Natural
Enemies
In this article...
the 3 P's
Living with the gypsy moth in Michigan is easier thanks to a little help from the 3 Ps - the Predators, Parasitoids and Pathogens of gypsy moth. These natural enemies take their toll on all stages of the gypsy moth life cycle.
Birds, insects, mice and shrews - all are important predators of gypsy moth.
Although many birds don't like feeding on gypsy moth caterpillars, a few species such as the yellowbilled cuckoo, seem to relish the big, hairy caterpillars. Small rodents like white-footed mice, shrews and voles feed on large caterpillars and pupae. Mice seem to prefer the larger female pupae to smaller male pupae. This selective munching may have an even greater impact on gypsy moth populations than random dining. Calosoma beetles also actively seek out and devour young gypsy moth caterpillars. Calosoma beetles are large, attractive ground beetles. Some species were imported into North America specifically for gypsy moth control. Both the larval and adult stages of this beetle are predaceous.
Parasitoids are small wasp-like insects and specialized flies that live by feeding in the body of another insect. Gypsy moth eggs, caterpillars and pupae are attacked by many different kinds of parasitoids. Some parasitoids are native, while others have been imported from overseas in hopes of finding the "silver bullet" to control gypsy moth.
One parasitoid that is very important in Michigan in Ooencyrtus kuvanae. This insect is a tiny wasp and was imported into Michigan by the Department of Agriculture. The young wasps feed on the developing caterpillars inside gypsy moth eggs. When the adult wasps emerge, they are attracted to the scent of gypsy moth females or new egg masses. Once they find an egg mass, they lay their eggs inside the gypsy moth eggs. Estimates made by Cora Gorsuch showed that an average of 35 percent of the eggs in an egg mass are parasitized. Other parasitoids may also feed in gypsy moth eggs, or in caterpillars or pupae. Although parasitoids won't control a gypsy moth outbreak by themselves, they certainly help.
Pathogens, or insect diseases, are the most important of the 3 Ps. One especially interesting pathogen is an organism that causes a virus disease in caterpillars. The virus disease Neucleopolyhederosis virsus (usually referred to as NPV) is the major factor that causes gypsy moth outbreaks to collapse. When caterpillars are killed by this virus, they hang in an upside-down V, turn a dark color and appear to melt as they decompose over leaves and branches.
NPV, always present in gypsy moth populations, is seldom noticed except during gypsy moth outbreaks. During a gypsy moth outbreak, there are usually two waves of disease in the gypsy moth population. Adult females can pass the virus to some of their offspring when they lay eggs. Young caterpillars may then pick up the virus as they chew their way out of the eggs. When these young larvae die (the first wave), they can spread the virus around foliage, tree trunks, and other areas.
Older larvae may eventually pick up the virus and die before pupating (the second "wave"). NPV is known to affect only gypsy moth caterpillars and does not affect other insects or other animals.
Gypsy Moth Populations -
Boom and Bust
Although the gypsy moth has many natural enemies, predators, parasitoids and most diseases can't keep up with a gypsy moth population explosion. In healthy gypsy moth populations, one female moth will lay 400-1500 eggs in one egg mass. Even if 90 percent of the eggs are killed by natural enemies, the ones that are left can build up populations in a single year. Because the virus disease is the major factor causing gypsy moth populations to collapse, gypsy moth managers must be careful not to interfere too much with the natural build-up of virus in the population. Gypsy moth populations usually have to build to high levels before the virus kicks in and drives populations down. At high population levels, caterpillars must compete with each other for food and space. Caterpillars get stressed, which makes them more susceptible to the virus disease. As more caterpillars get sick and die, the disease spreads through the whole population.
When insecticides are used, many gypsy moths are killed. However, there are always some gypsy moths that survive and populations can rise back to outbreak levels within a year. If insecticides are applied repeatedly, larvae won't get stressed and the virus may not kick in. This could cause high gypsy moth populations to occur year after year.
Gypsy moth managers in Michigan use Bacillus thuringiensis var. kurstaki (Btk) rather than broad spectrum chemical insecticides. Even Btk is only applied in residential areas and high-use recreation areas. The goal of gypsy moth management is to reduce the discomfort of people living in infested areas, without disrupting the virus disease and other natural controls.
- E-2604: Entomophaga maimaiga: A Natural Enemy of Gypsy Moth
- E-2622: Calosoma sycophanta: A Natural Enemy of Gypsy Moth Lavrae and Pupae
Document Author(s): Program Staff
Revised: September 10, 1997
URL: http://www.ent.msu.edu/gypsyed/docs/enemies.html