Exogenous and endogenous factors acting on the spatial distribution of a chrysomelid in extensively managed blueberry fields

Abstract

The role of endogenous (i.e. limited dispersal, intraspecific competition, aggregation) and exogenous (i.e. resource patchiness, heterogeneous landscapes, spatially structured habitat) factors on the spatial distribution of herbivores can be inferred from theoretical models in intensively managed or heterogeneous landscapes but not in extensively managed crops. In the present study, we examined aggregation patterns and the influence of environmental and spatial factors on the distribution of Altica sylvia Malloch larvae within extensively managed blueberry fields to determine how exogenous and endogenous factors affect this defoliator. Altica sylvia larvae and defoliation exhibited clumped within-field distributions. A lack of correspondence between larval density and defoliation indicated that oviposition habitat selection is occasionally suboptimal in this species. Clumped distribution patterns were not explained by endogenous factors or associations with spatially structured variables. Conversely, exogenous factors acting at the patch scale affected distribution patterns because larvae were less abundant in patches located close to forest edges, as well as in patches where weeds or a nonhost blueberry plant occurred. As a result of exogenously-produced variability in A. sylvia spatial distribution, we suggest that focused sampling methods be used to monitor this herbivore. We emphasize the need for similar assessments for herbivores that forage in extensively managed crops or somewhat heterogeneous monocultures.