Urban butterflies and moths have evolved adaptations that extend their flight season
Cities tend to be warmer than surrounding rural areas, which extends the growing season in urban environments. On the other hand, intense light pollution in urban environments may mislead organisms that use day length as an indicator of seasonal change. Many insects, including Lepidoptera (butterflies and moths), can take advantage of longer growing seasons by producing extra generations, but they also rely on day length when determining the time for entering diapause. Thus, urban environments potentially promote the evolution of novel seasonal adaptations in insects.
Thomas Merckx, Matthew Nielsen and colleagues from the University of Oulu, Stockholm University, Lund University, Finnish Environment Institute (SYKE) and Natural Resources Institute Finland (LUKE) studied the Green-veined White butterfly Pieris napi and the Latticed Heath moth Chiasmia clathrata to address the influence of the urban environment on flight phenology. They examined long-term data from standardized monitoring schemes (Finnish butterfly monitoring in agricultural landscapes, Finnish nocturnal moth monitoring, and Swedish butterfly monitoring) and citizen-science observation portals (www.laji.fi, www.artportalen.se) from the three largest cities in Finland, the three largest cities in Sweden and rural areas surrounding each of these six cities. Flight periods of both species tended to be longer and ended later in urban than in rural environments. This suggests that diapause is induced later in the growing season in urban than in rural populations.
Laboratory experiments using individuals deriving from Helsinki, Stockholm and surrounding rural areas confirmed that the urban populations of both species have evolved different responses to day length compared to the surrounding rural populations. Diapause induction took place under shorter day lengths in urban than in rural populations, which results in later diapause induction in the urban populations under field conditions because days get shorter towards the autumn. This result is consistent with adaptation to the longer growing season caused by urban warming.
This study suggests that the altered seasonality in urban environments is an important cause of evolution in urban populations. It is hence likely that urban adaptations in seasonal responses have evolved in many species that manage to live in urban environments.
Thomas Merckx, Matthew E. Nielsen, Janne Heliölä, Mikko Kuussaari, Lars B. Pettersson, Juha Pöyry, Juha Tiainen, Karl Gotthard & Sami M. Kivelä. Urbanization extends flight phenology and leads to local adaptation of seasonal plasticity in Lepidoptera. Proceedings of the National Academy of Sciences of the USA. https://doi.org/10.1073/pnas.2106006118
Photo: A Latticed Heath moth (Chiasmia clathrata) in an urban environment. Photo: Sami Kivelä