This is the second half of a two part series on gypsy moth. The following examines the insect’s predators, its impact on trees, and some of the most effective methods for controlling gypsy moth populations.
Lymantria dispar dispar, also known as gypsy moth, European gypsy moth, or North American gypsy moth, is a moth in the family Erebidae. Each year, Gypsy moth is responsible for defoliating millions of trees across Europe, Africa, Asia, and North America. Gypsy moth is of Eurasian origin. Carl Linnaeus, a Swedish botanist, physician, and zoologist, first described the species in 1758. Since its initial discovery, the species’ taxonomy has been disputed, with the insect being shifted between three families: Lymantriidae, Noctuidae, and Erebidae. Gypsy moth is classified as a pest. Its larvae consume the leaves of over five hundred species of trees and shrubs. It is considered one of the most destructive insects of hardwood trees in the eastern United States.
Natural predators of gypsy moth include parasitic and predatory insects such as wasps, flies, ground beetles, and ants. Four species of parasitic flies prey on gypsy moth larvae as they molt. Parasetigana silvestris and Exorista larvarum lay their eggs on the larvae. If the egg hatches before the larvae molts, the fly larvae will penetrate the host. Compsilura concinnata pierces the larvae, and inserts its own larva into their bodies. Biepharipa pratensis lays its eggs on leaves. As the gypsy moth larvae consume the foliage, they swallow the eggs, and the fly larva hatches within their stomaches. Eight species of parasitic wasps affect gypsy moth. Ooencyrtus kuvanae and Anastatus disparis are attracted to the eggs; Apanteles melanoscelus and Phobocampe disparis parasitize the early larval stages; Brachymeria intermedia and Monodontomerus aureus parasitize the pupae; Itoplectes conquisitor attacks and kills the pupae. Glyptapanteles portheriae and Glyptopanteles liparidis lay their eggs on the gypsy moth larvae.
Many species of spiders prey on gypsy moth. The Calasoma beetle, a ground beetle indigenous to Europe, feeds on the insect when populations are dense. Several species of birds, including bluejays, starlings, grackles, blackbirds, nuthatches, red-eyed vireo, rufous-sided towhee, northern oriole, catbird, and robins consume the larvae, pupae, and adults. Black-capped chickadee preys on gypsy moth throughout its entire life-cycle. Approximately fifteen species of common woodland animals, including white-footed mouse, shrews, chipmunks, squirrels, and raccoons utilize gypsy moth as a food source.
Diseases caused by bacteria, fungi, or viruses can cause populations to decline, especially during periods when populations are dense, and stressed by a lack of preferred foliage. One of the most lethal fungi to impact gypsy moth is Entomophaga maimaga. This fungus produces spores that infect the gypsy moth larvae. It is an important resource for limiting gypsy moth populations. Wilt disease caused by the nucleopolyhedrosis virus is specific to gypsy moth, and is the most devastating of the natural diseases. Wilt disease kills both the larvae and pupae, often resulting in a dramatic collapse in populations. Larvae infected with wilt disease are shiny, and hang limply in a v-like position.
Weather influences the development and survival of gypsy moth throughout the insect’s life-cycle. Temperatures ranging below -20 degrees Fahrenheit can kill exposed eggs if they persist for over 48 hours. Alternating periods of freezing and thawing in late winter and early spring may prevent overwintering eggs from hatching. Cold, rainy weather in spring inhibits the dispersal of newly hatched larvae, and can prevent them from feeding, effectively stunting their growth.
Effects on Trees
Gypsy moth defoliates susceptible trees. The effects of defoliation range in severity. They depend on the amount of foliage that is removed, the number of consecutive defoliations, the condition of the tree, the tree’s hardiness, and soil conditions. If less than half of the crown is defoliated, most hardwoods will experience a slight reduction in radial growth. If more than half of the crown is defoliated, most hardwoods will produce a second flush of foliage during summer. Aside from hardwoods, pines and hemlocks may be defoliated during gypsy moth outbreaks.
Trees that are healthy and vigorous can generally withstand defoliation. However, if subjected to continuous defoliation, trees will eventually become stressed. Trees that are weakened by stress may be killed if more than half of the crown is defoliated. Trees that have been weakened by defoliation, or other stressors are also vulnerable to disease pathogens and insect infestations. The Armillaria fungus often infiltrates the root system of weakened trees, while chestnut borer invades the trunk and branches.
Managing Gypsy Moth
- When planting in landscape settings, select a diverse composition of trees and shrubs, including species that are not favored by gypsy moth, such as honeylocust, ash, hickory, dogwood, mountain ash, tulip, yellow poplar, or many conifers, excepting pines and hemlocks. Enhance growing conditions for isolated trees by encircling them with ground cover plants that will not compete with them for moisture and nutrients.
- Remove and dispose of any dead branches, or flaps of bark. This will reduce the number of sheltered locations available to larvae and pupae.
- Eliminate egg masses found on trees, shrubs, buildings, fencing, vehicles, and woodpiles.
- Barrier bands composed of double-sided sticky tapes, or adhesive material like petroleum jelly or grease can be used to prevent larvae from crawling up the trunks of susceptible trees. These products should be applied to the surface of impermeable material, such as duct tape or tar paper. Avoid applying any material directly to the bark. Petroleum based products can cause swelling and cankering on trees with thin bark.
- Fertilize trees to maintain vigor, and promote healthy growth.
- Applying a layer of organic mulch around the base of trees and shrubs will improve soil conditions, maintain soil quality, and moderate soil moisture.
- Ensure that trees are sufficiently watered, especially during periods of drought.
- Avoid applying lime or weed killers near trees, particularly those with shallow root systems. Many commercially available chemicals can damage tree roots.
- Dense populations of gypsy moth can be reduced through the use of pesticides. Microbial and biological pesticides should be applied before the larvae reach the third instar. As they mature, larvae become more resistant to microbial pesticides. Bacillus thuringiensis is the most commonly used microbial and biological pesticide. When consumed internally, Bacillus thuringiensis paralyzes gypsy moth, preventing it from feeding. The insect eventually succumbs to starvation or disease. This pesticide is effective against gypsy moth, but may be used on many other pests as well, including spruce budworm, and tent caterpillar. The nucleopolyhedrosis virus, which is registered under the name Gypcheck, is a naturally occurring organism that functions as a microbial pesticide. The most commonly used chemical pesticides contain carbaryl, diflubenzuron, and acephate.
- Malathion, methoxychlor, phosmet, trichlorfon, and synthetic pyrethoids are registered for use against gypsy moth, but are not always as effective.
Photo courtesy of Didier Descouens under the Creative Commons Attribution Share Alike 4.0 International License