Plant defense theory provides insight into interactions involving inbred plants and insect herbivores
Inbreeding in the form of self-fertilization is widespread among plants and typically results in broad, detrimental changes in plant morphology and physiology. Phenotypic changes associated with inbreeding are likely to alter interactions between inbred plants and other organisms, but few studies have investigated this potential. We found that inbreeding in the entire-leaf morning glory, Ipomoea hederacea var. integriuscula, altered this plant's ability to resist and tolerate attack by insect herbivores. The effects of inbreeding on plant defense, however, varied among insect species, and plant defense theory helped explain this variation. If the effects of inbreeding on plant phenotype are analogous to those of environmental stresses, then the plant vigor hypothesis predicts specialist herbivores will perform better on outbred plants, and the plant stress hypothesis predicts that generalist herbivores will perform better on inbred plants. We conducted a series of greenhouse experiments in which we reared two species of specialist tortoise beetles, a generalist moth species, and a generalist aphid species on inbred and outbred morning glories to test these hypotheses. We found that specialist tortoise beetles performed significantly better when reared on outbred plants and that aphid populations grew significantly faster on inbred plants as predicted by the plant vigor and plant stress hypotheses, respectively. Beet armyworni caterpillars, however, performed better oil outbred plants, not inbred plants as predicted. These results suggest that plant defense theories may be Useful for predicting the effects of inbreeding on plant-herbivore interactions, but differences in herbivore feeding habit (leaf chewing vs. phloern feeding) may also help explain variation in the effects of plant inbreeding on insect herbivores.