Estimating the potential threat of increasing temperature to the forests of Turkey: a focus on two invasive alien insect pests

Abstract

Rising temperature can affect forests negatively through its impact on insect pests. The present study focused on two invasive alien insect species (Dryocosmus kuriphilus and Leptoglossus occidentalis) to understand how rising temperature might affect their damage in Turkish forests. For D. kuriphilus, the timing of chestnut budburst, gall induction and emergence of its introduced parasitoid, Torymus sinensis, were monitored between 2015 and 2019, and each phenological event was compared annually with fluctuations in temperature to observe the parasitoid-host synchrony. For L. occidentalis, cumulative degree days (CDD) were calculated, and the possible number of generations produced in 2020 in different regions of Turkey were predicted. The CDD calculations were repeated under increasing temperature and different photoperiod-diapause induction scenarios. Evaluation of the monitoring data on the D. kuriphilus system showed that gall induction occurred at the same time as budburst, whereas T. sinensis emergence was independent from the budburst, and that the parasitoid-host synchrony was disrupted after the abnormally warm winter in 2018. The CDD calculations estimated that L. occidentalis produced one to five generations from north to south in 2020. They also suggested a significant increase in the number of generations in the southern Turkey under temperature increase scenarios. Including photoperiod as a time-limiting factor reduced the highest possible number of generations from five to two. In conclusion, rising temperature has a potential to threaten the biocontrol against D. kuriphilus, and it can increase voltinism in L. occidentalis.