The Plant Stress Paradox: Effects on Pinyon Sawfly Sex Ratios and Fecundity

Plant stress theory predicts and many observational studies support the notion that abiotic stress can increase plant quality and benefit the performance of phytophagous insects. Paradoxically, numerous experiments contradict this assumption. To test the theory, we conducted experiments to determine the influence of pinyon pine (Pinus edulis) stress on pinyon sawfly (Neodiprion edulicolis) performance. In a natural pinyon population, we transferred first—instar sawfly larvae to 41 trees divided into four groups of 11; each group received either water, fertilizer, water and fertilizer, or no supplements. Upon completion of their penultimate larval instar, we collected sawflies and determined their fecundity and sex ratios. Although sex ratios are rarely used to estimate insect performance in natural systems, we believe that their sensitivity to changes in plant quality may play an overlooked role in insect—plant interactions. Most of our results refuted theoretical predictions, because sawflies performed better on trees receiving both water and fertilizer. However, in some cases, sawflies did benefit from pinyon stress. We contend that "simultaneous plant stress," such as low precipitation while the insect is feeding or ovipositing, can reduce performance. In contrast, "sustained plant stress," such as chronically poor soil conditions or prolonged drought, can benefit insect performance. These different effects on herbivores may parsimoniously explain some discrepancies between theoretical and empirical plant stress studies.