Compared with parasitoids, an even larger community of vertebrate and invertebrate predators feeds on gypsy moth. Very little is known about the impact of most predators, because predation is extremely difficult to measure. Many bird species worldwide feed on gypsy moth, but it is generally believed that most birds dislike the hairy cuticle of gypsy moths and avoid them.

Research groups led by H. Bess in the 1940s and R. Campbell in the 1970s concluded that predation by small mammals, particularly the white-footed mouse, Peromyscus leucopus, has a major impact on low-density gypsy moth populations. The mice feed on late instars and pupae, particularly on the forest floor. Both research groups demonstrated an increase in gypsy moth survival in forest plots from which small mammals had been removed or excluded. More recently, J. Elkinton and colleagues showed that predation on gypsy moth pupae was strongly correlated with density of mice and that gypsy moth densities increased when mouse densities declined. The density of mice, in turn, was correlated with the abundance of acorns, which, in oak-dominated forests, are their principal overwintering food. Indeed, there are many studies that link forest-dwelling mice to abundance of acorn crops. Poor acorn crops, which occur on a regional scale and are caused by a variety of weather events, could thus be the ultimate trigger of gypsy moth outbreaks. C. Jones and colleagues provided further experimental proof of these ideas. They removed mice from experimental plots and observed an increase in gypsy moth and they augmented food in other plots and observed an increase in mice.

There are a number of invertebrate predators of gypsy moth. One of these is the introduced ground beetle, Calosoma syco-phanta (Carabidae). R. Weseloh has shown that this insect becomes quite abundant in outbreak populations of gypsy moth and may cause substantial mortality. Several researchers have documented predation by ants as a significant source of mortality in low-density populations, but it is usually much less than predation by mice.

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