Landscape-scale host crop area and crop diversity as drivers of pest abundance and biological control in wheat agro-ecosystems of the northern Great Plains

Wheat is a dominant crop of arid ecosystems of the northern Great Plains, with the wheat stem sawfly, Cephus cinctus, a dominant pest in the region. Wheat stem sawfly, and its associated Bracon spp. parasitoids, are native to North America, originally associated with wild grasses in prairie landscapes. As such, semi-natural rangeland and CRP grassland habitats have the potential to be important sources of both pests and natural enemies spilling over into wheat. We combined landscape-scale sampling studies, and experimental work on host preference, to examine the potential importance of this cross-habitat movement on pest infestation in wheat. Counter to expectation, landscape surveys over the past decade suggest that wheat stem sawfly responds most strongly to wheat crop cover, rather than grassland cover, at the landscape scale, with increasing wheat cover consistently associated with higher infestation levels. Results further suggest that this pattern is driven by a combination of higher host stem densities in, and greater performance on, wheat relative to grassland. Parasitoid abundance was also not associated with grassland cover at landscape scales, but parasitism increased with increasing cover of broadleaf crops in some regions. Greenhouse experiments have shown that sugar resources associated with flowers and crop aphid honeydew can significantly bolster longevity of these parasitoids, which may be one mechanism via which increasing crop diversity increases parasitism in this system. Overall, our work suggests that diversifying crop rotations is an important complement to conserving semi-natural habitats, to bolster biological control at landscape scales.