Associational susceptibility of cottonwood to a box elder herbivore

Associational resistance, which refers to decreased herbivory experienced b y a plant growing with heterospecific neighbors, is a well documented ecolo gical phenomenon. In contrast, studies that describe increased herbivory du e to heterospecific neighbors (associational susceptibility) are relatively rare. In this study we document associational susceptibility among hosts o f the fall cankerworm (Alsophila pometaria). Cottonwoods (Populus angustifo lia X P. fremontii) located under box elder (Acer negundo) were colonized b y two to three times more cankerworms, and suffered two to three times grea ter defoliation than cottonwoods growing under mature cottonwoods, or cotto nwoods growing in the open. This associational pattern reflects fall canker worm'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 dif fer between the hosts. Based on these and other findings, we predict that, when generalist herbivores reach outbreak proportions and consume their pre ferred hosts, they will then move to nearby less-preferred hosts to complet e their life cycle. This "spillover" effect will result in associational su sceptibility for less-preferred hosts and is likely common in forest outbre ak situations where herbivore densities are high. With increased emphasis o n diversified plantings in agriculture and forestry, it is important to und erstand potential drawbacks such as associational susceptibility.