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

Background/Question/Methods

Most people live in cities for the first time in history, and urbanization continues at an unprecedented rate (UN, 2007).  Urban environments exert strong effects on the identity, abundance, and diversity of species that live along side us, the ecosystem services they provide, and the natural experience of urban-dwelling humans.  A particularly pronounced yet understudied aspect of urban environments is that they are hotter than surrounding areas due to the urban heat island effect. We might expect to exothermic species, such as arthropods, to exhibit especially pronounced responses to higher urban temperatures. Scale insects are sessile herbivores that frequently colonize and kill urban trees. We A. priori 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 satellite image of Raleigh, NC to locate willow oaks (Quercus phellos) growing in heat islands and at ambient temperatures throughout the city and surrounding suburbs.  Plant samples were collected throughout the year to determine scale abundance and identity.  We also reared parasitoids from additional plant samples to determine diversity of the parasitoid community and percent parasitism of the scale population. 

Results/Conclusions

We found that oak lecanium scale (Parthenolecanium corni complex), the most dominant scale group inhabiting urban willow oak in Raleigh, is significantly more abundant in urban heat islands than in surrounding areas, though species richness did not differ. In further support of our hypothesis, percent parasitism of oak lecanium scale was over 12 times less in urban heat islands, indicating negative effects of temperature on higher trophic levels.  We conclude the urban heat island effect disadvantages urban trees in the hottest, most developed areas of cities due to heavier scale insect infestations. Future research will test empirically the contribution of scale development time and disrupted top-down control as mechanisms for urban scale outbreaks.  Our results suggest that native scale insects are well suited for tracking future climate change. We predict scale insects could become more abundant and damaging in rural and natural areas as the climate warms.  Thus, urban areas may serve as canaries in the coal mine and models for predicting the many unforeseen consequences of climate change.