The population dynamics and host utilization of ... - Phthiraptera

Oecologia (Berl.) 15, 287--304 (1974) 

9 by Springer-Verlag 1974 

and 

The Population Dynamics 

Host Utilization of Geomydoecus oregonus, 

a Parasite of Thomomys bottae 

R. W. Rust 

Department of Entomology and Applied Ecology, 

University of Delaware, Newark, Delaware 19711 

Received January 28, 1974 

Summary. This study treats the population dynamics and host utilization of 

Geomydoecus oregonus Price and Emerson, a mallophagan parasite of the pocket 

gopher, Thomomys bottae (Eydous and Gervais). Over 135000 lice were collected 

from 393 gophers over a period of 20 months. Average infestation on all gophers 

was 357 lice. Bimonthly mean densities showed an increase in June-July of both 

years, and these data were statistically different from the rest. Population age 

structure remained relatively constant in time with 9.3% females, 7.2% males, 

47.4% nymphs and 36.1% eggs. Sex-age class separation of the gophers showed 

juveniles of both sexes to average 86 lice; subadult males averaged 210 lice; adult 

males averaged 544 lice; subadult and adult females averaged 255 and 296 lice, 

respectively. 

Lice were not randomly distributed on the gopher, but were most numerous on 

the head and anterior dorsal body. Lice eggs were restricted to hairs around the 

ears and eyes of the host. Over 80% of the animals sampled had eggs restricted to 

that region. 

Embryonic development and eelosion of G. oregonus proceeded over a wide range 

of environmental parameters. Over 80% of the ova tested survived and hatched in 

conditions between 33 ~ and 37~ and 22 % and 84 % relative humidity. The greatest 

survival was 98% at 35~ 75% R. H. and in a 3% COs atmosphere. 

The generation time of G. oregonus was 40• days. Duration of embryogenesis 

and nymphal stadia approximated 10• days for each. Adult lice lived 30-t- days 

on gophers. 

Age frequency mortalities were calculated as 0.02 for eggs, 0.18, 0.24, and 0.06 

for nymphal instars and 0.50 for adult lice. This indicates a Type II survivorship 

curve. 

There was a direct linear relationship between the number of female lice on a 

gopher and the number of lice eggs. The average number of eggs per female was four. 

Using the pivotal frequency for reproduetives, it was possible to calculate R o for the 

louse at 1.272. Thus, r was equal to 0.24 per generation or 0.006 per day, and the 

population doubles in 2.8 generations. 

Speculations regarding population regulation are also included. 

Introduction 

The relationshi p between an ectoparasite and its host is governed by 

a complex set of interacting biotic and abiotie parameters. For the ecto- 

1913 Oecologia (Bet1.), Vol. 10

288 R.W. Rust 

parasite, the biotic parameters are in general related to or originate from 

the host. The degree of adaptation of the parasite to the biotic parameters 

corresponds to the host specificity of the parasite. The degree of host 

specificity also indicates the adaptations of the parasite to the abiotie 

parameters influencing the host. Mallophaga, biting lice, are host specific 

(Hopkins, 1949; Werneck, 1950), and the restriction of all life stages to 

the body of the host refines the specificity and adaptations of a louse to 

its host and host's environment (see Clay, 1949; Ash, 1960; Baum, 1968; 

Foster, 1969, for avian lice; Ewing, 1924; Hopkins, 1949; Waterhouse, 

1953; Craufurd-Benson, 1941; Scott, 1952; Murray, 1957b-d, 1965; 

Hopkins and Chamberlain, 1969, for mammalian lice). For lice, the 

combination of interacting mortality factors and a species innate capacity 

for increase will determine the population level on a particular host. 

Evans and Smith (1952), in their classic demographic analysis of Pedi. 

culus humanus Linnaeus, were the first to study the reproductive potentials 

of an ectoparasite. Murray and Gordon (1969) modeled the population 

dynamics of Bovicola ovis (Linnaeus), a biting louse of sheep. They 

showed mathematically the development of the population from a low 

summer density to a winter high density. 

The genus Geomydoecus, parasites of the fossorial rodent family 

Geomyidae, possesses morphological adaptations not found in other 

trichodectid lice (Ewing, 1936). Degenerate adaptations as the absence 

of eyes and lack of abdominal spiracles and tergites might be a response 

to the altered conditions of the host's burrow environment (Kennerly, 

1964; McNab, 1966; Darden, 1970), and specializations of the antennae 

and well-developed hair-groove could relate to life on the gopher. Recently, 

Price and Emerson (1971) studied Geomydoecus and found 45 taxa 

on 29 of the 36 species of gophers. Most gophers have only one louse 

species; exceptions were hosts with extensive geographical distributions. 

The relative accessibility, small size, and peculiar habitat of the host 

provide a system for analyzing a parasite's population dynamics. Thus, 

Geomydoecus oregonus Price and Emerson, a parasite of Thomomys bottae 

(Eydoux and Gervais), was selected for analysis and interpretation. 

Methods and Materials 

Field 

A minimum of 20 pocket gophers per month as live-trapped from two irrigated 

alfaHa fields located on the University of California, Davis Farm, and 2 consecutive 

months of collection data were combined for analysis. The field sizes, agricultural 

practices, measurement of environmental parameters, gopher collecting and laboratory 

handling techniques and parasite recovery methods are treated in Rust 

(1973a, b). 

Three age classes were established for both sexes of gophers: juvenile, subadult 

and adult. Separation was based on reproductive condition, skull features, and body

Population Dynamics of the Gopher Louse 289 

measurements (Miller, 1946, 1962; Hansen, 1960; Thaeler, 1968). Juveniles had the 

following sutures unfused: parietal-squamosal, maxilla-alisphenoid, and alisphenoidsquamosal; 

the dorsal surface of the maxilla (in the zygomatic arch) and palatine 

showed distinct porosity. Juveniles had gray pelage and were still in association 

with their mother, as determined from trapping. Subadults had partial fusion of the 

cranial sutures, reduced porosity and brownish pelage. Subadult females had a 

closed pubic symphysis and limited follicular development; subadult male testes 

were only 3-6 mm long. Adults had skull sutures and nonporous bones. Adult 

females had the pubic symphysis open, placental scars, embryos, or were lactating. 

Adult males had testes from 7-15 mm long. Body length and maximum skull length 

and width showed overlap due to a continuous breeding population (Miller, 1946) 

but were also used in the separation. 

Time for the age categories would be approximately one and one-half to two 

months for juveniles, 2-5 months for subadults and the mean length of life for adult 

gophers has been estimated by Howard and Childs (1959) as 7.6 months for males 

and 12.3 months for females. 

Laboratory 

Eelosion and first instar survival in vitro were examined under various environmental 

conditions. 8 different temperatures (27-41~ were maintained in Percival 

environmental chambers. The chambers were set for total darkness with the 

photophase only when eggs or nymphs were checked. Eight different relative 

humidities (10-96%)were maintained in liter desiccator jars containing various 

saturated salt solutions (Winston and Bates, 1960). Eclosion was tested in normal 

air and a 3% COz atmosphere. The high CO 2 atmosphere was created by removing 

3 % of the gas volume from the jars and replacing it with CO 2. The CO 2 concentration 

was measured with a Unieo Gas Analyzer. 

Four replicates of 25 eggs each were tested at 64 different temperature-relative 

humidity combinations. Translucent eggs were plucked individually from gophers 

for the temperature-relative humidity tests. Eclosion and nymphal survival in 3% 

CO 2 atmosphere were recorded at 33 ~ and 35~ both at 62%, 75% and 84% R. It. 

Nymphal survival was also checked in normal CO 2 atmosphere. 

The duration of embryonic development was obtained by allowing female lice 

to oviposit in small petri dishes. This gave eggs of a known age (-4-12 h), and these 

eggs were placed in separate containers in various temperature-relative humidity 

conditions. 

Louse survival on the host was checked in a normal CO 2 atmosphere at various 

humidities. Humidities tested were 35 % • 5, 50 % =J: 5, 75 % • 5 and 100 % -- 2 and 

were maintained in environmental chambers at 21~1771. 

Louse demography was studied (in rive) in an artificial burrow system simulating 

natural conditions (Darden, 1970). Four artificial burrows were housed in 

adjoining chambers in a soil box (60.9 X 91.4 X 121.9 cm). Burrows were constructed 

of 7 • 7 mm hardware cloth welded together. A system consisted of an above ground 

holding cage (14.2 • 20.3 X 30.4 ore), an initial diagonal tunnel (30 cm), a horizontal 

tunnel (24 cm) and a nest chamber (19x24 era). The tunnels were 7.5 cm in diameter. 

Before an animal was placed in a burrow system, carrot pieces and fresh 

alfalfa were placed in the next chamber. The system was packed with soil and 

buried in the soil box, with 25 to 30 em of soil over the nest chamber. The surface 

was moistened with approximately 2 1 of water and allowed to stand for 24 h before 

a gopher was introduced. Additional fresh alfalfa was provided daily. Two Plexiglas 

tubes (6 mm O. D.) were attached to the roof of the nest chamber and horizontal 

tunnel. Both gas samples and temperatures were measured via the tubes. The entire 

system was housed in a greenhouse with a regime of controlled temperature and

290 R.W. Rus~ 

natural light. Every seventh day of an experiment an additional 21 of water were 

added to the surface. 

Two different treatments were designed for gophers placed in the artificial 

system. Newly eclosed first instar nymphs were introduced on a clean gopher, and 

the animal was released in a system for a set period--6, 10, 15, 20, 25, and 30 days. 

Freshly caught gophers were cleaned of parasites, empty lice eggs were plucked 

from the animal, and the remaining lice eggs were counted and mapped. These 

animals were released in a system for 20, 30 and 40 days. At the end of an experiment, 

the animal was sacrificed and cleaned. 

Statistical analyses used in this study were Student's t-test, linear regression and 

correlation, single class analysis of variance and curvilinear regression. 

Results 

Macro- and Microenvironmental Parameters 

During this study, average monthly temperatures varied from a low 

of 3.3~ in winter to a high of 25.4~ in summer. The Davis weather 

station mean maximum temperatures were 16.2 ~ and 30.2~ and the 

mean minima were 5.1 ~ and 11.8~ in winter and summer, respectively. 

Average monthly precipitation was from a high of 61.5 mm in winter 

months to 1 mm in summer months. Average monthly soil temperatures 

at 101 mm depth ranged from a low of 7.2~ in January to a high of 

32.3~ in July. The soil moisture averaged 6.6% in winter and 5.3% in 

summer. The high summer soil moisture level was maintained by irrigation. 

The alfalfa fields were flood-irrigated at approximately 30-day 

intervals from April to September, with water covering the fields, except 

irrigation levees, for nearly 24 h. 

The relative humidity of the burrow measured 97.4• (N-=96). 

Kennerly (1964), McSTab (1966), and Darden (1970) also recorded the 

near saturated atmosphere of the gopher burrow. Darden (1970) determined 

that the CO S concentration of the gopher burrow averaged 2.3 %, 

and 03 averaged 18%. MeNab (1966) found an average CO S concentration 

of 1.4 % in the burrow of Geomys pinetis Rafinesque, and Kennerly 

(1964) measured only 0.8 % CO S in Geomys bursarius (Shaw) burrows. The 

burrow temperatures showed less fluctuation than ambient temperatures 

and were out of phase with them. Rust (1973b) showed the relationship 

between ambient and burrow temperatures at two extreme times, 

August and January. 

In the artificial burrow system, the nest chamber temperature 

averaged 20.5-V0.6~ C (N-=30) and the CO S concentration was 2.4-4-0.1% 

(N=24). The soil moisture averaged 6.2% and ranged from 7.2% to 

4.1%. The greenhouse temperature was maintained between 14.4 ~ and 

25.0 ~ C. Thus, the conditions for experimentation are considered similar 

to those that would be found in a natural burrow system, except during 

the cooler winter months.


Population Density and Structure 

Over 135000 Geomydoecus oregonus were removed from the gophers; 

5 animals were clean of lice. The average infestation for all gophers was 

357.5q-13.9 lice, and 1941 was the maximum number of lice recovered 

from an adult. Five other gophers (2 females and 3 males) had more than 

1000 lice on them. The bimonthly mean densities (Fig. 1) showed an 

increase in June-July of both years, and these sample means were found 

to be statistically different from the rest (F=2.7417, P~0.005). The 15 

"dry land" gophers from August-September 1971 had a mean density of 

376.1-~66.4 lice. This was not found to be significantly different from the 

mean of the "irrigated land" sample for the same period or the overall 

mean (t=0.4253, 0.9~P ~0.5 and t=0.2709, 0.9~P~0.5, respectively). 

The population age structure remained relatively constant in time with 

9.3% females, 7.2% males, 47.4% nymphs and 36.1% eggs (Fig. 1). 

Juveniles had an increased proportion of eggs (44.8) and reduced nymphs 

(36.2), but adults were nearly equal. Geomydoecus oregonus has three 

nymphal instars as determined from head capsule measurements and 

increased number of abdominal setae. The proportion of the nymphs 

represented by each was 19.9% first, 13.3% second and 14.2% third. 

1Vlean louse populations on male and female gophers were strikingly 

different with males averaging over 430 and females under 290. Further 

analysis of the population based on gopher age classes of juvenile, subadult, 

and adult revealed a greater difference. Juvenile gophers (N~--18), 

regardless of sex, averaged only 86.3~ 13.7 lice. Subadult males (N=46) 

had 210.8~19.8 lice, and adult males (N----120) averaged 544.2-~25.6. 

However, subadult (2V=71) and adult females (N----123) averaged nearly 

the same number of lice 255.0q-26.3 and 296.0• respectively. The 

bimonthly sex-age class averages are plotted in Fig. 2. 

When the sex-age class averages are considered, the seasonal changes 

are at most slight although when combined, they show a statistically 

significant increase in June-July. This early summer increase in 1970 was 

a result of large louse populations on adult male gophers with 8 of 10 

having over 600 lice. The 1971 louse increase resulted from an increase in 

both adult female and subadult populations. Three of the adult females 

had over 800 lice, which if removed from the sample, reduces the mean 

to 299. 

Lice were not randomly distributed on the gophers, but were most 

numerous on the head and anterior dorsal body. Selective cleaning 

produced the following distribution of lice: 19.7% dorsal head, 14.0% 

ventral head, 49.5% anterior dorsal body, 7.5% posterior dorsal body 

and 9.3 % ventral body. Lice on the ventral body were restricted to the 

area between the forelegs and axillae. If the above were based on the

292 R. W. Rust 

600 

5OO 

9 g~ ~ 400 

~300 

2OO 

I 

Nymphs 

6O 

5O 

40 

.... ~ ~ "'"~"~ 30 

20 

Females 10 

-- Males 

A-IM J'-J A:S O'-N D% F'-M A:M J:J A'-S O'-N 

Months 

Fig. 1. Population structure of Geomydoecus oregonus from 378 Thomomys bottae 

collected from Davis, California, from April 1970 through November 1971. Left and 

above: mean population density for all gophers, vertical lines equal standard error. 

Right and below: population age structure 

800 

700 

600 

5OO 

~ 400 

g 

300 

200 

100 

Adutt Femc~le ~/"/'x"-, 

/, x, 

X ,,... ~ 

~ ~ ? "',",~"- "......... 7~ ""....... 

..~" " Subadu[[ Mole 

Mean Juvenile 

A"M J"J A~S ' O"N D~'J F~'M AIM J.J A"S O~'N 

Months 

:Fig. 2. Mean population density of Geomydoecus oregonus based on sex-age class 

separation of Thomomys bottae. Juvenile gophers represented by overall mean only


Fig. 3. Distribution of Geomydoecus oregonus eggs on 89 Thomomys bottae, above. 

Mean regional body temperatures in degrees centigrade, below 

proportion of the gopher's body inhabited, then 92.5 % of the lice were 

found on 55 % of the animal's surface. 

Egg Distribution, Numbers and Environmental Tolerance 

Distribution of the lice eggs was more restricted than thedistribution 

of nymphs and adults. The proportional distribution of eggs on 89 gophers 

is shown in Fig. 3. Over 80 % of the gophers had lice eggs located around 

the ears and eyes. If juvenile animals and adults with high infestations 

(800q-lice) were removed from the sample, the lice eggs on the mid-dorsal 

back were excluded from the distribution. An egg of G. oregonus is glued 

to the base of the hairs from one follicle and only one egg]hair group was 

observed. Although oviposition was not observed, the placement of the 

egg on the hairs suggests an oviposition behavior similar to that described 

for other lice (Murray, 1957a). The chorion remains attached after 

eclosion and until the hair group is shed with the subsequent molt. 

There was a direct linear relationship (Fig. 4) between the number of 

female lice on a gopher and the number of lice eggs (bz3.253=k0.208), 

with a correlation of 0.868. Thus 74% of the egg variation was explained 

by the presence of the female lice (r~=0.7396). The average number of 

eggs per female was 4 (3.93) and ranged from 0.6 to 11.2. If juveniles, 

females with litters, and old gophers were removed, the range was 

narrowed to 2.9 to 8.2 eggs/female. In terms of eggs per day, G. oregonus 

lays 0.2 eggs/day in an average life span.

294 R.W. Rust 

400 .', 9 ' '" 

9 ~6 

350 

3oo 

~" 250 

0 

~200 .... 

0 

-' 100 , ,,,-/:," , , r =0.868 

n 89 

50 ~.~, ," "~.X, ,: , : , 

, _ , , , , , , 

20 40 60 80 100 120 140 

Female Lice per Gopher 

Fig. 4. Regression of number of female Geomydoecus oregonus per Thomomys bo#ae 

to number of G. oregonus eggs per gropher 

Embryonic development and eclosion of G. oregonus proceeded over a 

wide range of environmental parameters. An 80 % or greater plateau of ova 

survival and hatching (Fig. 5) was found between 33-37~ and 22-84% 

relative humidity. The greatest survival was 96% at 35~ and 75% R. It. 

The mean rectal temperature of T. bottae was 36.5~0.4~ (N=26). 

Darden (1970) recorded an average of 36.7~ for the same species. This 

temperature was near the upper end of the survival range. However, 

surface temperatures (Fig. 3) of the cheek and head region were approximately 

1~ lower and corresponded to the highest eclosion rate. 

Development and hatching occurred in 3 % CO 2 atmosphere and with 

a slightly higher survival rate than eggs reared in a normal CO S atmosphere. 

Ninety-eight percent of over 500 eggs hatched in vitro and 

97.7% of the eggs hatched on gophers in the artificial burrow system. 

Darden (1970) determined the CO s concentration of the gopher burrow 

to average 2.3 %, and O 3 averaged 18 %. I found an average CO 2 concentration 

of 2.4% in the nest chamber of the artificial burrow system. 

Normal atmospheric CO 2 was determined to slightly reduce the survival 

of the louse. Eggs on 3 gophers maintained in normal atmosphere and 

exposed to a relative humidity greater than 75 % for 30 days hatched and 

developed to third instar and adult lice with an average mortality of 

56.3%.


100 100 

m "U 

zs 

zs 

D -I 

50 50 C 

| .7 

U ~. 

D- e~ 

25 25 

~'/ 

qb 

Fig. 5. Survival and eclosion of Geomydoecus oregonus eggs reared in vitro under 

constant temperature and relative humidity 

Population Development 

The embryonic development of G. oregonus averaged 10.5days 

(N=18) and ranged from 9 to 12 days. This time plus the duration of 

nymphal development (Table 1) indicates a generation time of 40=k6 

days. Duration of nymphal stadia approximated 10• days for each. 

The longevity of adult lice was not ascertained. However, 6 females and 

3 males out of a total of 80 lice lived 30~-days on gophers in the 

artificial burrow system. 

The mortality rate (Table 1) from cohorts beginning with first instar 

lice was extremely high as indicated by a 50 % mortality of first instar 

lice after 6 days. This high death rate was probably induced by handling

296 R.W. Rust 

Table 1. Recovery of Geomydoecus oregonus from gophers, each artificially infested 

with newly hatched first instar nymphs. Gophers maintained in artificial burrows 

Days Inoc- I~umber and stage of development Total % 

after ulum Mortality 

infesta- Immatures Adults 

tion 1st 2nd 3rd ~ 

6 36 18 . . . . 18 50.0 

10 58 1 16 -- -- -- 17 70.3 

15 49 -- 8 1 -- -- 9 81.7 

20 32 -- 2 2 -- -- 4 87.5 

25 31 -- -- 2 -- -- 2 93.5 

30 40 -- -- -- 2 1 3 90.0 

the lice and a delay (12 h) before they could feed or assume a correct 

posture on the gopher. The results (Table 2) obtained from cohorts 

starting with the egg stage, indicate a Type II survivorship curve for the 

louse population. Thus, age frequency mortalities can be calculated as 

0.02 for eggs, 0.18, 0.24, and 0.06 for the nymphal instars, and 0.50 for 

adult lice. Due to the 10 day age difference of eggs used to initiate these 

tests, females and new eggs were already present on the gophers in the 

30 day samples, and at 40 days, new first instar lice appeared in the 

samples. Using the pivotal frequency for reproductives, it is possible to 

estimate the net reproductive rate, Ro=Zlxm x. Thus, R o for G. oregonus 

is 1.272 (0.53 X 2.4= 1.272). This assumes a 1:1.2 sex ratio, equal survival 

of sexes and an average of 4 eggs per female louse. The intrinsic rate of 

increase (r) can be calculated from Ro=e% Thus, r is equal to 0.24 per 

generation or 0.006 per day, and the population doubles in 2.8 genera- 

tions. 

Duration 

days 

Table 2. Recovery of Geomydoecu8 oregonus from gophers, each naturally infested 

with eggs. Gophers maintained in artificial burrow systems 

Num- Inoc- Number and stage of development Mortality 

ber 

of 

ulum 

eggs Immatures Adults Eggs % % 

gophers eggs total 

1st 2nd 3rd ~ !~ Unhatched new 

%a 

age 

interval 

20 

30 

40 

2 295 65 127 34 7 -- 2.3 20.0 

4 551 -- 7 187 42 43 11 21 1.9 49.4 

5 914 13 b -- 30 150 180 ? 157 ? 60.7 

a Determined initially from 2 % egg mortality and rounded to nearest whole number. 

b First instars of second generation. 

18.0 

30.0 

ll.0


Discussion 

Population Structure and Development 

Coffman (personal communication) studied Geomydoecus geomydis 

(Osborn), a parasite of Geomys bursarius. He found nearly twice the mean 

density of lice as I did for G. oregonus as well as a seasonal shift with a 

spring increase for G. geomydis in South Dakota. Geomys bursarius is one 

and one-half to two times as large (body weight) as T. bottae from the 

Davis area (Hall and Kelson, 1959), breeds once a year, is subject to 

different climatic regimes and possibly different microenvironmental 

conditions such as lower CO S concentration (Kennerly, 1964). Size of host 

(decreased surface area), continous breeding (Miller, 1946), the effects of 

a different climatic regime, and flood irrigation are elements which 

probably reduce the population size of G. oregonus on T. bottae in the 

Davis area. Other trichodectid lice are found primarily on fissiped 

carnivores, hyraxes and ungulates (Hopkins, 1949; Emerson, 1964), and 

seasonal fluctuations in other small mammal mallophagans are unknown 

to me. Large mammal mallophagan species show a general late fall to 

winter increase and a summer decline (Crauford-Benson, 1949 ; Matthysse, 

1944, 1946; Scott, 1952; Enzie, 1956; Murray, 1960, 1963a, b). Vysotoskaya 

(1950) and Murray (1961) have suggested that the fall to winter 

increase may relate to the host's responses to the severity of the winter 

conditions. However, the general uniformity of G. oregonus population 

structure through time may result from the modified summer environmental 

conditions brought about by irrigation. The homogenous conditions 

in irrigated lands are also responsible for year-round reproduction 

of gophers (Miller, 1946). If the louse is affected by the environmental 

conditions in the late summer as is the host (Miller, 1946; Howard and 

Childs, 1959), then a population decline may be predicted. The small 

"dry land" sample mean, however, equalled the mean number on hosts 

from irrigated lands. Howard and Childs (1959) suggested that gophers 

may retreat to lower burrows and seal shallower ones to escape the 

summer heat. This could equalize environmental conditions and allow 

louse reproduction to proceed. 

The different louse densities on the sex-age classes of gophers result 

from at least two factors : the size of the animal and the loss of a portion 

of adult female's parasites to the young of each litter. Thus, the time of 

maximum gopher reproduction would also be the most disruptive period 

for the lice on female gophers. The reduced mean density of lice on adult 

females in February-March is concomitant with maximum gopher 

reproduction (Miller, 1946), and the increased density of lice in the early 

winter months corresponds to minimum gopher reproduction. The 

solitary habits of gophers (Ingles, 1952 ; Howard and Childs, 1959) suggest 

that the exchange of parasites takes place primarily between females with

298 1%. W. Rust 

litters and between litter mates. This explains the adult sex differences 

and small louse population (inoculum) on juvenile gophers. Miller (1946) 

found an average of 5 young/litter and 2 litters/year for gophers inhabiting 

irrigated fields in the Davis area. Thus, ideally 84% of the female's 

louse population could be lost twice a year. Actual measurements of the 

proportion of the population lost ranged from 56% to 66% for litter 

sizes of 2 and 4 (N=4). 

Lice are generally found on the head and foreparts of the small 

mammals and may spread over the dorsal and lateral aspects of the host 

as the population increases (Vysotoskaya, 1950; Dubinin, 1953; Murray, 

1961). In general, louse distribution is controlled by the grooming 

characteristics of the host (Murray, 1961). Observations of gopher 

grooming indicate that lice can be nipped from the ventral body and 

possibly the hind quarters. The short neck and large pectoral girdle 

prevent more extensive preening with incisors. Preening with the hind 

and fore claws in the form of scratching probably does not remove many 

lice because of their tenacity. This action could destroy stationary nits, 

but gophers were observed to rub the head region with their fore claws 

rather than scratch with claw tips. 

Preening is an important control agent in the distribution of nits 

(Murray, 1961), but hair size and configuration (Murray, 1957b-d; 

Hopkins and Chamberlain, 1969) and host regional body temperatures 

(Murray, 1957e, 1963b, 1965) also affect their distribution. Another 

factor on the gopher is the structure of the hairs on which eggs are glued. 

Most hair follicles on the gopher contain 9-18 hairs (Morejohn and 

Howard, 1956), but the character of hairs per follicle differs in the 

various body regions. Hair follicles on the dorsal and lateral aspects of 

the head bear 4-6 large guard hairs and 7-12 fur hairs, whereas follicles 

on the dorsal body lack the stiff guard hairs. Follicular separation is 

nearly equal over the body, with the hairs follicles being separated by 

0.25-0.33 ram. The response of female lice to a particular hair configuration 

for oviposition may result in the limited distribution as seen 

on over 80% of the gophers sampled. On juveniles, all hairs are of the 

fur type and thus the possible wider distribution of nits, but as the 

gopher loses its juvenile pelage the hair per follicle change to the fur 

hair-guard hair arrangement. The wider distribution of nits on gophers 

with large infestations (800-~) possibly indicates a spreading to less 

favorable sites. 

Molting restricts the availability of oviposition sites and may result 

in a loss of eggs already laid. Evidence for the latter is sparse, although 

Vysotoskaya (1950) considered molting a major factor for population 

decline. Three gophers with active molt lines in and around the ears and 

eyes had no unhatched nits in the area. When examined 30 days later the


molt line had moved posteriorly and the empty eggs were gone. Morejohn 

and Howard (1956) studied the molt patterns of captive and field gophers. 

They found two distinct seasonal pelages, winter and summer, and a 

constant molt period with a short non-molt period from December to 

February. It is therefore possible to have a reduction in the hair follicles 

available twice a year. The dense replacement hairs and thickened dermis 

during active molting renders an area inaccessible to lice and oviposition. 

Lice were not observed in this thick, new hair. 

Using an average juvenile gopher inoculum of 86 lice and no upper 

restriction on population growth, the average mean density of lice on 

subadults can be reached in about 200 days and for adult males in 300 

days. The mean louse density of adult female gophers would also be 

reached in 200 days, and this represents an approximation of sexual 

maturity (Miller, 1946). On male gophers, the louse density would reach 

nearly 1000 in 400 days, which is the mean longevity of male gophers 

(I-Ioward and Childs, 1959). The small sample of gophers with 1000 or 

more lice suggests that factors must be operating to maintain the density 

at a lower level than predicted. 

To examine the population growth curve, an arbitrary measure of time 

was necessary because of the difficulty in accurate aging of gophers. The 

measurement selected was the increase in surface area of the gopher 

(cm 2) and was obtained by combining body length and circumference of 

the thorax cavity, legs and tail excluded from the measurement. An 

accuracy check was made by measuring fresh, fiat skins. It has been 

shown that gophers continue to grow as adults (Miller, 1952 ; Howard and 

Chflds, 1959). Therefore, a clumping of older animals in a younger age 

group is not expected. 

The average densities on categoric age groups showed nearly equal 

numbers of lice on subadult and adult female gophers. Thus, the louse 

population on a female gopher should reach an asymptote at some body 

size or point in time. The relationship between female gopher area and 

lice per area was curvilinear (Fig. 6) and the curve was defined by a 

second degree polynomial, Y=--8.9990+O.15884X--O.OOO56X 2. On 

female gophers, the louse population leveled at 2.2 lice per cm ~ over 

140 cm 2 of body area, or 308 lice. The mean densites of lice on male 

gophers differed according to age groups, and the relationships were 

analyzed for both linear and eurvilinear regression. The test for curvilinear 

regression (Snedecor, 1956) was highly significant (F--~7.1604, 

P>0.001), and a linear relationship was rejected. The curve (Fig. 6) was 

defined by a second degree polynominal, Y------5.4723-~0.09352X-- 

0.0026X 2. The louse population on male gophers leveled at around 2.9 lice 

per cm ~ over an area of 175 cm ~, or 507 lice. The respective asymptotes 

are equal to the overall mean densities for adult animals.

300 R. W. Rust 

% 

o. 

._J 

4 

3 

2 

Juveniles ..;'if"'" Female 

Adult Mate 

SubaduIt ..,.'J............................................ 

Male ..,'": ............ 

,.'..;~;:'~"~ j Adult Ferna[e 

4O 

, I " | , t I , i .... , 

80 100 120 140 160 180 200 

Gopher Body Area (cm 2) 

Fig. 6. Population growth curve for Geomydoecus oregouus on male and female 

Thomomys bottae as determined from regression analysis of the relationship of the 

number of lice per body area to the total body area available 

Population Regulation 

Population growth as presented above can be controlled by both 

density dependent and density independent factors. Food is not considered 

to be an important factor, since it is in constant supply on the host 

(Waterhouse, 1953). One dependent factor appears to be the availability 

of a proper oviposition site. This was indicated by the narrow distribution 

of nits on gophers and the inaccessibility of a site until the hairs 

were replaced. Cannibalism has been reported for avian mallophagans 

(Waterston, 1926; Rothschild and Clay, 1952; Nelson, 1971). Hopkins 

and Chamberlain (1969) found destruction of nits, as high as 25% in 

in vitro colonies of B. crass@es. I found no evidence for cannibalism in 

G. oregonus. Empty chorions on the gophers appeared intact and undamaged. 

Lice held in small containers for 5 days did not destroy or consume 

newly laid eggs or immature individuals. This does not, however, exclude 

the possibility of cannibalism for G. oregonus. 

Density independent factors such as preening, molting, and social 

interaction of the host can lead to population reduction or control. 

Environmental conditions in the Davis area and especially in irrigated 

fields do not appear to restrict the population densites. Irrigation may 

destroy a proportion of the population at the time of flooding. Wet 

gophers were found in the fields during the initial stages of irrigation, but 

more commonly, dry animals were found on field levees, murray (1963a) 

showed that adults and nymphs of B. ovis can survive immersion in


water for 1 h with less than 20 % mortality. Eggs could be immersed for 

up to 7 days before a substantial reduction in hatching occurred. It is 

therefore possible to have a reduction in the nymph and adult population 

with each irrigation. The distribution of the lice on the gopher and the 

inability of the gopher to withstand immersion for even one hour probably 

reduces the severity of irrigation. 

Interaction and competition with other ectoparasites may influence 

the upper densities (Hopkins, 1949; Vysotoskaya, 1950). Geomydoecus 

oregonus co-inhabits the gopher in the Davis area with three other 

parasites (Rust, 1973 a, b). The small size and restricted distribution of 

Geomylichus n. sp. and the nidicolous habits of Haemolaelaps geomys 

Strandtmann remove them from interactions with the louse (Rust, 

1973 b). Hirstionyssus ]emuralis Allred is found both in the nest and on the 

host in low numbers and are not considered to pose any serious deterrent 

in the louse density increase. 

Finally, the physiological state of the host, either environmentally or 

pathologically induced, may influence population size. Sick animals have 

been reported by several workers as having unusually heavy infestations 

of lice (Thompson, 1936; Spencer, 1939; Craufurd-Benson, 1941; and 

Eichler, 1942). The state of health of the 6 gophers with heavy infestations 

could not be determined. However, they appeared healthy. The 

only "unhealthy" animals encountered were wounded males, but this 

is probably a common condition resulting from their pugnacious behavior 

(Howard and Childs, 1959). Increased louse reproduction induced by 

ingestion of host hormones, as in the classical case of Spilopsyllus cuniculi 

Dale and its host the rabbit (Mead-Briggs and Rudge, 1960; Rothschild 

and Ford, 1964 a, b), is not considered important. The food and feeding 

habits of trichodectid lice (Waterhouse, 1953) would prevent contact with 

host's blood. If estrogens enter the dermal tissue fluids, then they may be 

ingested and could then be considered an important factor. 

Acknowledgements. I thank K. Emerson and 1%. D. Price for their identification 

of Geomydoecus oregonus. 1%. F. Denno, D. L. McLean, R.W. Thorp and R.K. 

Washino read various stages of the manuscript. Their criticisms and suggestions 

were appreciated. Charles Coffman provided unpublished data which was most 

helpful. Aileen K. Rust assisted throughout the study. Her comments and suggestions 

are sincerely appreciated. 

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20*

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Dr. R. W. Rust 

Department of Entomology 

and Applied Ecology 

University of Delaware 

Newark, Delaware 19711, USA

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