A measure of the changes in soil water which induce water deficits–and thus physiological stress–in the tissues of plants is described, and named the stress index. Known outbreaks of psyllids throughout Australia were strongly correlated with suddenly increasing high levels of this stress index. It is postulated that physiological stress of plants at these times increased the amount of nitrogeneous food available to the psyllids, thus greatly increasing the chances of young surviving and reproducing. A similar correlation of outbreaks of other sorts of phytophagous insects with the appropriate stress index has been found, and it is suggested that herein may lie an ecological generalization of some significance.