Trees have a huge arsenal of ways to keep the pests that attack them under control. In the course of evolution, for example, some English oaks (Quercus robur) have developed the ability to release volatile signaling substances when attacked by oak moths (green oak leaf roller, Tortrix viridana), whose caterpillars can completely defoliate the trees.
Click here for original story, Bitter substances spoil the appetite of oak moth caterpillars
Source: Phys.org