Entomophaga maimaiga

Although Entomophaga maimaiga was introduced in the United States from Japan in 1910 and 1911, its 1989 appearance in Connecticut, New Hampshire, Vermont, northeastern Pennsylvania, New Jersey, southeastern New York, and Massachusetts is a mystery: if its presence is due to the original introduction, why wasn't it detected during the years between 1911 and 1989?

Presence of E. maimaiga may be determined by late instar gypsy moth larvae which, when infected with this fungus, die hanging vertically from tree trunks with prolegs extended laterally. The cadavers subsequently fall to the bases of trees.

Hyphal bodies are found within infected insects and spore-producing mycelium can grow on the outside of cadavers but decomposes after conidia are produced and actively ejected from cadaver surfaces. Resting spores are about 30 micrometers in diameter and may be found inside late instars of infected gypsy moths.

Trees, both broadleaf and coniferous, of forests and urban and suburban areas (see distribution map below).

High titers of E. maimaiga resting spores have been documented at the bases of trees where infected gypsy moth larvae have died.

Based on lab results, E. maimaiga appears to be quite specific to the family that includes gypsy moth, although it can cause low levels of infection in a number of other species.

During a field study, the only cadavers that were found on trees were gypsy moth larvae killed by E. maimaiga. At locations where there were active epizootics of E. maimaiga occurring in gypsy moth populations, more than 1500 insects of 53 difference species were collected. Of those 1500 individuals, only one individual of a lasiocampid and one individual from the Noctuidae were infected with E. maimaiga.

The conidia are relatively short-lived. Spores are actively discharged from cadavers and can immediately germinate and cause infection.
Hyphal bodies occur within infected insects and mycelium grows on the outside of cadavers.
Resting spores are produced internally in late instars and must undergo a period of dormancy.

From E. maimaiga epizootics occurring in North American gypsy moth populations in 1989 and 1990, it became clear that this fungus was capable of becoming an important mortality factor. Whereas nuclear polyhedrosis viral epidemics in gypsy moth populations require a high population density, E. maimaiga can cause epizootics at lower densities. In release studies done in 1991 and 1992, this fungus appeared to spread readily, even in less wet years, and persisted 3 years later. Gypsy moths were also still present in these studies, but there was a high degree of fungal infection documenting long-term persistence of this fungus in an area with the potential for long-term control. In a 1995 study of E. maimaiga released against gypsy moth in residential areas, only the control plots had high levels of defoliation.

Data from 1996 indicates that gypsy moth defoliation declined more than 85% from 1995 levels over 11 states. The most dramatic case is in Virginia where 1996 defoliation was nonexistent compared with 850,000 acres in 1995. There is general consensus among scientists and pest managers that E. maimaiga is probably responsible for the decline. However it is not known if this level of fungus activity will continue. It appears that E. maimaiga has spread throughout much of the area considered to be generally infested by gypsy moth, the only exception being the transition area into which the insect is spreading for the first time.

E. maimaiga has been highly variable and unpredictable. Consequently, the use of environmentally safe and effective insecticides will continue to be important tools to reduce damage caused by gypsy moth outbreaks, and several points should be emphasized:

It is important to continue studies to fully understand how E. maimaiga will affect gypsy moth populations in the long term. As a new member of a group of natural control agents that regulate gypsy moth population in North America, it is not known how the fungus will fit into the cycle in the long term or what effect it might have on other natural control agents of the gypsy moth.
Gypsy moth outbreaks will continue to occur and may initially be severe as the insect spreads into uninfested areas. However, it is hypothesized that as the fungus becomes established in these new areas of gypsy moth infestation it will have a moderating influence on the gypsy moth population.
People will continue to inadvertently move the gypsy moth to uninfested areas of the country on outdoor household articles, nursery stock, Christmas trees, and other commodities. Detection, monitoring, and suppression of these isolated infestations will continue to be important to prevent establishment outside the generally infested area.

Each spring, infection by E. maimaiga is initiated when resting spores germinate to produce infective germ conidia. Because high titers of resting spores have been found in the organic layer of the soil at the bases of trees, it is assumed that disruption of this layer, e.g., turning over the soil at the tree base, might decrease the titer of resting spores near the soil surface, and thus would decrease the level of infection from these spores.

Although E. maimaiga has not yet been tested for its susceptibility to fungicides, there is the potential that use of fungicides especially around the bases of trees could cause mortality among E. maimaiga resting spores.

Studies are planned to evaluate the susceptibility of E. maimaiga to the fungicides to which it might be exposed in the forest and urban landscape.

Thanks to Ann Hajek for reviewing this page, supplying photographs and suggestions, and making valuable additions.

Schneeberger, Noel F. 1996. Gypsy moth populations plummet in 1996 while "The Fungus" skyrockets. Gypsy Moth News, 42: 1-2.