Associational susceptibility of cottonwood to a box elder herbivore

Associational resistance, which refers to decreased herbivory experienced by a plant growing with heterospecific neighbors, is a well documented ecological phenomenon. In contrast, studies that describe increased herbivory due to heterospecific neighbors (associational susceptibility) are relatively rare. In this study we document associational susceptibility among hosts of the fall cankerworm (Alsophila pometaria). Cottonwoods (Populus angustifolia x P. fremontii) located under box elder (Acer negundo) were colonized by two to three times more cankerworms, and suffered two to three times greater defoliation than cottonwoods growing under mature cottonwoods, or cottonwoods growing in the open. This associational pattern reflects fall cankerworm's strong preference for box elder over cottonwood: egg densities were 26 times greater on box elder than cottonwood, first instar larvae consumed 75 times more box elder than cottonwood in larval palatability trials, and fourth instar larvae consumed three times more box elder than cottonwood. In terms of larval performance, first instar larvae exhibited approximately six times greater mortality and 40% slower development time on cottonwood relative to box elder, whereas fourth instar larval performance did not differ between the hosts. Based on these and other findings, we predict that, when generalist herbivores reach outbreak proportions and consume their preferred hosts, they will then move to nearby less-preferred hosts to complete their life cycle. This 'spillover' effect will result in associational susceptibility for less-preferred hosts and is likely common in forest outbreak situations where herbivore densities are high. With increased emphasis on diversified plantings in agriculture and forestry, it is important to understand potential drawbacks such as associational susceptibility.