Pine pest aphids of the genus Eulachnus (Hemiptera: Aphididae: Lachninae): how far can their range extend?

Species of the aphid genus Eulachnus (Hemiptera, Aphididae), associated with pine trees, are an example of insects reported as rare over their native geographical range in Europe and serious pine pests far from their natural distribution. To predict the risk of invasion of spotted pine aphid Eulachnus agilis (Kaltenbach, 1843), green pine aphid Eulachnus brevipilosus Börner, 1940 and narrow brown pine aphid Eulachnus rileyi (Williams, 1911) on a global scale, ecological niche modelling was applied. We used the maximum entropy model, based on associations between unique occurrence localities and a set of environmental variables. We obtained models of potentially suitable habitats, based on climatic variables and distribution of host plants, suggesting that favourable conditions for each species may be present on every continent. However, E. agilis and E. rileyi appear to be potentially the most widespread species. In Europe, models showed that the potential distribution of the representatives of the genus Eulachnus overlapped with that of the host plants. In places where they have been introduced, these aphid species have readily adapted to indigenous or introduced pines, including trees grown commercially for timber. A jackknife test indicated that annual mean temperature and mean temperature of the coldest quarter were the most important environmental variables restricting the expansion of the species under investigation. Moreover, the most important factor in the decrease in population density of the feeding aphids is the rainy season and, on a smaller scale, the presence of their natural enemies.