Gall-forming insect attack patterns: a test of the Plant Vigor and the Resource Concentration Hypotheses

We tested the following two hypotheses to investigate the attack patterns by gall-inducing insects on their host plant Eriotheca pubescens: (i) vigorous modules are more often attacked by galling insects; and (ii) E. pubescens trees associated with a higher density of co-specific individuals have a higher gall abundance. We collected terminal branches from the host plant to measure their module lengths and gall richness and abundance and determined the E. pubescens densities in a 5m radius around the focal individual. There was a higher availability of short branches. Among the four gall morphotypes, two preferred the higher branches, and the other two morphotypes had no shoot length preference. Gall attack did not present a relationship with the host plant density. Thus, the vigor of E. pubescens was a determining factor only for some gall morphotypes. In contrast, the resource concentration hypothesis was not important at an intraspecific level for gall attack in this system.