Hot in the city: scale insects, parasitoids, and the future of warmer urban trees

Most people live in cities for the first time in history, and urbanization continues at an unprecedented rate. City environments exert strong effects on urban species, the ecosystem services they provide, and the natural experience of humans. One understudied aspect of urban environments is that they are hotter than surrounding areas. We might expect to exothermic species, such as arthropods, to exhibit pronounced responses to higher urban temperatures. Scale insects are sessile herbivores that frequently colonize urban flora. We predicted scale are more abundant and diverse in hot urban areas, in part due to a depauperate parasitoid community that exerts weak top-down control. We used a thermal image to locate willow oaks (Quercus phellos) in heat islands and at ambient temperatures across Raleigh, NC. Plant samples were collected throughout the year to determine scale abundance and parasitism rates. We found lecanium scale (Parthenolecanium corni complex) is significantly more abundant in urban heat islands than in surrounding areas, though species richness did not differ. Further, parasitism of oak lecanium scale was over 12 times less in urban heat islands. We conclude heavy scale infestations disadvantage urban trees in the hottest, most developed areas of cities. Our results suggest native scale are well-suited for tracking future climate change. We predict scale insects could become more damaging in rural and natural areas as the climate warms. Thus, urban areas may serve as canaries in the coalmine and models for predicting the many unforeseen consequences of climate change.