Landscape structure alters the abundance and species composition of early-season aphid populations in wheat fields

Agricultural landscape composition is reported to have effects on the occurrence of natural enemies in fields, but the responses of herbivorous pests to landscape features have rarely been studied. Our previous landscape-level study in northern China found that natural enemies had no significant effects on the population growth of wheat aphids at the colonization stage. Hence, we hypothesised that the initial aphid population in wheat fields may largely depend on immigration events from other habitats. In this study, we investigated the effects of landscape pattern on wheat aphid abundance and species composition at the colonization stage by surveying population densities of different aphid species along a landscape gradient. We found that noncrop habitats such as woodlots, fallow lands and vegetation around dwellings or wetlands in the landscape all had positive correlations with aphid abundance in wheat fields because they provided a source for aphid colonization. More specifically, the proportion of woodlots, fallow lands, dwellings, and other crops increased the abundance of Rhopalosiphum padi (L.), and the existence of water and dwellings in an agricultural landscape facilitated the occurrence of Sitobion avenae (Fabricius). Likewise, the abundance of Schizaphis graminum (Rondani) in wheat fields was found to be significantly promoted by the presence of water. The habitats around water and dwellings usually provide the overwintering sites for the aphids. Furthermore, wheat production acreage had a significant negative correlation with the abundance of R. padi, S. avenae and S. graminum. Landscape characteristics had a significant effect on the species composition of wheat aphids. Our study suggests that noncrop habitats in a landscape enhance aphid occurrence at the population colonization stage, and differences in overwintering host plants among aphid species may be responsible for the differing responses of the abundance of different aphids to landscape variables, which greatly altered aphid population composition across different landscapes. These results highlight the need for taking landscape effects on the pest itself into consideration when designing landscape-level pest management strategies.