Warming and drought combine to increase pest fitness on urban trees

Urban habitats are characterized by impervious surfaces, which increase temperatures and reduce water availability to plants. Three primary hypotheses have been proposed to explain why multiple herbivorous arthropods are more abundant and damaging in cities, and support has been found for each. First, less complex vegetation may reduce biological control of pests. Second, plant stress can increase plant quality for pests. And third, urban warming can directly increase pest fitness and abundance. We test the hypothesis that urban warming and drought stress combine to increase the fitness and abundance of the scale insect, <i>Melanaspis tenebricosa</i>, a pest of urban trees the southeastern U.S. We did this by manipulating drought stress across an existing mosaic of urban warming. We found support for the additive effect of temperature and drought stress such that female embryo production and body size increased with temperature and was greater on drought-stressed than watered trees. This study provides further evidence that drivers of pest insect outbreaks act in concert, rather than independently, and calls for more research that manipulates multiple abiotic factors related to urbanization and climate change to predict their effects on ecological interactions. <br>