Landscape simplification associated with agricultural intensification has important effects on economically important arthropods. The declining cover of natural and semi-natural habitats, in particular, has been shown to reduce natural-enemy attack of crop pests, but also in some cases reduced crop colonization by such pests. In this study, we examined the influence of changes in two elements of landscape composition, natural grassland cover and cover of a highly suitable crop host, on infestation by a generalist insect pest in wheat, and parasitism of this pest by its dominant natural enemies. Surprisingly, we found no significant influences of increasing natural grassland habitat, at either local or landscape scales, on infestation by the wheat stem sawfly, Cephus cinctus, or parasitism of this pest by the native parasitoid wasps, Bracon cephiand Bracon lissogaster. In contrast, we found significant increases in levels of C. cinctus infestation with increasing wheat cover at the landscape scale. This pattern was consistent across six study regions spanning three states in the northern Great Plains of North America, despite large differences in cropping systems and pest population densities across regions. Regional variation in pest infestation was best explained by long-term averages in precipitation, with higher C. cinctus infestation rates found in drier regions. Results suggest that landscape-mediated variation in pest pressure in this system is better explained by a direct response of pest insects to increasing cover of a highly suitable crop rather than an indirect response via reductions in natural enemies as natural habitat declines. The implication is that habitat diversification at the landscape scale could play a role in suppressing agricultural pest populations via reductions in area of suitable crop hosts. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. b s t r a c tLandscape simplification associated with agricultural intensification has important effects on economically important arthropods. The declining cover of natural and semi-natural habitats, in particular, has been shown to reduce natural-enemy attack of crop pests, but also in some cases reduced crop colonization by such pests. In this study, we examined the influence of changes in two elements of landscape composition, natural grassland cover and cover of a highly suitable crop host, on infestation by a generalist insect pest in wheat, and parasitism of this pest by its dominant natural enemies. Surprisingly, we found no significant influences of increasing natural grassland habitat, at either local or landscape scales, on infestation by the wheat stem sawfly, Cephus cinctus, or parasitism of this pest by the native parasitoid wasps, Bracon cephi and Bracon lissogaster. In contrast, we found significant increases in levels of C. cinctus infestation with increasing wheat cover at the landscape scale. Thi...

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is a key pest of wheat in the northern Great Plains of North America, and damage resulting from this species has recently expanded southward. Current pest management practices are inadequate and uncertainty regarding geographic origin, as well as limited data on population structure and dynamics across North America impede progress towards more informed management. We examined the genetic divergence between samples collected in North America and northeastern Asia, the assumed native range of C. cinctus using two mitochondrial regions (COI and 16S). Subsequently, we characterized the structure of genetic diversity in the main wheat producing areas in North America using a combination of mtDNA marker and microsatellites in samples collected both in wheat fields and in grasses in wildlands. The strong genetic divergence observed between North American samples and Asian congeners, in particular the synonimized C. hyalinatus, did not support the hypothesis of a recent American colonization by C. cinctus. Furthermore, the relatively high genetic diversity both with mtDNA and microsatellite markers offered additional evidence in favor of the native American origin of this pest. The genetic diversity of North American populations is structured into three genetic clusters and these are highly correlated with geography. Regarding the recent southern outbreaks in North America, the results tend to exclude the hypothesis of recent movement of damaging wheat stem sawfly populations from the northern area. The shift in host plant use by local populations appears to be the most likely scenario. Finally, the significance of these findings is discussed in the context of pest management.

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Providing sugar resources for parasitoids is an important component of habitat management approaches to bolster biological control. We screened three flowering cover crop species, and one aphid species, for their potential to increase the longevity of the parasitoid wasp, Bracon cephi (Gahan) (Hymenoptera: Braconidae), an important biological control agent of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae). We found that buckwheat and honeydew from the cereal aphid, Rhopalosiphum padi (Linnaeus) (Hemiptera: Aphididae), increased longevity of B. cephi females by over threefold, while longevity on sunflower and coriander was not significantly different from controls on wheat. The results suggest that incorporating buckwheat into cover crop mixes could enhance parasitoid performance. However, the finding that honeydew associated with a common aphid in wheat provides a suitable resource suggests that a better understanding of the varying quality, and spatial and temporal availability, of aphid honeydew will be a critical consideration in evaluating the potential benefits of managing floral resources for parasitoid conservation in this system.