Soybean HPR virulence and parasitism by Aphelinus

Invasions by insect species that become pests are an increasing problem
for agriculture. Introductions of parasitoids from the regions of pest
origin and breeding plants resistant to invasive pests can reduce invasive
pest abundance and impact. Whether interactions between parasitoids and
plant resistance are positive or negative depends on the detailed
mechanisms. We studied the effects of susceptible versus
resistant soybean and avirulent versus virulent soybean aphid on the
parasitoids Aphelinus certus and Aphelinus glycinis, the first
with a broad host range and the second with a narrow host
range. Both parasitoid species parasitized virulent aphids at
least as much as avirulent aphids on susceptible and resistant soybeans.
Aphelinus certus parasitized fewer avirulent aphids than virulent aphids
on resistant soybean and fewer avirulent aphids on resistant versus
susceptible soybean. The number of aphids parasitized by A. glycinis did
not vary with the treatments. Emergence rates of parasitoid
progeny were high for both parasitoids and did not vary with the
treatments. Progeny sex ratios of the parasitoids did not vary with plant
resistance or aphid virulence. However, for A. certus, there was
a small effect of the interaction between plant resistance and aphid
virulence. Body masses of female and male progeny of A.
certus did not vary with the treatments. However, body masses of female
progeny of A. glycinis were larger on susceptible versus resistant
soybeans and on virulent versus avirulent aphids. Body masses of male
progeny of A. glycinis were larger on susceptible versus resistant
plants. Parasitism by A. certus is known to be
density-dependent, and given that density-dependent parasitism has been
found in several other species of Aphelinus, parasitism by A. glycines is
very likely also to be density-dependent. Given the higher densities of
virulent versus avirulent aphids on resistant soybean reported in the
literature, these parasitoids should parasitize more virulent than
avirulent aphids on resistant soybean and would limit the abundance of
virulent soybean aphid if much of the soybean acreage had resistant
plants. Little research has been published previously on the
combined impacts of plant resistance and herbivore virulence on
parasitoids. If our results hold true in other systems, plant resistance
and biological control may be more compatible in suppressing herbivore
virulence than has sometimes been proposed.