Increasingly favourable winter temperature conditions for major crop and forest insect pest species in Switzerland

Abstract

With global warming, recent winters in Switzerland have been milder than in previous decades, and this trend is predicted to continue. Survival during the cold season could increase for insect species sensitive to winter cold events. Forest pests, such as the pine processionary moth (Thaumetopoea pityocampa) and green spruce aphid (Elatobium abietinum), as well as some crop pests, such as the southern green stink bug (Nezara viridula), could overwinter more easily. These species are affected by temperatures below –12°C or below –8°C. In this study, we examined whether changes in winter minimum temperatures (Tmin) could potentially favour the winter survival of these pest species in various places in Switzerland. We analysed long-term daily Tmin data from the period 1980–2019 at 67 locations. We then used two climatic scenarios (RCP2.6 and RCP8.5) to analyse daily Tmin in 2020–2099. We determined the number of days with Tmin below –8°C or –12°C and the frequency of years with at least one day below these thresholds. Our results show that the frequency of cold days has decreased over the last 40 years, even though winter Tmin has increased less than yearly Tmin. However, the –8°C threshold was still reached in most years, except on the Southern side of the Alps. The –12°C threshold was reached almost every year above 800 m, but infrequently at lower elevations. Our results indicate that, by the end of the 21st century, temperatures below –12°C will occur only infrequently up to 1700 m in Switzerland, and years with occurrences of temperatures below –8°C will become rare at lower elevations. Future temperature conditions can thus be expected to favour some crop pests, by enabling them to overwinter more easily on the Swiss Plateau, as well as some forest pests, which will likely reach higher elevations.