This study was designed to examine the mechanisms underlying the response of herbivores to variation in the size of host plant patches. In order to explain why identical changes in patch size affect closely related herbivores in different ways, I separated the effects of patch size alone from the confounded effects of the surrounding plant community. Further tests determined whether herbivore densities were directly affected by surrounding nonhost plants (via changes in movement patterns) and/or indirectly affected by changes in host plant size/quality caused by the surrounding nonhost plants.