Montana Field Guide

The Gypsy Moth is a moth species native to Europe and Asia and considered a problematic and invasive species across its introduced range. The species is a generalist and readily consumes hundreds of plant species causing defoliation and occasionally host death. In North America, the species is well established in the Northeastern US and adjacent areas of Canada. Control of extant populations and containment of spread are high priorities within this area. Small populations have been found outside of the core range across the US, but to date these have been controlled.

The species appears distinctive across its life stages. Adults are sexually dimorphic and females are larger with a whitish coloration and males are smaller and dark brown in color. Although females have wings, within North American populations, they do not fly. Adults live long enough to mate and in the case of females, lay eggs. Egg masses are placed unselectively on natural and man-made objects, which if moved, contribute to the spread of the species. Larvae are distinguished from other caterpillars by the five pairs of blue and six pairs of red spots arranged in two rows. High larval densities can cause defoliation of local trees and shrubs sometimes resulting in host death, especially when trees are stressed by other factors such as drought.

Larvae are around 2-3 mm in length upon hatching and grow to 60 mm before pupation and have numerous hairs to aid in defense from predators. Larvae are distinguished from other caterpillars by the five pairs of blue and six pairs of red spots arranged in two rows along the back. Pupa are dark reddish color and found in cracks and crevices of bark and in litter. Adults are moderate sized typical moths with a wingspan of 3-4 cm for males and 5.6-6.7 cm in females. Males are smaller in size, are a darker brown in color, and have feathered antennae. Females are larger, lighter in color, and in North America, flightless. Both sexes have dark scalloped lines across forewings. Adults are only active for a brief period after emergence to mate (See USDA Aphis 2014 for in depth description of each life stage).

The species is non-migratory, but is colonizing new areas of North America at a rate of between 3 and 29 km/yr (Tobin et al. 2007).

Gypsy Moth larvae feed on over 500 plant species (USDA 1981), but oak species (Quercus) are preferred (Gansner and Herrick 1985)

Gypsy Moth eggs are typically deposited in protected locations, but females are generalists in their selection of substrate. In natural systems, eggs are typically deposited in crevices on the bark or trees or on rocks and other debris. In urban areas or areas occupied by people, eggs may be deposited on outdoor furniture, vehicles, and other objects that provide some level of shelter. Unfortunately, the propensity to deposit eggs on objects that may be moved to other areas contributes to the spread of Gypsy Moths. Egg masses are deposited in late summer and larvae emerge the following spring (as reviewed in USDA 1981).

Predation on larvae and egg masses can have a significant effect on control of small or low density populations (allee effect). The primary vertebrate predators are Peromyscus mice such as the Deer Mouse and White-footed Mouse, and Sorex shrews. Invertebrates such as ants also consume larvae, but are not thought to consume enough to regulate populations (Elkinton and Liebold 1990).

Larvae emerge from egg masses in the spring or early summer. Individuals disperse from their natal site using strings of silk to catch the wind. Once a larvae has dispersed to a suitable site it will begin feeding on local vegetation. This species is a dietary generalist and has been shown to feed on over 500 plant species from over 100 families. In high enough densities, larvae are capable of defoliating tree and shrubs which in severe cases can cause host mortality. Between mid-June and early July larvae pupate and become adults. Larvae select a sheltered site such as cracks and crevices in the bark of trees or under objects on the ground, and form a chrysalis. After 2-3 weeks the adult moth emerges from the cocoon (as reviewed in USDA 1981).

Adult moths live around a week, and during this time seek to mate. Emergence is typically in late July or August. Male moths seek mates by following pheromones emitted by females. Upon finding a mate, the male transfers a spermatophore to the female and may seek additional mates. Females then stop releasing pheromones and seek suitable sites to deposit their eggs, selecting sheltered sites and releasing up to 1500 eggs (USDA 1981, USDA APHIS 2014).

Females of the European and Asian subspecies of Gypsy Moth differ in their ability to fly. European Gypsy Moth females have wings but do not fly, which limits the distance they can disperse from their pupation site. In contrast, females of the Asian subspecies are strong fliers capable of traveling up to 25 miles from their pupation site to mate and deposit eggs (Savotikov, 1995).

Eradication efforts have been successful for isolated populations of this species outside of its core range in North America. Pheromone baited traps are extremely effective in detecting isolated populations and disrupting mating. The USDA APHIS and state governments deploy around 100,000 traps per year for initial detection efforts (Kauffman et al. 2017). Once detected, eradication efforts including application of the pesticide Bacillus thuringiensis var. kurstaki (Btk) are conducted with high success rates. Btk application has proven so successful for this species it is now the most common biopesticide in use (Solter and Hajek 2009).

In contrast to the successful eradication of isolated populations, management of Gypsy Moths in their established range has proved problematic. However, efforts to reduce the rate of spread including the US Forest Service “Slow the Spread” campaign have resulted in a 60% reduction in spread since implementation. The program relies on detection and targeted control methods including traps and pesticides to reduce expansion of the species (Tobin et al. 2012).

Further containment efforts focus on transport of moths and their eggs and larvae. Currently transport of this species outside of a quarantine zone established by USDA APHIS is prohibited. Legally, all outdoor items within the quarantine zone must be inspected prior to transport outside of the quarantine zone and a record of inspection retained for the duration of the move. APHIS Inspection Service provides a map of the zone and additional details.

For information and resources on plant pests and diseases see the USDA's Animal and Plant Health Inspection Service's Plant Pests and Diseases Profiles