More phylogenetically diverse polycultures inconsistently suppress insect herbivore populations.

Because the diet of many herbivorous insects is restricted to closely related taxa with similar chemistry, intercropping with diverse plant communities may reduce both pest populations and reliance on chemical pesticides in agroecosystems. We tested whether the effectiveness of intercropping against herbivorous insects depends on the phylogenetic relatedness of neighboring crops, using butternut squash (Cucurbita moschata) as a focal crop species in a series of different intercropping combinations. We found that increased phylogenetic divergence of neighboring plants could reduce abundance of herbivorous insects, but the effect was only detectable mid-season. In addition, we tested two hypothesized mechanisms for a negative association between phylogenetic distance of neighboring plants and reduced herbivore populations: one, we tested using Y-tube olfactometer and choice cage trials whether diverse volatile cues impede host-plant location by the dominant pest of butternut squash in our experiment, striped cucumber beetle Acalymma vittatum. Two, we recorded predator and parasitoid abundance relative to crop phylodiversity to test whether diverse crops support larger natural-enemy populations that can better control pest species. Our results, however, did not support either hypothesis. Striped cucumber beetles preferentially oriented toward non-host-plant volatiles, and predator populations more often decreased with phylodiversity than increased. Thus, the mechanisms driving associations in the field between phylogenetic divergence and herbivore populations remain unclear.

Parasitoids, Nematodes, and Protists in Populations of Striped Cucumber Beetle (Coleoptera: Chrysomelidae).

The striped cucumber beetle, Acalymma vittatum (Fabricius), is an important pest of cucurbit production in the eastern United States, where most commercial producers rely on insecticides to control this pest species. Biological control provides an alternative to insecticide use, but for A. vittatum, top-down control has not been well developed. In the northeastern United States, two parasitoid species, Celatoria setosa (Coquillett) (Diptera: Tachinidae) and Centistes diabroticae (Gahan) (Hymenoptera: Braconidae) have been reported from A. vittatum, but their distribution is poorly known. To determine whether these parasitoid species are attacking A. vittatum in Pennsylvania and the amount of mortality they provide, we characterized the parasitoid dynamics in two distinct efforts. First, we reared parasitoids from beetles captured at two research farms. Second, we focused on one of these farms and dissected beetles to quantify both parasitoid and parasite species attacking A. vittatum. Both efforts confirmed Cl. setosa and Cn. diabroticae, and parasitism rates varied widely between locations and among years (4-60%). Unexpectedly, our dissections revealed that a potentially undescribed nematode species (Howardula sp.) as the most common parasite in the community. We also discovered gregarine protists. Despite being smaller than females, males were more commonly attacked by parasitic species, but we detected no relationship between the size of beetles and abundance of parasitic species in A. vittatum. This work provides a baseline understanding of the parasitoid and parasite community attacking A. vittatum and advances opportunities for conservation biological control using these natural-enemy species.