Various natural environmental factors help control gypsy moth in North America. A disease-causing fungus known as Entomophaga maimaiga was first introduced in 1910-1911 to control gypsy moth. This fungus only affects select families of moth caterpillars that encounter infected soil and plants or through contact with other infected caterpillars. The spores of the fungus germinate in the spring and work best if rain is abundant. E. maimaiga was responsible for widespread gypsy moth mortality in 1989 and 1990, when wetter than normal conditions were reported in May. Since this time, E. maimaiga has become a significant regulator of gypsy moth populations at both low and high densities. Researchers are unsure whether the increased prevalence of the fungus is due to its initial introduction or if it is the result of a more recent reintroduction into the US. Older gypsy moth caterpillars that die as a result of the fungus die in a vertical position with their legs sticking outward.
Gypsy Moth caterpillar that has died from Entomophaga maimaiga
A nucleopolyhedrosis virus (NPV) kills enough gypsy moth caterpillars when populations are high to eventually end an outbreak. Caterpillars must eat the viral particles in order to become infected. Caterpillars infected with the NPV die in an inverted V position, which explains why the common name for the NPV is "the wilt". The activity of NPV is specific in that it only kills gypsy moth caterpillars.
Gypsy moth caterpillar that has died from NPV