Entomophaga maimaiga (The Gypsy Moth Fungus)

What is it and how does it work?

Entomophaga maimaiga (Em) is a common disease in the gypsy moth in Japan that kills large numbers of larvae. [a] It has been imported both intentionally and accidentally and is established in a large area of the North-East and is considered a major factor in controlling the population density of the Gypsy Moth. 

The resting spore stage of Em winter over and germinate into the conidia stage after heavy rains in the Spring. The conidia enters around the hair follicles of the gypsy moth larvae ( or caterpillar ) with the assist of enzymes. The protoplast stage develops and uses nutrients in the larvae to reproduce, killing the larvae within a week. Again water is needed, so damp conditions are required. The next generation of conidia are expelled through the shell to infect nearby later stage larvae or may be wind born or tracked to new areas. Those other larvae die producing both more conidia to attack even later stage caterpillars and also produces resting spores, which are able to winter over. [b] 


Natural distribution of Em lags the spread of the gypsy moth, so attempts have been made to study introducing Em into areas beginning to be infested.

Although two early attempts were made to introduce it to infested areas, the first in 1910, and again in 1984 both were considered unsuccessful and some believe the fungus was established accidentally around 1989 imported on pallets or travelers shoes. [c]

Experimental releases have been made by two methods. The first method is distributing soil which contains the Em spores but not other microbes which should not be transported to new areas. Movement of soil is strictly regulated by state and federal agencies including the USDA.

The other, more benign, method is to collect cadavers from tree trunks and distribute them in areas where gypsy moths are expected. They have to be collected during a few weeks of July and stored carefully over the winter. [d]


Difficult to measure as control plots are difficult to keep uncontaminated. 

E.m. may be responsible for reductions attributed to other control measures, and other natural enemies (virus, dear mice, stinkbugs, and birds) may account for some of the kills attributed to Em. There has been some co-infection reported with the virus. 

It has been found to be very effective in reducing populations of gypsy moths in areas of both high or low infestation. Follow up checks at release sites years later found a high degree of fungal infection in host populations. 

Em is not a silver bullet. Other management techniques are needed in many cases. 

In some areas where the fungus has been found or placed, gypsy moth populations completely collapse, some show a population reduction, while others show little impact. Usually takes 2-4 years to establish itself naturally, so the initial outbreak will have already occurred which causes much of the tree mortality.

Even the infected caterpillars will continue to defoliate the trees for up to 10 days. 


There is no commercial distribution of Em, as it is difficult and expensive to produce in the laboratory and to distribute effectively. There also would not be a repeat market to support profitable production. Controlled experimental distribution has supplemented the natural spread of the fungus, including 8 releases in Ohio and 57 in Michigan by 1994. Unofficial harvesting of infected soil has been observed. 


No commercial price. An abundant supply from infected areas and a fairly simple process for testing soil would support a fairly low cost to release Em into new areas, once the process is perfected. The USDA Forest Service and the Michigan Department of Natural Resources did not charge for the material used there. 

Human Health Risks

Since there is no commercial production, the tests required of a product have not been conducted. It is naturally occurring in Japan and has been in the environment of the North East US for several years. It is not known to pose health risks to people or pets. [c] 

Environmental Factors

Experiments to date have found no threat to other life forms other than a couple of other pests; a tobacco hookworm and the Douglas fir tussock moth. Artificial inoculation of Em into areas without the gypsy moth but where an infestation is anticipated is not recommended due to lack of sufficient research, however. [c] 

Anecdotal note from a correspondent in Michigan:

 "About five years ago Lansing MI had a severe gypsy moth outbreak underway. We held off treatment with B.t. in favor of releasing E.m. The fungus was released in swampy, gypsy moth infested areas in May. In July after a very wet week hundreds of thousands of gypsy moth were dead. The people who knew about the release apparently told their friends in surrounding states because there has been a constant stream of people digging soil in the swampy area. We haven't had a problem since the release." 


[a] Entomophaga maimaiga A Natural Enemy of Gypsy Moth, MSU Extension Bulletin E-2604. May 1996 

[b] Pathology and Epizootiology of Entomophaga maimaiga Infections in Forest Lepidoptera, Ann E. Hajek, Cornell University. Microbiology and Molecular Reviews, Dec 1999, p 814-835 

[c] Forest Health Technology Enterprise Team, Morgantown, WV, Technology Transfer Pamphlet FHTET-97-11 June 1988 

[d] Entomophaga maimaiga: A Fungal Pathogen of Gypsy Moth in the Limelight, Ann E. Hajek Cornell Community Conference on Biological Control, April 11-13 1996