Associational Susceptibility of Cottonwood to a Box Elder Herbivore

Abstract

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 × 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.

Variation in Herbivore-Mediated Indirect Effects of an Invasive Plant on a Native Plant

“…Although R. conicus oviposition on wavyleaf thistle can be high in areas without musk thistle , the results here are consistent with other recent studies that showed associational susceptibility with host plant co-occurrence in an area; in these studies, as in ours, the secondary host species suffered more damage from shared insect herbivores near versus far from the preferred host (White andWhitham 2000, Blossey et al 2001). …”

“…In our study, R. conicus use of C. undulatum, averaged across all distances to the focal musk thistle patch, was higher at sites with greater densities of adult R. conicus in the focal musk patch. This result may provide partial support for the resource limitation hypothesis of White and Whitham (2000) and Blossey et al (2001). However, contrary to this resource limitation hypothesis, we found that variation in R. conicus adult density in patches of the preferred musk thistle did not predict either the presence of associational susceptibility or the magnitude of decrease in R. conicus egg load on the acquired secondary host plant, C. undulatum, with increasing distance from the patch.…”

“…Differences in R. conicus egg loads on native wavyleaf thistles, within versus outside the focal musk patches, could occur either because wavyleaf thistles within the focal musk thistle patches were closer to the preferred host (musk thistle), or because they had higher densities of acceptable neighbors (regardless of the species composition of those neighbors) than did wavyleaf thistles outside of the focal musk thistle patches (White and Whitham 2000). To evaluate the possibility that local density of palatable neighbors was driving variation in R.…”

“…Given the large annual variation in musk thistle density observed at our sites, active dispersal by R. conicus adults that allows aggregation in dense musk patches, may explain why wavyleaf thistle experiences associational susceptibility, rather than associational defense, amid its preferred musk thistle hosts. White and Whitham (2000) and Blossey et al (2001) hypothesized that variation in the density of the shared insect herbivore drives variation in the strength of associational susceptibility. In fact, they predicted that associational susceptibility would occur only where the shared herbivore is sufficiently abundant to deplete its preferred host.…”

“…White and Whitham (2000) and Blossey et al (2001) hypothesize that associational susceptibility only occurs where insect herbivore populations are large enough to deplete their preferred host, forcing them to attack secondary hosts. This resource-dependent hypothesis suggests that the ratio of insect herbivores to preferred host plant resources drives associational susceptibility of secondary hosts.…”