Insect population dynamics in willow plantations managed for bioenergy

Insect pest outbreaks are common in willow plantations grown for biomass production in Northern Europe. One of the most damaging pests in central Sweden is the leaf beetle Phratora vulgatissima. During 1999-2005, we studied the population dynamics of leaf beetles in 20 willow (Salix viminalis) plantations and 20 natural (S. cinerea) willow habitats. We found that the temporal population variability and outbreak risks were higher in plantations than in natural habitats. The greater outbreak risk in plantations seems to be due to (1.) relatively low densities of natural enemies in plantations, and (2.) the planting of trees in dense stands, which facilitates for leaf beetles moving between trees when foliage is consumed. Omnivorous Heteropteran bugs, which attack eggs and larvae of the leaf beetles, are sometimes very abundant on natural willows but are removed from willow plantations during harvest. Recent studies on the ecology and population dynamics of heteropteran bugs may reveal how to enhance the abundance of natural enemies in willow plantations and thereby provide sustainable pest control.