Finding and Losing Host Plants by Phyllotreta: Patch Size and Surrounding Habitat

Abstract

This study examined the effects of size and surrounding habitat of food plant patches on the density, immigration, and emigration of two specialized phytophagous insects. Two flea beetles, Phyllotreta cruciferae and P. striolata, were censused for 3 wk in host plant patches of four sizes, with background vegetation of three types. Densities of both beetle species declined with decreasing patch size. Single isolated host plants (collards) were without flea beetles because of high rates of emigration. By releasing and recapturing marked flea beetles, I found that the emigration rates of both beetle species consistently increased with decreasing patch size. This sensitivity of emigration to host patch size may be common among phytophagous insects because of simple changes iin perimeter:area ratios. When this is the case there may be a “critical patch size” below which host islands do not sustain herbivore populations. Immigration rates were also affected by patch size and habitat, but only in P. striolata, not in P. cruciferae. Even in P. striolata, variation in immigration was less consistent and less dramatic than was variation in emigration. Beetles wandering out of patches apparently do not readily return to the host patch. To quantify how far beetles would have to wander before “losing” their host patch, I released marked beetles in two different habitats (a goldenrod field or a cultivated garden) at varying distance (2, 4, 6, or 8 m) from a target patch of collards. For both beetles species, the percentage of individuals discovering the collards after 1 d declined rapidly beyond 2 m and was dramatically reduced by the persence of goldenrod between released beetles and the collard target.