Factors contributing to the poor performance of a soybean aphid parasitoid Binodoxys communis (Hymenoptera: Braconidae) on an herbivore resistant soybean cultivar.

Host plant resistance and biological control are important components of integrated pest management programs. However, plants expressing resistance to herbivores may also have direct or indirect negative effects on natural enemies simultaneously providing pest suppression. Soybean aphids (Aphis glycines Matsumura) are invasive and serious pests of soybean (Glycine max L.) in the United States. Several soybean lines with aphid resistance have been identified, but the long-term impact of these resistant plants on soybean aphid biological control agents is uncertain. In a previous study, we reported that a soybean aphid parasitoid, Binodoxys communis (Gahan) had lower mummy production on resistant plants compared with a near isogenic susceptible soybean line, but the reason for this was unclear. Therefore, we examined three possible mechanisms to explain these findings: 1) resistant plants directly impact wasp emergence and longevity, 2) varying aphid density influences parasitism rates, and 3) resistant plants indirectly affect wasp development through reduced aphid longevity. We found that parasitoids in this study were not directly influenced by resistant cultivars, as there was no difference in wasp adult emergence or longevity between resistant and susceptible plants. There was also no significant effect of aphid density on mummy production over the range of aphid densities we tested. However, aphids on resistant plants had significantly shorter lifespans and were unable to survive long enough to develop into mummies compared with aphids on susceptible plants. We discuss these results and possible implications for integrating biological control and host plant resistance within soybean aphid integrated pest management programs.

Genetic structure of Graphocephala atropunctata (Hemiptera: Cicadellidae) populations across its natural range in California reveals isolation by distance.

The genetic structure of 23 populations of Graphocephala atropunctata (Signoret) (Hemiptera: Cicadellidae: Cicadellinae), a vector of the plant pathogenic bacterium Xylellafastidiosa Wells et al., was investigated using ribosomal 28S and mitochondrial cytochrome C oxidase I gene sequences. The 28S sequences were identical across all G. atropunctata specimens and populations, but 16 mitochondrial haplotypes were detected and significant interpopulation differences were found in the distribution of these haplotypes. Pairwise estimates of Fst correlated positively with geographical distance between populations, a phenomenon known as genetic isolation by distance. Thus, despite potential for widespread movement of G. atropunctata through nursery and agricultural industries, isolated populations of G. atropunctata have remained genetically distinct, suggesting that negligible numbers of G. atropunctata are actually transported or population interbreeding rarely occurs. The phylogenetic relationship between G. atropunctata and two additional congeners, Graphocephala cythura Baker and Graphocephala flavovittata Metcalf, which have overlapping distributions with G. atropunctata, also was investigated. Although 28S sequences of G. flavovittata were strikingly similar to those of G. atropunctata, mitochondrial DNA (mtDNA) suggests that both species are genetically distinct from G. atropunctata.