Human health risks of invasive caterpillars increase with urban warming

Development and survival vary across a species’ geographic range and are also affected by local conditions like urban warming, which may drive changes in biology that magnify or reduce the risks of hazardous organisms to people. Larvae of the pine processionary moth (Thaumetopoea pityocampa Schiff; PPM) are covered with setae (hair-like structures) that cause allergic reactions in warm-blooded vertebrates upon contact with the skin, eyes, or respiratory tract. Our objective was to determine whether PPM larva development, phenology, and survival change with urban warming in ways that affect the risks of this organism to people. In Orleans, France, we conducted a field study of PPM larvae across a gradient of urbanization from forests to city center to measure winter survival and the timing and duration of the life stage that poses the greatest risk to people. Larvae in the city spent more time in the fifth, most dangerous, instar than larvae in the forest. Urban warming indirectly increased larva survival by advancing phenology of urban larvae to a more cold-tolerant life stage prior to the winter cold period. Our results indicate that local urban warming drives changes in larva biology that increase the risks the organisms pose to people. In recent decades, the PPM has expanded its geographic range to higher latitudes with rising temperatures. Our study highlights that local landscape variation, such as a mosaic of warmer and cooler temperatures in cities, can alter the effects of this type of range expansion for people.