Dealing with growing forest insect pests: the role of top-down regulation.
Forest insect pests are becoming increasingly common in Europe. A recent paper published in this journal dealt with a particular case of pest spreading in Portuguese coniferous forests. The authors discussed the effect that mass trapping and clear-cut belts had on slowing down the invasion and suggested that likely causes of pest spreading are stress due to global warming and the introduction of exotic species. Inspired by that paper, we defend here the point that forest communities are not only regulated by bottom-up mechanisms, such as water stress, but also by top-down mechanisms, such as predation. We provide information from European metadata showing that forest biomass grew by 38% (1991-2015) whereas forest insectivorous passerine trends were stable (1980-2016). Synthesis and applications. We suggest that rebounding European forests are too young to support large communities of insectivorous birds and that an effective way to solve pest problems could be to promote the use of nest boxes for them (and for other insect predators), as it was done in the past in large reforestations implemented in Spain. Additionally, the presence of large mammalian herbivores is needed to reduce the density of trees in forests. Priority should be given to this ecological management in comparison with massive trapping and clear-cut belt construction or at least it should be used together with those engineering practices.