Phytohormone Pathway Crosstalk and the Relationship Between Defoliating Herbivores and Phloem-Feeding Insects on White Oak Trees

Sarah Timbie
Sarah Timbie

Sarah Timbie is a rising junior (’23) who grew up in Madison, Wisconsin before moving to North Carolina and attending Carrboro High School. She is majoring in Biology with a linked major in Environmental Studies and a minor in Chemistry. After Wesleyan, she plans on attending Veterinary School. In her spare time, she loves crocheting, gardening, and listening to music.

Abstract: Ecosystems are full of complex interactions between species ranging from direct, such as an insect consuming a leaf, to indirect, like the indirect interactions between two insects that share the same food source. One of these many interactions is the relationship of defoliating herbivores and phloem-feeding insects. Phloem-feeding insects trigger a defensive response in plants that is regulated by the phytohormone salicylic acid (SA). The induction of SA can suppress another defensive pathway: the jasmonic acid (JA) pathway, which is induced by feeding from leaf-chewing herbivores. The crosstalk between hormone pathways can result in the plant being more vulnerable to leaf-chewing herbivores, such as caterpillars. The inhibition of plant resistance traits by phytohormone crosstalk has been documented in many herbaceous plants. Previous research from the Singer Lab made an observation of forest insects that could be explained by phytohormone crosstalk in oak trees. Caterpillars were found in greater abundance on branches of white oak (Quercus alba) trees that also harbored phloem-feeding insects, compared to those without. We hypothesize that caterpillars prefer feeding on oak leaves proximate to phloem-feeders because the latter inhibit induced plant resistance to caterpillars. This summer, we tested this hypothesis with two experiments. First, we experimentally manipulated the presence of phloem-feeding insects on white oak tree branches in the field and measured the effect of leaf palatability for caterpillars in the lab. Second, we experimentally applied volatile derivatives of SA and JA to white oak branches in the field and measured the effect of leaf palatability for caterpillars in the lab. Our hypothesis predicts that caterpillars will feed and grow more on leaves from branches with either phloem-feeders present or SA applied compared to controls. The results of both experiments did not support our hypothesis. Neither the presence of phloem-feeders nor the phytohormone treatments affected the feeding or growth of the oak-feeding caterpillars. 

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