New urban trees/pests interactions in Europe.

The tree species have long been chosen for street use on the ground of the following criteria: they should be easy and inexpensive to produce, they had to be hardy and withstand stresses and finally they should not bear undesirable pests. Recently, in order to bypass pests and diseases problems that could have arisen, or simply to introduce aesthetic changes, more and more exotic tree species have been used. Some inappropriate practices were developed, for instance the transport of cuttings or of adult plants with soil, sometimes on long distances. Such practices and the increase of international trading resulted in an exponential growth of the unintentional introductions of new pests. In this fast-changing situation, some street-trees are abandoned or doomed, even before being used as such. The best solution for this problem should be classical biological control, which consists in the safe acclimatization of specific natural enemies of introduced pests; this method should be used systematically on a larger scale.