Induced resistance mitigates the effect of plant neighbors on susceptibility to herbivores

Plant susceptibility to a shared herbivore is reduced by belowground competition with neighbors

Spatial variation in plant community composition is an important driver of variation in susceptibility to herbivores. In close proximity, certain neighbors can attract or repel herbivores to a focal plant ("associational effects"). Neighboring plants may also compete for resources, modifying their phenotype in ways that affect susceptibility to herbivores. To test whether and how competition contributes to associational effects, we manipulated the sharing of belowground resources among plant neighbors (spotted Joe Pye weed and common boneset) that serve as alternate hosts for an herbivorous beetle. In the field, the beetle Ophraella notata laid more eggs and inflicted more damage on plants of both species that were released from belowground competition with neighbors. Competition also weakened the effects of neighbor identity during field trials, reducing associational susceptibility. When beetles were forced to choose between the two host species in cage trials, competition again reduced beetle use of Joe Pye weed as a secondary host. To test the role of plant traits related to herbivore defense and nutrition, we quantified leaf protein, specific leaf area, and trichomes, and conducted behavioral assays on leaf disks. Beetles did not distinguish between Joe Pye weed treatments at the leaf disk level, and competition did not impact specific leaf area and protein. Trichome density was higher in both species in the preferred treatment. Overall, our results suggest that belowground interactions between plants may mediate the strength of associational effects, as secondary hosts become more attractive when released from competition with primary host plants.

Host interaction mechanisms in herbivorous insects – life cycles, host specialization and speciation

Why are most herbivorous insects so specialized? How do generalists and specialists differ ecologically? And what sensory and cognitive processes determine which host species an insect recognizes and uses? We review sensory cues involved at different stages of interaction with host plants and outline a unified scheme of insect–host interaction mechanisms. Generalist species may be best understood as specialized organisms with a longer tail of secondary host species that they accept to a lessening extent. Secondary hosts have fewer attractive cues, and hence a lower probability of meeting the stimulation threshold required for egg laying. We then discuss the alternative theories of co-evolution and sequential evolution used to explain the diversification of insect herbivores. We conclude that ‘escape and radiate’ co-evolution rests on unrealistic assumptions and empirical evidence favours sequential evolution of plants and herbivores, and the unique life cycles of insect herbivores may sufficiently explain their extreme diversity within the animal kingdom. We also outline a new speciation-oriented hypothesis to explain why specialists are far more diverse than generalists. Generalists tend to have large, contiguous distributions and therefore lower speciation rates. Natural selection in novel environments may also result in narrowing of the host range more often than broadening, further increasing specialist diversity.