Plant module size and dose of gall induction stimulus influence gall induction and galler performance

Abstract

Several different hypotheses attribute large variations in gall abundance and galler performance to variations in plant growth rate and/or module size. The plant vigor hypothesis (PVH) predicts that galler performance will be greatest on large, fast‐growing plant modules due to their large, rapidly dividing cells that allow them to make large galls, where survival and size of survivors are usually greater. The optimal module size hypothesis (OMSH) predicts that galler performance will be greatest on intermediate‐sized modules, based on a tradeoff between ease of gall induction on small modules and increased resource availability in large modules. Here, we evaluate predictions of the PVH and OMSH during a two‐year field study by examining gall induction and full gall development success, as well as subsequent galler performance in successfully developed galls, using a shoot galling adelgid, Adelges abietis, on white spruce Picea glauca. In addition, we tested whether increased dose of gall induction stimulus on different sized modules, achieved by observing differing A. abietis densities per bud, could increase the rate of successful gall induction, as predicted by the OMSH. Galls of A. abietis appeared to be induced by a dose‐dependent stimulus from fundatrices in spring. Furthermore, the critical stimulus dose required to induce a gall appeared to increase with shoot length. These findings support the OMSH and not the PVH. Galler performance (gall volume and the number of gallicolae that emerged from each gall) in successfully developed galls was positively related to shoot length, supporting both the PVH and OMSH. We conclude that the PVH represents one component of the more inclusive OMSH, which considers both ease of gall induction on different‐sized modules and the effect of plant module size on gall size.