What Are the Factors Behind Sparse Gypsy Moth Populations?
Natural enemies play an important role during periods when gypsy moth populations are sparse. Natural enemies include parasitic and predatory insects such as wasps, flies, ground beetles, and ants; many species of spider; several species of birds such as chickadees, blue jays, nuthatches, towhees, and robins; and approximately 15 species of common woodland mammals, such as the white-footed mouse, shrews, chipmunks, squirrels, and raccoons.
Predation by small mammals (mice and shrews) is the largest source of mortality in low density gypsy moth populations and this mortality is apparently critical in preventing outbreaks. Calosoma (ground beetles of European origin), cuckoos, and flocking birds, such as starling, grackles, and red-winged blackbirds, are attracted to infested areas in years when gypsy moth populations are dense.
Diseases caused by bacteria, fungi, or viruses contribute to the decline of gypsy moth populations, especially during periods when gypsy moth populations are dense and are stressed by lack of preferred foliage. Wilt disease caused by a particular nucleopolyhedrosis virus (NPV) that is specific to the gypsy moth is the most devastating of the natural diseases. NPV causes a dramatic collapse of outbreak populations by killing both the larvae and pupae. Larvae infected with wilt disease are shiny and hang limply in an inverted “V” position. Infection with NPV is the most common source of mortality in high density populations and NPV epizootics usually cause the collapse of populations.
Since the 1980s, the fungus Entomophaga maimaiga has also had a large impact on gypsy moth populations in North America. Weather affects the survival and development of gypsy moth life stages regardless of population density. For example, temperatures of -20°F, (-29°C.) lasting from 48 to 72 hours can kill exposed eggs; alternate periods of freezing and thawing in late winter and early spring may prevent the over wintering eggs from hatching; and cold, rainy weather inhibits dispersal and feeding of the newly hatched larvae and slows their growth.