Galls are atypical plant growths that provide nourishment and shelter for gall-inducing insects. Previous studies could not determine whether insect galls are induced by mechanical or chemical stimuli because gall formation occurred at the sites where the insects were active.
In this study, feeding by the spruce gall adelgid, which measures about one millimeter in length, caused large galls to form up to 800 millimetres away. The effects of chemical stimuli were therefore unambiguously separated from any mechanical influence due to feeding or egg-laying. Initiation and growth of galls was inversely correlated with distance of the insect from buds (potential gall sites), strongly suggesting that galls were induced by a chemical stimulus transported to buds via vascular tissue, and that its efficacy was dose-dependent.
Ecology Letters