Interspecific and Intraspecific Interactions Via Plant Responses to Folivory: An Experimental Field Test

Abstract

I tested the hypothesis that phytophagous insect species compete interspecifically on shared host plants by inducing chemical, physical, and phenological changes in the plant that either directly reduce insect growth and survival or indirectly alter attack by natural enemies. I manipulated levels of simulated folivory on branches within eight trees of Quercus emoryi in a complete randomized block design. Survival and causes of mortality, including attack by natural enemies, of a dominant leafminer, Cameraria sp. nov., were monitored over two growing seasons. Survival varied significantly among trees but not among treatments. Causes of mortality also varied significantly among trees. Death from premature leaf abscission increased significantly with increasing levels of branch damage, while mortality from other causes, including death from bacterial and fungal attack, declined significantly with increasing levels of damage. However, neither of these changes in mortality was sufficient to cause changes in overall survival. Leaf damage did not influence attack by macroscopic natural enemies (hymenopteran parasitoids or vertebrate and invertebrate predators). By far, the most important factor affecting survival and causes of mortality within trees was the presence of co—occurring larvae on leaves (i.e., intraspecific competition). Simulated folivory in the first growing season did not affect folivory by Cameraria or other folivorous arthropods in the next season. Colonization, survival, rate of attack by natural enemies, and amount of herbivory by other arthropods in the second growing season were not affected by levels of leaf damage in the first one. Neither survival nor causes of mortality of Cameraria in the second growing season were associated with folivory by endemic leafchewing arthropods. Simulated folivory in the first growing season did, however, reduce leaf production in the second season. These results do not support the hypothesis that insect species complete interspecifically via induced plant responses and their effects on the third trophic level (natural enemies of the herbivores). The effects of simulated (first growing season) and insect (second growing season) folivory via induced plant responses on the Cameraria population was negligible relative to intraspecific interactions. Further, my results do not support the notion that folivore—induced responses are defensive and that plants have evolved these responses to manipulate the third trophic level. In natural settings, other factors that influence population dynamics of phytophagous insects, such as intraspecific competition and interactions with microparasites, may obscure any negative effects of herbivore—induced plant responses on interactions among herbivore species and their macroscopic natural enemies.