Differential Larval Predation by Ants Can Influence Willow Sawfly Community Structure

Abstract

We experimentally tested the ability of generalist predatory ants to structure sawfly gall communities through differential predation on sawfly—galler larvae in relation to their life history patterns. Galler larvae of three species exit their galls in late summer and are exposed to predatory ants, while larvae of one species (stem—gallers) remain within their galls to overwinter. Many ant species (Formicidae) are patchily abundant in willows in northern Arizona. The abundances of four galler species and the activity levels of ant assemblages were assessed in census studies performed on variable numbers of the willow clones between 1986 and 1989. Two distinct types of sawfly gall communities were found on willow clones in the study area, communities composed of four galler species (stem, leaf, and petiole galls, and leaf folds), and communities composed primarily of stem galls. Clones with low ant activity levels support the former, whereas those with high activity levels support the latter. Experimental introductions of larvae of two existing species (leaf— and petiole—gallers) into and beneath natural willow clones with high and low ant activity levels showed that larvae are quickly captured by ants in and under high activity willows. Ants were not selective to larval species in these introductions, and were clearly shown to be generalist predators when experimentally offered two species of larvae. Experimental introductions of leaf— and petiole—galler larvae into four clones with high ant activity in 1987 and 1988 did not result in higher gall populations in the following year, indicating that larvae did not survive to recruit into those clones as adults. Experimental caging of leaf— and petiole—galler adults onto branches of high activity clones resulted in the establishment of modest populations of both species, followed by seemingly normal gall development patterns and larval survival rates inside galls. We conclude that the absence of leaf and petiole galls from willows with high ant activity is due to intensive larval predation by generalist ants, and not to plant resistance factors. This means that generalists and opportunistic predatory ants can significantly structure insect gall communities through differential predation that is dependent upon specific life history traits of some gallers.