Three major hypotheses have been advanced for the adaptive nature of plant galls: the nutrition-, enemy-avoidance-, and microenvironment hypotheses. The microenvironment hypothesis has frequently been assumed to favor selection for gall induction by insects; however, it has rarely been tested directly. Here, I report two lines of evidence supporting this hypothesis from a population of Andricus californicus wasps inducing galls on Valley Oak (Quercus lobata) in Northern California. First, relative humidity and temperature data were gathered from both fresh and dried galls in the field, indicating A. californicus larvae significantly modify their microenvironments by raising and stabilizing relative humidity levels inside galls. Second, larvae removed from galls and subjected to laboratory-controlled temperature and humidity conditions survived significantly longer when water vapor levels approached saturation. Taken together, these data suggest gall induction by A. andricus can be viewed as adaptive manipulation of its environment; thus, A. andricus acts as a miniature ecosystem engineer.