During outbreak years, gypsy moths, an exotic pest of oak forests in eastern North America, can defoliate thousands of hectares of forest and kill millions of trees. White-footed mice are by far the most effective predators on the pupal (cocoon) stage in the life cycle of the gypsy moth.
Our experimental and observational studies strongly support the hypothesis that mice regulate low-density populations of gypsy moths by voraciously attacking pupae. In fact, given the intensity of mouse predation on moth pupae, our research now focuses on how gypsy moths escape extinction.
Preliminary results suggest that extinction is avoided only when the probability of surviving mouse predation is highly variable in space, and when gypsy moth larvae disperse only short distances away from sites in which their mothers survived. A crash in the mouse population - typically caused by an acorn failure - appears to be important in allowing moth populations to grow.
In addition to mouse population density, mouse foraging behavior plays an important role in gypsy moth dynamics and is currently being researched. Although individual mice tend to become less efficient in attacking moth pupae when pupal density is low, their attack rates still seem sufficient to further reduce moth populations, and their ability to forage more efficiently, via learning, seems strong.