Document: BRI-3-48-30

Aphids, galls, and fruit: Things are better when sinks pull together.

REHILL, B.J.* and J.C.SCHULTZ

The Pennsylvania State University, University Park, PA 16802, USA ¹

Abstract:
In a field experiment, we tested the hypothesis that the leaf-galling aphid Hormaphis hamamelidis competes for resources with fruit formed by its host plant, witch hazel (Hamamelis virginiana).We compared the fecundity of fundatrices near developing fruit with those more than 10 cm. away. Fundatrices that formed galls on leaves within 10 cm. of developing fruit had a 13% greater mean fecundity (mean SE = 46.28 1.86 offspring) than fundatrices in galls formed on leaves greater than 10 cm away from developing fruit (P = 0.0522; df = 172). Thus it appears that aphid performance is enhanced by gall proximity to reproductive sinks of the host plant, inconsistent with results from other systems. Also different from other gall-aphid systems, these results were inconsistent with the predictions of both the plant vigor and sink competition hypotheses that gall insect performance is enhanced by increased growth rate or size of the galled plant part. Leaf size did not limit resources or affect the fecundity of fundatrices; there was no correlation between leaf area and fecundity (p = 0.993; n = 174). Also, there seemed to be no exploitative competition among the aphids, since fitness of fundatrices in single galls (alone on the leaf) didn't differ from those in multiple galls (two or more galls per leaf) (p = 0.1033; df = 172). Neither did gall position on leaf influence aphid fecundity: there was no significant difference in fecundity between fundatrices in galls in the apical and basal halves of leaves (p = 0.2096; df = 172). Lastly, it appeared that the ability to create a larger gall to act as a sink conditioned the fecundity of fundatrices. The gall reached 80% of its final volume before the appearance of offspring, and gall volume was significantly correlated with fecundity (Pearson correlation coefficient = 0.527; p < 0.0001; n = 174). This evidence suggests that the gall of H. hamamelidis acts as a mobilizing sink independent of the leaf on which it is found, but influenced by the host plant's source-sink relationships such that local plant sinks can help a fundatrix acquire resources. Further, fundatrix reproductive performance depended on the magnitude of the induced response (gall size), but unlike other aphid gall systems, H. hamamelidis reproduction appeared to be facilitated by nearby plant sinks.

Keywords: aphid; gall; sink-source relations; facilitation; sink competition; plant vigor

This abstract is being presented at: 3:30 PM in session:
HERBIVORE EFFECTS ON PLANTS