Probability of outbreaks of forest insects in Europe: a generic model calibrated on six forest insect profiles

Insect pests are one of the major threats to forests. Although invasive species cause more and more impacts, native species could also generate temporary very high damages. The population dynamics of insects relies on several factors, going from weather to stand conditions. Due to global change, insects could face conditions they have never encountered before, leading to unusual population outbreaks. Forest managers need to consider these possible emerging pests but predicting insect outbreaks is still very challenging. In this context, we have developed a mathematical model at the crossroad of statistical and mechanistic models to describe the likelihood of outbreaks for a set of 6 insect profiles: bark beetles, longhorn beetles, tortrix moth, other moths, aphids, and Hymenoptera. This model describes the probability of occurrence of an outbreak at a given time and at a given area, based on several conditions. It has been built and parametrized on the most documented orders of European forest pests. Parametrization for these species' profiles can be used as a baseline to explore the risk of outbreaks for closely related pest species. We provide an illustration of the model application for the oak processionary moth, Thaumetopoea processionea, which reach epidemic levels in north-western Europe. This generic outbreak model is particularly performant to point out some years or areas as unlikely for an outbreak, and thus targets correctly factors that inhibit outbreaks. It is still at an exploratory level and should be further improved for an operational use in forest stand surveillance and management.