Insects that have evolved over long periods of time with their plant hosts may play an important role in maintaining healthy forests. For example, heavy defoliation of certain tree species by defoliators or the weeding out of weak individuals by bark beetles results in forest containing vigorous trees that are well-adapted to the site they are growing on. A good example is the Douglas-fir tussock moth (slide) whose larvae (slide) defoliate grand fir trees growing on sites that are more suitable for Douglas-fir, and defoliates Douglas-fir growing on sites more suitable for ponderosa pine (Slide). Heavy defoliation results in insect frass and dead bodies covering the forest floor (Slide), releasing nutrients into the soil which then becomes available to the surviving trees. This is why the growth rate of forests often increases following defoliator outbreaks (Slide).

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© 1998 Alan A. Berryman