Hot in the city: Urban warming drives pest insect abundance

Background/Question/Methods

Urbanization is one of the most potent forces of ecological change in the world today. Urbanization has strong effects on the composition of animal communities, though the mechanisms that govern these changes are unknown. Herbivorous arthropod pests are frequently more abundant in urban than rural areas. Hypotheses for pest outbreaks on urban plants include reduced biological control due to depauperate predator and parasitoid communities in urban areas, increased host plant quality due to greater plant stress, and anthropogenic inputs. However, neither of these hypotheses hold true in every case, nor do they explain variation in the abundance of pests within urban areas. 

Results/Conclusions

Here we show that heat, a ubiquitous feature of urban areas, increases the abundance of a common insect pest when habitat, natural enemy, and plant effects are held constant. The scale insect, Parthenolecanium corni, was 13 times more abundance on willow oak trees in the hottest parts Raleigh, NC than in cooler areas that had similar levels of parasitism and impervious surface cover. We further isolated the effect of heat from natural enemies and plant quality in a greenhouse reciprocal transplant experiment and showed that P. corni collected from hot urban trees becomes more abundant in hot greenhouses than cool greenhouses, while the abundance of P. corni collected from cooler urban trees gains no advantage from the warmer environment. Our results provide the first evidence that heat is a primary factor driving insect pest outbreaks on urban trees. Moreover, the temperature increase at which we found scale outbreaks to occur is similar to that predicted to occur worldwide in the next 50 years. Rising urban and global temperatures could have strong negative consequences for urban and natural forests by increasing pest abundance.