Support for the microenvironment hypothesis for adaptive value of gall induction in the California gallfly, Andricus quercuscalifornicus (Hymenoptera: Cynipidae)

Three major hypotheses have been advanced for the adaptive nature of plant galls: the nutrition-, enemy-avoidance-, and microenvironment hypotheses. Of these, the microenvironment hypothesis has been frequently invoked, but rarely tested directly. We tested this hypothesis in a population of Andricus quercuscalifornicus (Bassett) wasps inducing galls on Quercus lobata Née in Northern California. Relative humidity and temperature data gathered from both fresh and dried galls in the field indicated that A. quercuscalifornicus larvae modify their microenvironments significantly by raising and stabilizing relative humidity levels inside galls to near saturation. In addition, excised larvae maintained under experimental conditions survived significantly longer under levels of high relative humidity. These data support the hypothesis that A. quercuscalifornicus adaptively manipulates its environment through gall induction.