlon If the bags<br />
are opened frequently for the purpose of sellmg beans<br />
For many farmers no adequate method of control of<br />
bruchld pests m cowpea IS therefore avatlable What IS<br />
needed IS a sustamable method of control that IS (1)<br />
effective, (2) practically and economIcally feasIble, (3)<br />
environmentally safe, and (4) flexible Wlthm a changmg<br />
SOCialand agncultural environment BIOlogicalcontrol of<br />
bruchlds may meet all these reqUIrements<br />
Biological control<br />
The most Important natural enerntes of bruchlds of cowpea<br />
are parasltOlds Larval parasltOlds (Dtnarmus basalzs and<br />
EupelrH/8 spp ) and egg-parasltOlds (Uscana spp ) have<br />
been proposed as candidates for biological control of bruchlds<br />
m tropical storage (Monge & HUlgnard, 1991) We
Proceedmgs of the 7th Iniernatumal Workmg Cmlference on Stored-product Protectvm - Volume 1<br />
concentrate on Uscana larwphaga Steffan (Hymenoptera<br />
Tnchogrammatidae ) Egg parasitoids have the advantage<br />
that they kill their host already before It has done any<br />
damage (contrary to larval parasrtoids)<br />
U larwphaga ISindigenous to West Africa In the field<br />
and m storage It IS responsible for a substantial amount of<br />
mortality of C maculaius (Lammers & Van HUlS, 1989,<br />
Sagma, 1994) Its hfe-lustory, functional response and host<br />
location have been mvestigated m the laboratory (Van<br />
Alebeek, 1996) We are mvestigatmg possibilmes to<br />
mcrease the Impact of U larwphaga on C maculaius<br />
populations, usmg a 'conservation strategy' of biological<br />
control Tlus strategy ISaimed at conservmg the populations<br />
of natural enemies that are already present m the system<br />
(rather than mtroducmg exotic natural enemies, or<br />
producmg and regularly releasmg large numbers of natural<br />
enemies) (Arbogast, 1984, Van HUlS, 1991, Van HUISet<br />
al , 1991, Waterhouse, 1992) Conservation biological<br />
control IS especially mterestmg for resource-poor farmers<br />
(Waterhouse, 1992) In this particular case, prormsing<br />
options are (Van Alebeek, 1996) ( 1) marupulatmg the<br />
storage environment, such that conditions for U<br />
larwphaga are optirruzed, (2) offenng food to the adult<br />
parasitoids to enhance their longevity and fecundity, (3)<br />
moculatmg the cowpea storage With Uscana from locally<br />
available resources, and (4) addmg plants or plant extracts<br />
to the stored beans which kill or repel the bruchids but do<br />
not harm the natural enenues These strategies can of<br />
course be combmed, for example, plant extracts (option 4)<br />
and Uscana (option 3) could be added at the same time<br />
The four options, and the possible role of simulation<br />
modellmg, are discussed below<br />
(1) Manipulation of the storage environment<br />
Temperature IS one of the mam driving forces of msect<br />
development and population dynanucs Insect species differ<br />
m theIr temperature responses It IS therefore poSSiblethat<br />
at certam temperature regImes, a natural enemy perfonns<br />
relatively better, and ItS host worse, than at other<br />
temperatures A SImulationmodel, based on experImentally<br />
detennmed temperature-dependent development rates of<br />
both host and parasltOld, could help establIsh whether such a<br />
temperature regime eXIsts If It eXiSts, It would be<br />
worthwhIle to mvestlgate whether such temperatures can be<br />
achieved by SImple changes m the placement and<br />
construction of tradItional grananes (Van HUIS et al ,<br />
1994) For example, a granary can be placed m the shadow<br />
or m the sunlIght (Van HUISet al , 1994), or a roof which<br />
allows for mcreased ventilation m the granary could be<br />
constructed (J Mumford, personal commurucatlon, see also<br />
Hames, 1984) Meanwhile, Van HUISet al (1994) have<br />
ShOWllthat, when U lanophaga IS offered an excess of<br />
eggs, Its rm value at 35"C ISdouble the value at 20'C , whIle<br />
the rm of C nmculatus remams constant and IS always<br />
74<br />
below that of U lariophaqa However, when the<br />
availabihty of bruchid eggs IS hmited. the I'm value of U<br />
larwphaga may be lower<br />
(2) Offenng food to parasrtoids Access to honey<br />
mcreases the fecundity of Uscana larwphaga by three<br />
times, and ItS longevity by five times, as compared to<br />
parasitoids that are starved (Van HUISet al , 1991) If such<br />
a result could be achieved m storage, It would probably exert<br />
mfluence on the bruchid population (see Wakers, 1998,<br />
Wakers, m prep) Whether enhanced pest control as a<br />
result of additional food ISto be expected, and how much the<br />
longevity and fecundity should be mcreased m order to<br />
obtam an adequate effect, can be assessed usmg a computer<br />
simulation model If simulations mdicate that feedmg the<br />
parasitoids has a sufficient negative effect on the bruchid<br />
populations, 'field' expenments could be set up in order to<br />
test this prediction However, practically feasible ways of<br />
adrrurustenng food to the parasitoids should be found<br />
(3) Inoculating the cowpea storage With Uscana There<br />
are many bruchid species which infest pulses of legummous<br />
trees (e g Woegan, 1995) Those bruclnds are not<br />
associated With cowpea, and many have only a few<br />
generations per year The same bruchids are also hosts to<br />
Uscana spp , which in turn are also capable of attackmg the<br />
bruchids of cowpea (A Van HUlS, C Schutte & S Sagnia,<br />
unpublished results) These resources of natural enemies<br />
could be used m bruchid control by adding pulses of these<br />
trees to cowpea stocks, at a time when the bruchid<br />
population IS most vulnerable to an mcreased density of<br />
parasitoids Farmers could even set up small temporary<br />
reanngs of natural enemies, based on this leguminous plant<br />
matenal, and release them mto their granary Legummous<br />
trees are relatively abundant m the dry Sahel zone, and<br />
farmers are fanuhar With these trees because they use the<br />
leaves for medical purposes, flavourmg food, and as food<br />
protectants, and the pulses are used as arumal feed (D K<br />
Kossou, personal commurucatlon) Reanng of parasltOlds at<br />
VIllage level IS feasible, as demonstrated by farmers m<br />
MeXICOwho rear parasltOlds for the control of the coffee<br />
berry borer (Galvez, 1992) Reanng of parasltOlds on<br />
bruchlds m pulses from legummous trees IS currently under<br />
mvestlgatlon m Togo (I A GlItho & S F Bolevane,<br />
personal commumcatlon) Computer SImulations, together<br />
With data obtamed from population-dynamics expenments,<br />
could help decide at which time m the storage season<br />
moculatlons With parasltolds are most lIkely to be effective,<br />
and how many parasltOlds should be used These predIctions<br />
can then be tested m expenmental grananes<br />
(4) Dned leaves and other parts of many plants are used<br />
m traditional control methods Some of these plant products<br />
may not be hannful to the parasltOldsof bruchlds, dependmg<br />
on the properties of the plant speCIeS The parasltOld<br />
population that IS left behmd after treatment With these
Proceedmgs of the 7th Internatumal Work1lLgConference 011 Stored-product Protectum - Volume 1<br />
products might be able to prevent resurgence of the beetle<br />
population This can be tested with a model, based on<br />
mortality data of both host and parasitoid This option IS<br />
especially interesting m combmation With inoculation of<br />
grananes With Uscana<br />
Research questions<br />
From the above, It appears that questions like, 'At which<br />
moment during the storage season should grananes be<br />
moculated with extra natural enemies m order to be effectrve<br />
m biological control?' , and 'Can additional food to natural<br />
enemies reduce the damage mfhcted by bruchrds" ' are of<br />
practical relevance<br />
However, other questions have also emerged from work<br />
on tlus system These questions are more fundamental m<br />
nature, but they are also Important for our understanding of<br />
the level of control U lanophaga may offer For instance,<br />
m the absence of food, U larwphaga has a very short<br />
adult longevity and generation time compared to C<br />
maculatus At 30'C and Without access to food, U<br />
Iariophaqa has an adult longevity of 1 - 2 days and a<br />
generation time of 8 days, while adults of C maculaius<br />
live about a week, and their generation time IS<br />
approximately 3 weeks ThiS Imphes that, If C rnaculatus<br />
populattons would occur m age cohorts which are more than<br />
two weeks apart, extmctton of Uscana m those<br />
commumttes IS hkely to occur (Van Alebeek, 1996, Van<br />
HUlset al ,submitted) Separate age cohorts may occur m<br />
msect populattons as a result of actlVlty of larval parasltOlds<br />
(Godfray & Hassell, 19<strong>87</strong>, 1989), or as a result of a<br />
dlfferenttal response to relattvely low temperatures of<br />
different Immature stages The questton IS whether thiS<br />
occurs m C maculatns populatIOnsm stored cowpea Such<br />
temporal separatton of generations would have Important<br />
Imphcattons for the expected results of Uscana as a<br />
bIOlogicalcontrol agent Another questton IS whether the<br />
metabohc heat that IS developed by the beetles (larvae and<br />
adults) IS of such a magmtude that It affects the<br />
developmental ttme of the both the beetles themselves and<br />
the parasltOlds There are mdlcattons that thiS might be the<br />
case (Van HUls et al ,submitted) If It IS, It should be<br />
taken mto account m predlcttons of the efftcacy of bIOlogical<br />
control We Willget back to tlus m the fmal sectton<br />
Models with a Theoretical and<br />
with a Practical Aim<br />
Models of stored-product pests have been developed from<br />
two qUlte different backgrounds Theorettcal bIOlogistshave<br />
used stored-product msects for studies of populatton dynamiC<br />
phenomena and the mechamsms that are responSible for<br />
those phenomena They have developed mathemattcal<br />
models that descnbe these processes (Mertz, 19<strong>72</strong>)<br />
75<br />
Ouestions that are central to theoretical ecology are, 'What<br />
causes oscillations m population density?' and' what causes<br />
populanons of a host and a parasitoid to coexist?" Storedproduct<br />
insects (especially Triboiium. and Callosobruchns<br />
species) have often been used in ecological studies because<br />
they show interesting population dynamics, they are easy to<br />
rear, easy to handle, and because their biology IS welldescnbed<br />
(Mertz, 19<strong>72</strong>, Hassell et al , 1989, Throne,<br />
1995) Moreover, the larvae of many bruchid species use a<br />
discrete food resource to feed upon, which IS selected by<br />
their mother (as m parasitoids) This has provoked studies<br />
on optimal foragmg and hfe-history evolution m these<br />
species (Wilson, 1988, 1994, Messina, 1991, Sibly et al<br />
1991, Colegrave, 1994, Horng, 1994) In this paper we<br />
Willemphasize' ecological' and population dynamical models<br />
because they are more relevant to the Issue of biological<br />
control in storage The models that are used m theoretical<br />
ecology are mainly analytical, explanatory models that<br />
contam biologically meaningful parameters which are<br />
incorporated into mechanistic relatronships These models<br />
mainly Yieldqualitative predictions<br />
Researchers that are mterested m control of storedproduct<br />
pests, on the other hand, have also developed<br />
models that Simulate the development of msect populattons<br />
m stored gram (Throne, 1995) However, these 'pest<br />
management' models have been developed to answer<br />
questtons hke, 'Can we expect a pest to develop m a gram<br />
storage With these speCifiCImttal condlttons?' and 'When<br />
should a storage manager start cooltng hiS gram?' (e g<br />
Thorpe et al , 1982, Smclalr & Alder, 1985, Maler et al ,<br />
1996, Flmn et al , 1997, Woods et al ,1997) These<br />
models are mamly phenomologlcal, deSigned m order to<br />
accurately and quantttattvely predict populatIOn<br />
development, based on accurate sampltng and temperature<br />
and hUlmdlty data from the gram store They are almost<br />
always slmulatton models, and often draw upon empmcal<br />
relattonshlps Temperature IS often mcluded as a dnvmg<br />
vanable, and developmental rates depend on ambient<br />
temperature. Models of thiS type can also be mcorporated<br />
mto 'expert systems' or other 'deCISionsupport' tools (see<br />
e g Norton & Mumford, 1993) These are software<br />
products which help the storage manager deCide what he<br />
should do to prevent or control msect mfestattons m the<br />
stored product Models have been developed for most msect<br />
pests that are of Importance m gram storage m mdustrlaltzed<br />
countries<br />
The models that have been developed m both dlsclplmes<br />
(theorettcal and practtcal) are generally very different m<br />
purpose and appearance Hlstoncally, both types of models<br />
have been labeled 'strategic' and 'tacttc', respecttvely<br />
(Nisbet & Gurney, 1982) In the next two secttons, we Will<br />
diSCUSSthe models m both categones III more detail<br />
Subsequently, we Willfocus on the relevance of the eXlstmg
Proceeunqs oj the 7th Internauonai Workmg Conference 011 Stored-product Protection - Yolume 1<br />
models to the subject of our mterest The literature on<br />
'theoretical' models of stored-product pests has also partly<br />
been reviewed by Mertz (19<strong>72</strong>) and Throne (1995) The<br />
literature on 'practical' simulation models has been<br />
reviewed by Longstaff (1991) and Throne (1995) The use<br />
of expert systems and decision support m stored product<br />
protection has been discussed by Wilkin & Mumford (1994)<br />
and Longstaff (1997) For an explanation of some baSIC<br />
terms we refer to textbooks and general papers on modellmg<br />
m biology (e g Rabbmge et al , 1989, Renshaw, 1991,<br />
Brown & Rothery, 1994)<br />
Models from Theoretical Ecology<br />
Theoretical ecology models are here defmed as those that<br />
deal With the question, 'What regulates the dynamics of<br />
population densities?" Many different mecharusms have<br />
been proposed to explam OSCillations m population densrties,<br />
or the absence of them Three major ones can be<br />
distinguished (either alone or m combmation ) Densitydependent<br />
processes, mterations With natural enemies, and<br />
spatial heterogeneity<br />
Density-dependent reproduction and survival<br />
Density-dependent reproduction or survival has been<br />
mcluded in many models to account for the fact that<br />
population growth IS not unlumted (Hassell, 1975) At high<br />
population densities, mortahty rates mcrease and/or<br />
reproduction rates decrease However, density dependent<br />
processes can also create mterestmg OSCillations m<br />
population Size These OSCillations tend to disappear after<br />
some time, unless there IS a time-lag associated With the<br />
density-dependent relationship (Hastmgs & Constantino,<br />
19<strong>87</strong>, Shimada & Tuda, 1996) In such so-called delayed<br />
density dependent systems, one life stage exerts at high<br />
densities a negative effect on another life stage, rather than<br />
on Its own life stage (e g adults eatmg juveniles when adult<br />
densities are high) Tlus creates a time-delay m the effect<br />
of high densrties, because It takes some time for the cohort<br />
of affected life stages (e g , the juveniles) to develop mto<br />
the cohort which can Itself produce denSity dependent effects<br />
(e g , the adults) The OSCillations m delayed denSity<br />
dependent systems typically have a penod of two to four<br />
times the maturation penod (Gurney & Nisbet, 1985)<br />
However, direct denSity dependence may m some cases also<br />
produce strong fluctuations (Gurney & Nisbet, 1985) They<br />
can be dlstmgUlshed from delayed denSity dependence<br />
because they have a penod which approXlmately equals the<br />
maturation penod We Will dtscuss m thiS section some<br />
examples of both direct and delayed denSity dependence m<br />
stored-product pests<br />
Mathematically, denSity-dependent relatIOnships are often<br />
of the form of the logistic dtfference equation, or<br />
76<br />
modifications of It (Utlda, 1967, Hassell, 1975, Shimada,<br />
1989) However, the mechanism underlymg such densitydependence<br />
IS not always specified These models do not<br />
give a stnctly mecharustic explanation but rather a<br />
phenomenological description of processes observed at the<br />
population level (e g Utida , 1967, see Gordon et al ,<br />
1988) Explicitly formulated mechamstic density dependent<br />
processes mclude canrubalism, competition for energy,<br />
nutnents and space, and a reduction m fecundity as a result<br />
of crowdmg Note that these processes are not of the same<br />
mtegration level (Fig 1), competition for energy, for<br />
instance, may result m a reduced fecundity Canmbahsm<br />
can be seen as a special form of competition<br />
Fig. 1 Mecharusms that produce density-dependence III<br />
stored-product insect populations (partly based on<br />
Gurney & Nisbet. 1985) An astensk (* )<br />
mdicates the methamsrns<br />
text<br />
which are treated III the<br />
Canrnbahsm has mamly been studied m Tribolium ,<br />
where mobile stages canrnbahze on rmmobile and/or younger<br />
stages (Mertz, 19<strong>72</strong>, Desharnais & LlU, 19<strong>87</strong>, Hastmgs &<br />
Constantmo, 19<strong>87</strong>) In Tribolium , cannibalrsm occurs<br />
randomly, such that the nsk of bemg canrubahzed mcreases<br />
at higher msect densities (Hastmgs & Constantmo, 19<strong>87</strong>)<br />
There IS convmcmg eVidence that thiS IS mdeed respolllsble<br />
for fluctuations m denSIties of laboratory populations of<br />
TnbollUm, althought FUJll (1976) argued that food<br />
renewal schedules also create CyCltClty, espeCially m the<br />
mnnature stages<br />
In the bruchld species Callosobruch ns chmenstS and C<br />
macnlatns, whose larvae develop mSlde a smgle bean,<br />
canlllbahsm does not playa role of any importance Instead,<br />
denSity-dependent surVival through competition between the<br />
larvae, and decreased reproductIOn as a result of crowdmg<br />
has received more attention m models of these species<br />
(Bellows, 1982a,b, Shimada, 1989, Tuda & Shimada,<br />
1993, Tuda, 1993, Kuno et al , 1995, Shimada & Tuda,
Pmceedmgs of the 7th Iniernatumai Workmg Canference an Stored-product Protechor~ - Volu:me 1<br />
1996) At high larval densrties. the larvae compete for<br />
nutrients, energy and space (direct density dependence)<br />
At high adult densities, females lay fewer eggs, and eggs<br />
are damaged by trampling adults and because of tOXICeffects<br />
of the oviposition marker pheromone (delayed density<br />
dependence) (Bellows, 1982b, Oshima et al m Kuno et<br />
al , 1995) Although both processes contribute to damped<br />
osciiiations or a monotonic approach towards a stable<br />
eqtnhbnum density, only the increased egg mortality as a<br />
result of large numbers of old adults m overlappinggeneation<br />
systems of C chrnensis prolonged the<br />
OSCillations(Shimada & Tuda , 1996)<br />
Gurney & NISbet (1985) have shown that, If the effect of<br />
larval competition IS expressed directly on the larvae<br />
themselves (direct density dependence), strong fluctuations<br />
Witha penod approximately equal to the maturation time can<br />
anse In other words, separate generations may occur m<br />
populations whose larvae suffer an mcreased death rate or<br />
maturation time at high larval densrties Discrete<br />
generations m msect populations can therefore be an<br />
mtnnsic property of the system, they are independent of<br />
external cues (such as food renewal schedule) or nutial<br />
conditions They have been observed m expenmental<br />
populations of the Indian meal moth, Plodia mterpunciella<br />
(Gurney et al , 1983 m Gurney & NISbet, 1985), and m<br />
models and expenmental systems of Ca llosobnu:h us spp<br />
(Bellows, 1982a, Hassell et al , 1989)<br />
Most of the models mentioned so far are capable of<br />
producing chaotic dynamics at certam parameter values<br />
However, Hassell et al (1976) have shown that m most<br />
smgle-species cases denSity-dependence Will probably<br />
produce monotomc dampmg, rather than chaotic or unstable<br />
OSCillations Indeed, the models discussed m thiS sectIon,<br />
usmg realIstic bIological parameter values, show either a<br />
monotomc or OSCillatory approach towards a stable<br />
eqUIlIbnum, or stable lImit cycles Constantino et al<br />
(1995, 1997) have shown that quasipenodic cycles and<br />
chaos, which IS predIcted by their model of Tl'1bolmm for<br />
certam parameter values, can be achieved by artifiCIally<br />
Imposmg certam adult recrUItment and death rates m<br />
laboratory population of Tr1bohurn<br />
Nuttall (1989) developed a very detailed model of the<br />
population dynamiCs of the stored product beetle Ptmus<br />
tectu,:" HIS model Illustrates the dynamiC processes that<br />
have been treated m thiS section, mcludmg denSity<br />
dependence, delayed denSIty dependence, and all types of<br />
fluctuations m population numbers<br />
Interaction with a parasltOid or predator<br />
Numerous studies have been devoted to the dynamiCSof<br />
systems With one or more herbivores and one or more<br />
natural enemies In contrast to the denSity-dependence<br />
lIterature, the question that IS behmd most herblvore-<br />
77<br />
natural enemy models IS .how can hosts and natural enemies<br />
coexist, or how can their populations become stable?' rather<br />
than 'how can population oscillations anse? ' (Hassell &<br />
May, 1973, Reeve, 1990) Reviews of the literature on<br />
host-parasitoid models have recently been published<br />
(Godfray & Hassell, 1994, Jones et al , 1994, Mills &<br />
Getz, 1996) Although many of these models have not been<br />
developed specifrcaily for stored-product systems, the theory<br />
ISso general that It can be applied to such systems as well<br />
Here we WIllrefer mainly to the reviews, in addition we Will<br />
CIte some examples from work on stored-product msects<br />
Numerous mechamsms that promote stability have been<br />
Identified (Reeve, 1990, Godfray & Hassell, 1994, Jones et<br />
al , 1994, Mills & Getz , 1996) For convemence we have<br />
combmed the most Important mto four groups In general,<br />
stability or long-term coexistence of hosts and parasitoids IS<br />
promoted by the following mechamsms<br />
1 A relatively low efficiency of the parasitoid at low host<br />
densrties (Godfray & Hassell, 1994) Thrs can be<br />
related to learning behaviour of the parasitoid, or to<br />
refuges for the host Learmng behaviour imphes that<br />
parasrtoids search more efficient at higher host<br />
densities, because they become fanuhar With this<br />
particular host Such parasitoids search less efficiently<br />
at low host densities Physical refuges provide shelter<br />
for a number of hosts, which also results m a reduced<br />
attack rate at low host densities However, host<br />
refuges need not be only physical Phenological and<br />
physiological refuges are also possible (Godfray &<br />
Hassell, 1994) Phenological asynchrony (the host<br />
emerges before or after the penod m which parasltOlds<br />
are active) results m a temporal refuge, and the term<br />
phYSiologicalrefuge ISused to mdlcate that some hosts<br />
may escape parasItism as a result of e g encapsulation<br />
of the parasltOld eggs These mechamsms, too,<br />
promote stabilIty Low parasltOld effiCiency at low<br />
denSities of the host IS graphically represented by a<br />
SigmOIdalfunctional response curve<br />
2 A relatively low effiCiency of the parasltOld at high<br />
parasltOld denSIties (Hassell & May, 1973, Godfray &<br />
Hassell, 1994) At mgh parasltOld denSities the<br />
encounters between parasltOldsbecome more frequent,<br />
and thiS results m relatively fewer hosts bemg<br />
parasItized Even Without phySical encounters, many<br />
paraSltOlds notice the presence of conspeClflcs by<br />
chemical cues (Godfray & Hassell, 1994) It has been<br />
observed that thiS leads to a reduced net effiCiency<br />
3 InclUSionof more species Oones et al , 1994) The<br />
mtroductlon of an extra parasltOld or host speCIesmay<br />
contribute to stabilIty Competition between a<br />
speclaltst and a generalIst parasltOld, for mstance, can<br />
result m a stable eqUIlIbnum where the mteractlOn<br />
between the host and the generalIst alone wnld not be
Proceedmqs of the 7th Iniernaiumal Workmg Conference on Stored-product Protectum. - Volume 1<br />
stable. Similarly, a system of two hosts and a host themselves Tuda & Shimada (1995) mvestigated the<br />
parasrtoid which switches to the host which IS most persistence of the host-parasitoid system at different<br />
abundant at any particular time IS potentially stable temperatures Persistence of host and parasitoid appeared to<br />
4 Spatial heterogeneity m host density, combmed With depend very precisely on ambient temperature<br />
environmental<br />
attack (Reeve.<br />
variation or<br />
1990, Jones<br />
variation m<br />
et al 1994.<br />
parasitoid<br />
Godfray &<br />
Spatial processes in single-species systems<br />
Hassell. 1994) Spatial heterogeneity or patchmess Compared to spatial aspects of the interaction between<br />
may contnbute to stabrhty of a system. because If the hosts and parasitoids, relatively little attention has as yet<br />
host and/or the parasrtoid become extmct m one been paid to spatial aspects of smgle-species systems of<br />
patch. the same patch can be colomzed agam m the storage pests Hassells et al ( 1989) investigated the<br />
next time step Of the four muchamsms mentioned. stability of patchy systems WIth C maculaius or C<br />
this one IS also a very powerful persistence-promoting chmensts m an expenmental and model context Stability<br />
mechamsm (Jones et al 1994) It may be the most appeared to depend on the reproduction rate and the spatial<br />
Important force behmd coexistence m successful distnbunon of the females A high reproduction rate,<br />
biological control SItuations (Beddmgton et al 1978. combmed With little aggregation (low patchiness) results m<br />
MIlls & Getz, 1996)<br />
mstabihty of the system A high degree of patchiness can<br />
A general cntenon for stabrlrty. based on these cause the system to stabilize (and also results m a smaller<br />
mecharusms, could be heterogeneity or vanabihty m the risk population SIZe due to unequal resource utihzation combined<br />
of bemg parasitized (Godfray & Hassell, 1994) It should be With density-dependent effects) However, the<br />
noted. however, that stability also depends on the model expenmental systems were all in the zone of damped<br />
context Some of the mechamsms mentioned abouve do not oscillations, the degree of patchiness m tills case had little<br />
create stability m discrete-time models (Nicholson-Bailey), effect on stability<br />
willie they do so for contmuous-ume models (Lotka- Tuzmkevich (1991) developed a model which could<br />
Volterra) (Reeve, 1990. Godfray & Hassells, 1994, MISS& quantitatively predict the results of an expenment wich<br />
Getz , 1996) ThIS IS due to the time-delay WhIChIS inherent Newman. Park & Scorr had carred out m 1956 In tlus<br />
to the discrete-time models As was concluded m the expenment, the spatral dIstrIbution of Trtboltum beetles m<br />
preVIOUSsection. delayed denSIty dependence often creates a CUbIC contamer. fIlled With flour. was studied The<br />
mstabilIty<br />
hIghest concentrations of beetles occurred near the walls of<br />
Ecologral theory concermng host-parasitOied relatIonshIps the contamer. and near the surface of the flour mass The<br />
IS Illustrated by several examples WIth Callosobruchus model of Tuzmkevich (1991). of willch baSIC features were<br />
chmensu; and C mnculatus Hassell et al (1985) showed mortaity as a result of competition With nearby mdlvlduals,<br />
that Amsopterornalus calandrae and Heteros]nlus and random movement of beetles. descnbed the<br />
prosopulu; both show mverse denSIty dependence m an expenmental data well He also dIscussed other models<br />
enVIronment With patchIly distrIbuted hosts (the fraction of WhICh had been developed to explam the observations<br />
host larvae paraSItIZed m a patch decreased With mcreasmg There are many models that do not fIt mto any of the<br />
host denSIty m a patch) Tills was due to a lack of denslty- categones that have been dIscussed so far (e g Joneset<br />
dependent aggregation of the parasitoid and a relatlvely low al 1990) However, the models descnbed m tlus sectlon<br />
maxImum attack rate per parasltold WIthm a patch show WhICh aspects of storage pest ecology have receIved<br />
Accordmg to Hassell et al (1985), thIS mechamsm, lIke most attention<br />
normal denSIty dependence m a patchy enVIronment. can<br />
contrIbute to perSIstence of the system Hassell & May 'Pest Management' Models<br />
( 1988) mention that A calandrae and C chmenstS<br />
coexIsted m a patchy expenmental system. whereas the Pest management models have been developed pnmanly by<br />
homogeneous system was unstable The dynamICS of both research teams m AustralIa, North-Amenca and the Umted<br />
brucilld speCIes and the larval parasitOid Larwphagus Kmgdom (Throne, 1995) They deal mamly With storage of<br />
dlSttnguendUS were mvestIgated m a model context by wheat. but also With storage of maIze (Subramanyam &<br />
Bellows (1988) Tuda & ShImada (1995) dId the same for Hagstrum. 1991, SmIth. 1994, Throne. 1994, Maler et<br />
C chtnenstS and H prosopults In both studIes. age- al 1996) or nce (Ryoo & Cho. 1988) Models that deal<br />
structured models were used The model of Bellows (1988) With other types of stored produce, such as frUit (Jang,<br />
predIcted erratic fluctuations WhICh were SImIlar to those 1996). are not conSIdered here because the system IS very<br />
found m laboratory systems These fluctuations were the different from cereal storage<br />
result of the age structure and complex mteractIons between Most attentIon has been paId to frve coleopteran pests,<br />
parasitOid attack of host stages and competition between the namely Oryptolestes ferrugtneus , Oryzaephtlus<br />
78
Prcceeduuu. of the 7t11 lutes uatiouo! WOIhlllg Conteience 0/1 Stcn ed-prodnct Protection - Volume<br />
'1011I1/(l/ilell'I. (Smith. 199.:1) and<br />
Cepluilouonnu uutei stuu, against Ci uptolesie-;<br />
Models to Storage in Developing<br />
Countries, Particularly Stored<br />
fl'l I uotuetu ( Fhnn & Hagstrum , 1995)<br />
Cowpea in West Africa<br />
Temperature IS included Ii1 most models .h the mam<br />
vanable drr, mg insect development Many ot these models Aspects considered essential to tropical storage are (1) the<br />
can therefore be used for prediction of insect population specific pest organisms involved. (2) high temperatures and<br />
growth based on actual temperatures measured m the sometimes high humidity. which create an Ideal<br />
granary In addition, some models also predict the environment for many pest msects (Jones et al 1993.<br />
temperature inside the granary (e g . at the center of the Hames. 1995), (3) socio-econormc constraints. due to<br />
gram mass) based on ambient temperature and solar which gram coolmg , the apphcation of pesticides, and other<br />
Irradiation The latter models mclude physical sub-models techniques commonly used m mdustnahzed countnes are not<br />
wluch descnbe the conduction ot heat through a gram mass teaslble. (.:I) the Impact of natural enemies on pest<br />
(e g Thorpe et al , 1982, Flmn et al 1992. Maler et populations All these aspects should be taken mto account<br />
al , 1996, V-v'oodset al . 1997) These phYSical models are m a relevant model that can be used to develop a sound<br />
not mcluded m thiS overView, for a review see Jayas bIOlogICal control strategy for stored-product pests m<br />
(1995 )<br />
developmg countlles<br />
Models which predICt populatIOn size as a functIOn of<br />
temperature normally mcorporate models which predICt rates<br />
Pest orgamsms<br />
of development. mortality. and OViposItion as a functIOn of In pest management models. most attention has been<br />
temperature These models are sometimes pubhshed gl\ en to bve coleopteran storage pests However. the<br />
sepalatelv (e g Hagstrum & Mllhken. 1991) They are msects willch cause most problems m developmg countnes<br />
only mcluded If It IS clear that they were developed to be are not necessanly the same (see Allotey. 1991. Khan &<br />
used m pest-management models<br />
Mannan. 1991, Gahukar. 199.:1. Hames. 1995) For<br />
Most models predict vanables such as the number of mstance. after ItS mtroductlOn from Central amenca m the<br />
msects present per kilogram of gram In general, the 1980s. PlVl>tep!lll/lIUo tllllicatw, (Coleoptera<br />
practical' models are sm1UlatlOn models whICh are Bostnchldae) has become a se\ere pest of stored maize m<br />
charactenzed by empmcallelatIonshlps Some exceptIOns to Atnca (Boxall. 1Y91 Gahukar, 199.+) Smlliarly.<br />
tl1lS rule mclude the parasltOid-host mteractlOns m Smith llposcehds (Psocoptera) seem to be a much bigger problem<br />
( 199.:1) and Fhnn & Hagstrum ( 1(95) which ha\ e teatures of m tropICal gram stores than m temperate zones (Hames.<br />
mechamstlc models, phYSICal submodels, which are 19(5) Fmallv. bruchlds are knowledge. no pest<br />
mechamstlc and often also analvtlc. and models of Longstaff management model that deals With bruchlds has yet been<br />
(1981. 1(88) and Thorpe et al (191\2). m whICh an developed However. a number of models that deal With<br />
analytical approach to mvestIgate pest-management Issues bruchlds have been developed m theoretical ecology (e g<br />
was used In additIOn. some models of msect development Bellows. 1982a. b. 191\8)<br />
rate employ bIOchemically<br />
rendel s them mechamstlc<br />
meanmgful parameters<br />
(e g Rvoo & Cho,<br />
which<br />
19S5,<br />
High temperatures and high denSIties of msects<br />
Subramanyam & Hagstrum. 1991. I(93)<br />
Many of the models that have been developed m<br />
79
Pvoceedruq- 0/ tin' Til: Iutei uatiouo! Wl!lhlllg Coutevence Oil Sttn ed-proli«t Protect ion - Yoiun,« 1<br />
theoretical ecology take into account high densities of<br />
insects Tlus IS not surpnsmg , because theoretical<br />
ecologists au, inu rested m the tactors wluch regulate the<br />
dynamic size ot a population However, in these studies the<br />
densities of insects are generally so high that they are not<br />
relevant tor (tropical) gram stores ( see e g Hassell et al .<br />
19t;9. Jones et al 1990) Moreover, in the studies on<br />
Tllbo!1II1Ji and Callowbl uclut--, the tood source IS tvpicallv<br />
renewed at regular intervals (e g Fuju , 1976. Hassell e-t<br />
al 1989) Tlus does not resemble a normal storage<br />
situation the storage manager. on the other hand. wants to<br />
control pest populations below levels which are of interest of<br />
theoretical ecologists As a consequence. the tvpical pest<br />
management model deals pnmarily wtth msects at \ery low<br />
densttLes where populattOn dvnanuc processes ate<br />
qualtatt\-elv dttferent from those at htgher densttLes<br />
Temperatures m cowpea grananes m West Afnca (an<br />
exceed -iOe (Van Huts et al . submttted. C Stolb. et al<br />
unpubltshed results) These temperatures need not be lethal<br />
for the adult brucluds (see e g Hemtlch. ILJLJ3) but<br />
mortaltty of the larvae may mcrease Tuda & ShImada<br />
(1993) have shown expenmentally and theoretIcally that a<br />
rather small mcrease m ambIent tempetatute (trom 30°C to<br />
32°C) lesults m a stgmftcant decltne m the equtltbtlum<br />
populatton stze ot Chllo,obl III II/Ii) dllllCII;"lb Tlus may be<br />
due to a decreased lanaI suntval at 302C compared to 30°C<br />
(Tuda. 1993) Tuda & Shtmada ( 1095) ha\ e shown ( both<br />
expenmentallv and m a model context) that the perststence<br />
of a larval parasttOld of thts bruchtd. Hetcm\pil u\<br />
PI'ObOP/(ZI\ IS gleater at 3WC than at 32'C AddttLonally.<br />
their model predtcted that the parastt01d would go e"tmct<br />
ear Iter at 28°C than at 32C These results suggest that<br />
relatt\-ely small dtfferences m temperature may hm-e a<br />
stgmflcant eflect on btOlogtcal control. as we reterred to m<br />
the secon secttOn of thiS paper Most practIcal pestmanagement<br />
temperatures<br />
models. howe\er. are not valtdated for htgh<br />
50('10-1'(01101111(' lO/l\fiWIl t\<br />
Compton et al (1992) and Jones et al (1993) ha\e<br />
develped an expert system for pest control m bagged matze<br />
m the troptcs Tlus computer-based model tS mtended fO!<br />
large-scale stores Oones et al . 1903) and tS not applOpnate<br />
for Saheltan subststence falmers who store thetr cereals and<br />
beans prt\-ately The lack of tmanctal means and the poor<br />
state of mfrastructute (electncttv. lOads) tmposes<br />
constramts on the methods that can be used to control<br />
st01age pests m thess systems The control methods that are<br />
constdered m most pest management models are not<br />
applOpnate m tlus case<br />
Rather than a dectstOn support tool tor farmers. we want<br />
to develop a model that enables lesearchers and posstbley<br />
extenStOn workers to e g select a natural enemy 01 a yearly<br />
'release date' for the natural enemy whtch wtll prove most<br />
80<br />
ettectr, e m supressing the pest popula tions<br />
Natural enemies<br />
While a large pat t ot theoretical ecology deals with natural<br />
enenues , the type of questions that tS treated there are often<br />
not appropnate to storage condrtions For instance. the<br />
paradigm ot an ecological equthbrium has dormnated for the<br />
past decades (Mtlls & Cetz , 1096) However. it tS<br />
questionable whether the 'search for stabihty ' can be<br />
justified from the viewpoint of pest control m stored<br />
product It may be benefrcal if the natural enemy and its<br />
host CoeXIStat a very low equihhnum density in storage. but<br />
a high equilibnum densitv tS obviously destructive to the<br />
stored product Nonetheless. concepts and techniques which<br />
hm-e been de\ eloped m theoretLcal ecology may be used m<br />
deternunmg wluch aspects of a storage sYstem need to be<br />
mcluded m a sunulattOn model For mstance. Godfray &<br />
Hassell ( 19<strong>87</strong>. 10t;LJ) show how natural enenues may cteate<br />
sepatate generattOns m troptcal tIlsect spectes As menttOned<br />
III the second secttOn of tiltS paper. such dlstmct genera ttOns<br />
may lumt the abtltty of U<br />
populattOns<br />
lclilOplwga to suppress bruchld<br />
:-.Jatural enemtes have only been constdered m a pestmanagement<br />
model m two cases ( Table 2) However. there<br />
tS now mcreased attenttOn for btologtcal control m stored<br />
products m mdustnaltLed countnes (Arbogast, 198-i.<br />
Brower et al . 1l)LJ6) Thts may accelerate the deYelopment<br />
of pest management whtch mcorporate natural enenues<br />
Conclusions and Perspectives<br />
for the Future<br />
Thele tS a dtstmcttOn between models from theoretLcal<br />
ecologv and pest management models m terms of the type of<br />
questLons and the methods they employ (Ftg 2) both<br />
approaches ha\-e prO\ en to be vet y useful and fruttful areas<br />
of lesearch. and m order to be able to develop gutdelmes for<br />
btOlogKal contlOl m tradtttonal storage m mdustnaltzed<br />
countnes. we need to combme the' ecologtcal realtsm' of<br />
theoretLcal ecology wtth the accurate. quantLtatlve<br />
predtcttOns of pest management models Extstmg pestmanagement<br />
models have mamly been constructed for use m<br />
mdusttlaltzed counttles They do mostly not entad natural<br />
enenues. wlule these are tmportant for stored-product<br />
protecttOn m developmg countnes Natural enemies of<br />
stored-product pests are ftrmly embedded m ecologtcal<br />
the01 y. however. the type of research questLons and<br />
predtcttOns that are ytelded by these models do not meet the<br />
needs of people who are mterested m practtcal btologtcal<br />
control m developmg countnes The same tS true for othel<br />
aspects<br />
counttles<br />
that are spectftc to tladtttOnal storage m developmg
Pvoceediuq« of tile 7t11 lutei natuntu! WUlhllltj Couiereuce Oil SfOled-]JI'(x/nd Piotection - VO/IIllle 1<br />
THEORETICAL PEST<br />
mass. metabolic heat production sornetnnes results m so-<br />
MODEL AIM<br />
fEATURES<br />
•<br />
..<br />
~<br />
-NATURAL ENEMIES<br />
-HIGH INSECT<br />
DENSITIES<br />
ECOLOGY<br />
QUALITATIVE<br />
PREDICTIONS<br />
MANAGEMENT<br />
QUANTITATIVE<br />
PREDICTIONS<br />
RELEVANT<br />
FOR<br />
called 'hot spots' 111gram (Howe. 19··[\. 1962. Sinha &<br />
Wallace 1966) In such hot spots. insects and fungi<br />
develop more rapidly. causing severe deterioration of the<br />
gram quahtv the amount of metabolic heat production under<br />
different Circumstances has been measured m several storedproduct<br />
pests (e g Cofie-Agblor et al 1995. 1996)<br />
-MECHANISTIC<br />
RELATIONSHIPS<br />
-NO -NATURAL- - - - - -<br />
ENEMIES<br />
-LOW INSECT<br />
TROPICAL<br />
STORAGE<br />
------------~--------------<br />
,<br />
I,<br />
(LIJ<br />
Longstaff ( 199 J 1997) and Throne (1995) have both<br />
pointed out the need to incorporate the mfluence of<br />
metabolic heat production on gram temperature models of<br />
stored-product protection Nevel theless, a robust model<br />
DENSITIES<br />
-EMPIRICAL<br />
expoIlIfIII8l<br />
growth)<br />
that pi edicts insect population<br />
based on ambient temperature<br />
growth and heat production<br />
and nunal gram conditions IS<br />
RELATIONSHIPS<br />
not yet available Such a model would both be useful for the<br />
A model that qu.mut.rrrvelv piedic h the ie--ult uf<br />
development of pest-management models. and mterestmg<br />
dt ffcrent m.magcme nt pi.u ucc«. .umed at from a theoretical point of View It would be useful because<br />
con-ervmg the n.uui II enenuc- III d n opn al ~tordge It could hep decide whether the heat generation for a grven<br />
-v-u m h nut "",dctbk Requu cuu nt-, dIe msect species and climatic range IS such that It should be<br />
111lrctt!ono{ I1ct(11lctlenemle~. vcthdd 1
PWCf'e(//lf(j, of the Tth Iutei uution«! \TO/hilf(} (()J/frlel/((' ()/I "t()/erl-jJJ'(x/lict Protection -Volllllle I<br />
also possible Filed' mdiv iduals regularly enter storages and<br />
interbreed there with storage' mdividuals Knowledge<br />
about the evolution of storage pests and their natural<br />
enenues could give Important mforrnation concerning<br />
(bilogical i control of storage pests<br />
Several insects develop easily on stored cereals and<br />
pulses. For that reason. they are both a nuisance to the<br />
storage manager. and an opportunitv to the SCientist who<br />
seeks to understand population ecologv Storage managers<br />
and theoretical ecologists need collaborate for safe and<br />
effective control ot those insects m storage As haines<br />
(1995) has pointed out. a thorough analvsis ot the ecology<br />
ot tropical stored-product pests IS needed If we want to<br />
control those pests Natural enenues will hav e to be included<br />
111 such an analysis for the development of biological control<br />
of stored-product pests m developing countries<br />
Acknowledgements<br />
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PhD students ot the Laboratory for Entomology for valuable Hagstrum (eds ). Integrated management of insects 111<br />
comments on previous versions ot this paper Many of the stored products Marcel Dekker. Inc New York (etc )<br />
Ideas presented m this paper have emerged from the work of pp 223 - 286<br />
Frans van Alebeek We thank Felix Wackers tor mterestmg Brown. D and Rothery. P 1994 Models m biology<br />
discussIOns Gerard Pesch assisted With the literature Mathematics. statistics and computll1g John Wiley and<br />
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