Responses of Alder and Willow to Attack by Tent Caterpillars and Webworms: Evidence for Pheromonal Sensitivity of Willows

Red alder (Alnus rubra) and Sitka willow (Salix sitchensis) trees subjected to attack by tent caterpillars (Malacosoma californicum pluviale) or webworms (Hyphantria cunea), respectively, exhibited a change in foliage quality such that bioassay insects fed leaves from the attacked trees grew more slowly than those fed leaves from unattacked control trees. In contrast, bioassay of leaf quality of S. sitchensis, subjected to attack by tent caterpillars, indicated that altered leaf quality had been induced not only in the attacked trees but also in nearby unattacked control trees. This suggests that S. sitchensis is sensitive to and can respond to signals generated by attacked trees or the caterpillars. Since no evidence was found for root connections between attacked and control willows, the message may be transferred through airborne pheromonal substances.