1 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. 2 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. 3 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. 4 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. 5 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.

Traits such as larval growth rate and head capsule width are often measured in economically important insects to determine their developmental stage. However, these traits have the potential to vary between genotypes or in response to several ecological factors. To determine whether geographic or ecological factors cause variability in the head capsule width of Altica sylvia Malloch (Coleoptera: Chrysomelidae), and to verify whether measures of head capsule width are adequate to identify larval instars in this species, A. sylvia larvae were recovered from 35 fields of Vaccinium angustifolium Aiton (lowbush blueberry; Ericaceae) of eastern New Brunswick, Canada. The distribution of head capsule widths varied in response to accumulated degree-days, A. sylvia larval density, and latitude. An overlap between measures of head capsule width of first-instar and second-instar larvae in fields supporting a high density of A. sylvia larvae suggested that intraspecific competition caused a reduction in larval growth rate that affected head capsule development and may have induced developmental polymorphism. Based on these results, we stress that the sampling protocol of studies conducted to determine head capsule width intervals in a species should include diverse ecological settings as well as several locations within the range of the species.