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Pecan domestication has been a gradual process, initially consisting of thinning riverine woodlands to pure stands of
pre-existing pecan trees. Wild pecan nuts are commercially attractive and usually preferred by bakers and confectioners
because they are typically priced about 25% less than selected varieties; they are about 40% smaller, allowing the whole
halves to be pleasingly presented on small cakes, candies, etc.; and they are comparable, if not superior, in taste to selected
varieties. More than 60% of current nut production in Texas comes from such trees with the remainder produced by vegetatively
propagated varieties. Thus, the pecan landscape throughout the indigenous range consists of wild trees growing in
mixed species woodlands, native pecan that has been thinned from woodlands, and vegetatively propagated orchards often
found adjacent to one another. Management programs like fertilization, pesticide application, irrigation, and pruning range
from being most intensive in orchards to virtually nonexistent in mixed species woodlands. This variety of habitats provides
unique opportunities to examine many questions including effects of plant domestication on arthropods. Most investigations
of the pecan arthropod complex have been spurred by economic considerations and have concentrated on species that pose
economic threats or economic relief (Harris 1983). However, given the long, close association of the arthropod complex with
the pecan and the physical proximity of the wild and cultivated varieties, the opportunity to interpret results of pecan arthropod
interactions from other perspectives invites attention. The arthropod complex consists of more than a 100 phytophagous,
predator, and parasite species that have been associated with pecan from the earliest of times (Harris 1983).
The blackmargined aphid (BMA) is a monophagous, multivoltine, obligatorily alate, phloem feeder that overwinters
as eggs placed in bark crevices and parthenogenetically infests pecan from shortly after budbreak until the fall some 7-8
months later, when males are produced, mating occurs, and the overwintering egg population is established (Harris 1983).
Seasonal phenology on wild trees typically consists of BMA densities initially below 1/leaf increasing to between 1-10
aphids/leaf for a 2-3 week period during the summer, and then returning to densities below l/leaf for the remainder of the
season. Orchard trees typically experience _>rivefold higher densities and exhibit BMA population increases earlier in the
season that last for 3-4 weeks (Liao et al. 1984).
BLACKMARGINED APHID AS A KEYSTONE SPECIES
Effects of BMA on nut production appear to be negligible in wild trees (Liao and Harris I984), and with one exception
(Wood et al. 1987), on orchard trees. However, routine use of insecticide in the early season typically results in epidemics
of pecan aphids (PA) (Monelliopsispecanis Bissell), mites, and leafminers (Harris 1988, 1991). The pecan aphid complex
consisting of BMA and PA has particularly been considered a primary threat to nut production (Dutcher and Htay 1985,
Beshears 11988). Population densities of BMA are typically an order of magnitude lower than those of PA in such epidemics
and, if the epidemic proceeds to defoliation, BMA is virtually absent during the latter stages (Bumroongsook and Harris
1992).
BMA outbreaks routinely occur once per season in nature, or can also be induced once following the use of broad
spectrum insecticides. Cage studies show that BMA outbreaks occur 2-3 weeks after introducing BMA into natural enemy
exclusion cages throughout the season on previously unexposed foliage (Edelson 1982, Liao and Harris 1984, Liao et al.
1984, Liao et al. 1985, Bumroongsook and Harris 1992). These outbreaks naturally subside after 3-4 weeks, leaving intact
photosynthetically active foliage (Bumroongsook and Harris 1992). BMA populations cannot reinfest the foliage for at least
a month after the first induced outbreak (Liao et al. 1984). BMA outbreaks can be curtailed by opening the exclusion cages
before the infestation has run its course or by introducing spiders or lady beetles into the cage at densities of about 1 per 10
leaves (Liao et al. 1984). Lacewing, Chrysoperla rufilabris (Burmeister), oviposition increases as BMA densities increase.
Lacewing egg densities are also higher on foliage exposed by opening cages containing an incipient outbreak of BMA (Liao
et al. 1984). Spider densities increase as BMA densities increase, and spiders readily accept BMA as prey (Bumroongsook et
al. 1992). Spider densities are also higher on foliage exposed by opening cages containing an incipient outbreak of BMA
(Liao et al. 1984). M. pecanis is slower to outbreak on mature bearing trees (Bumroongsook and Harris 1992), and high
densities result in defoliation whether the leaves have been conditioned by BMA or not, indicating these aphids are capable of
causing more loss of foliage than BMA.
These studies indicate that BMA may mitigate the impact of disruptions or natural declines of natural enemies in the
pecan ecosystem by outbreaking and then attracting and reestablishing natural enemies to the system before more insidious
phytophages increase in density (Bumroongsook and Harris 1992). The effects of such a role would not be limited to the
pecan aphid complex, but would extend to all phytophagous species serving as prey to the polyphagous lacewings, lady
beetles, spiders, etc. (Liao et al. 1984), that respond to M. caryella.
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