The timing of insect life-cycle events such as the annual spring flight is highly influenced by temperature. For predicting how future climate change will affect the phenological responses across insect species, it is crucial to investigate both the local temperatures and large-scale climatic indices such as the North Atlantic Oscillation (NAO) that drive the timing of biological events. In the northern hemisphere, the NAO exerts significant influence on the climatic fluctuations across Europe and eastern North America. As the NAO index shifts from its positive (linked with warmer, milder winters) to its negative phase (linked with colder, harsher winters), this could then affect the local temperatures which can consequently lead to the advancement or delayment in spring phenology.
In this study, we examined the relationship between the annual spring flights of five tortricid moth species and the climatic variables with trap data collected at the Fruit Research and Extension Center (Biglerville, Pennsylvania) from 1981 to 2016.We used a moving-window correlation to investigate the monthly temperatures which best predicted the timing of the annual spring flight across all species. Similarly, with a moving average of the monthly NAO index, we investigated which periods, if any, affect the species’ spring phenology.
For all tortricid species, we found the timing of spring flight to be affected by shared monthly temperature-windows and the North Atlantic Oscillation. Locally, March temperatures were the most important predictors with colder temperatures leading to later spring flights in all species. The multivoltine species appear to use supplementary cues such as January and February temperatures possibly due to their earlier spring flights suggesting that variation in life-history traits can influence phenological responses. Finally, November temperatures also significantly influenced the spring phenology of the tortricid moths with lower temperatures correlating to a delay in the timing of spring flights.
At the regional scale, the NAO has significant effects on four of the tortricid species with the timing of their spring flights affected by either the autumn (September to November) or winter period (January to February). The mechanism for how these climatic variables drive spring phenology is mostly likely due to the temperatures affecting the insect development rates, overwintering survivorship, and the induction and breaking of diapause.
In conclusion, our results show that the spring phenology of agricultural pests is best predicted by climatic variables at both the local and regional scale.