Entomophaga maimaiga: 
A Fungal Pathogen of Gypsy Moth
in the Limelight

Ann E. Hajek
Department of Entomology
Cornell University
Ithaca, NY 14853

(prepared from the videotaped presentation*)

The use of fungi to control gypsy moth is a much younger field of study than many of the systems we've talked about so far in this conference. I have been working with fungal pathogens, primarily of gypsy moths, for the past 10 years.

Gypsy moths are very destructive, and they impact not only forests but also urban and suburban areas. In 1981, a peak year for gypsy moth populations in northeastern North America, gypsy moth larvae caused 13 million acres of defoliation.

The purple sections of the above map indicate areas of infestation where gypsy moth larvae are found. New England has an established population of gypsy moth, as does Michigan and Wisconsin. The orange shows areas of spread, which usually experience heavy damage when the gypsy moth first enters the area. However, gypsy moths are also caught in pheromone traps across the country (yellow and orange areas), indicating that they are continually found at very low levels in scattered locations across the United States.

Gypsy moth is a native of temperate Asia and Europe. It was accidentally introduced into the Boston area in 1868 or 1869. Over the past 125+ years, gypsy moth has been of concern to many people in North America. The generally accepted hypothesis about natural regulation of gypsy moth populations has been that at low densities, the most important mortality factors are vertebrate predators, at medium densities parasitoids are most important (usually Diptera or Hymenoptera), and at high densities, the gypsy moth nuclear polyhedrosis virus (NPV), a baculovirus, is the most frequent agent regulating populations. NPV causes big crashes in the gypsy moth population (=epizootics) and people often tended to wait for these epizootics to control high populations.

This general theory about gypsy moth population regulation has been challenged. In 1989, gypsy moths in the northeastern U.S. were seen dying of what appeared to be a viral epizootic. What made this remarkable was that the populations were not at high density but were just beginning to increase and were not yet dense enough for an NPV epizootic. When cadavers were dissected, fungal spores were found inside. No such fungus was previously known as an important gypsy moth pathogen in North America. In fact, only in Japan was any fungal pathogen known to be important in gypsy moth control.

To determine what fungus was attacking the gypsy moths, we did isozyme studies and RFLPs. We knew the fungus was in the Entomophthorales and we compared strains of the fungus found on gypsy moth cadavers from Connecticut, New Hampshire, and Massachusetts with Entomophaga  species infecting forest Lepidoptera in North America as well as with the Japanese species that infects gypsy moth, E. maimaiga.  These molecular analyses both showed a perfect match between northeastern isolates and E. maimaiga,  the fungal species known only from the Japanese gypsy moth.

This fungus was known to have been introduced from Japan in 1910 and 1911, but no one had seen it since its introduction until 1989. It is quite a mystery whether the fungus was established at the time of its original introduction, and if so, why it wasn't detected during the years between 1911 and 1989?

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. The question arose regarding potential use of this fungus for biological control but what would we release? There are several life stages of species in the Entomophthorales:

• The conidia are relatively short-lived. These 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. However, these are vegetative growth stages and are not environmentally resistant.

• The resting spores are relatively long-lived. If you dissect infected late instars, you find resting spores inside that are about 30 micrometers in diameter.

  Based on what we knew about the life stages of this fungus, the resting spores appeared to be the best stage of the fungus to work with.

  In releasing this fungus, we established these objectives:

  1. To establish E. maimaiga  in new areas. As gypsy moth spreads into new areas, the fungus also spreads, but we believe there is the opportunity to establish E. maimaiga  faster through manipulation.

  2. To augment the fungus in areas where it was not well established so that it could reach higher densities sooner. To increase fungal populations in areas where the fungus is established and thereby promote the occurrence of epizootics.

  We initially obtained E. maimaiga  resting spores for fungal releases by collecting soil that contained resting spores. Infected gypsy moth larvae die hanging onto tree trunks and cadavers subsequently fall to the bases of trees. We've documented high titers of resting spores at the bases of these trees. Concerns regarding this strategy for obtaining resting spores are that

  1. the soil containing the resting spores needs to come from areas without deleterious microbes also inhabiting the soil, and

  2. the soil must be processed to confirm the presence of resting spores and to quantify them.

  We could also collect and distribute the cadavers, but this strategy also has a specific problem: the gypsy moth has only one generation per year, and the larvae are generally present from April through July (varying by year and latitude). Cadavers do not hang on trees very long and, once they fall to the ground, they are not recognizable for long. Therefore, we would need to know in July the extent to which one would want to be distributing resting spores the following May. Unfortunately, gypsy moth populations are difficult to predict and one does not always know in July, before all adults have finished laying eggs for the next generation, the extent to which gypsy moth populations would need to be controlled the following spring.

  At present we cannot produce resting spores in vitro  - we can only produce them in insects. There is a constitutive dormancy and we've been studying this dormancy to discover in what ways we might be able manipulate this fungus while still releasing resting spores that have satisfied the dormancy requirements.

  In 1991 after 2 years of observing these epizootics of E. maimaiga,  the Forest Service provided funds to investigate establishment of E. maimaiga  after resting spore release. This was a large study including almost 50 release plots in 4 states over 2 years.

  

  The gray area on the above map shows where the fungus was found in 1989-1990 surveys. The open dots show our 1991 releases of resting spores and the black dots show 1992 releases. We made the releases along the leading area of gypsy moth spread.

  

  The lines on the graph above show gypsy moth egg mass density in control plots (blue lines) and release plots (yellow lines). The concentration of egg masses at the beginning of the study is shown on the left. We tried to control for egg mass density when choosing our plots.

  During 1991, there were 34 fungal release plots in 4 different states. After the release there was a drought, and fungal activity is, of course, decreased during dry conditions. Nevertheless, we did achieve establishment of E. maimaiga  that same year in the majority of sites.

  The second release year, 1992, was a more normal year with regards to precipitation. The fungus showed up in almost all of the plots where we had released it in 1991 but it also spread into many control plots. During 1992, gypsy moth populations throughout the area declined due to fungal infection. There were also population decreases in some control plots where the fungus had moved in, but we documented increases in other control plots.

  In follow-up checks in 1994, the fungus had persisted well in the seven 1991 or 1992 release plots studied. Gypsy moths were still present in these plots also, but there was a high degree of fungal infection in the host population, providing long-term fungal control.

  Number of E. maimaiga  Release Sites
  State	1990	1991	1992	1993	1994
  Maryland	0	5	0	0	0
  Michigan	0	23	1	27	6
  New York	5	6	0	0	0
  North Carolina	0	0	1	0	0
  Ohio	0	0	0	8	0
  Pennsylvania	0	27	4	4	0
  Virginia	0	18	5	0	0
  West Virginia	0	6	0	0	0
  	5	85	11	39	6

  Researchers and land managers in several different states have been involved in releasing E. maimaiga . At the time of our 1991 releases, people in Michigan and Pennsylvania were actually releasing E. maimaiga  to areas recently colonized by the gypsy moth with the idea that it would be a long time before E. maimaiga  spread on its own into these areas. People in Wisconsin and Indiana are presently talking about releases in their states, even though the gypsy moth is not yet established in Indiana.

  

  This map shows the spread of E. maimaiga  in 1989, 1990, and 1992. We don't believe the spread was only caused by our small dispersal programs; long-distance wind dispersal of fungal spores has also been hypothesized as very important for spread of E. maimaiga . Since this fungus has such potential to cause epizootics, we're wondering if another application would be augmentative releases for control.

  

  In Maryland during 1995, we looked at releasing E. maimaiga  on a small-scale to fight gypsy moth in high populations in homeowners' trees. We measured early season and late season infection, evaluating spread and residual effects of treatment. We collected larvae from the trees in a bucket truck and also from the ground and we saw a treatment effect (lower densities of gypsy moth larvae in release plots) early in the season. We think our fungal releases might be associated with the fact that only the control plots had high levels of defoliation. Thus, homeowners saw what they wanted - decreased defoliation.

  

  In Maryland, where our 1995 study plots were located, there were epizootics throughout the eastern shore. E. maimaiga  made big headlines and people who lived in that area became very aware of it.

  There are many other Lepidoptera that inhabit forests with gypsy moths, and there is always concern about impact of gypsy moth control tactics on non-target hosts. We conducted lab bioassays challenging field-collected insects from West Virginia forests with E. maimaiga . In all we evaluated 78 different species of Lepidoptera. In the lab, given optimal conditions and doses, we found a smattering of infection across different super-families. For the most part, the levels of infection were pretty low when you consider that we optimized infection conditions.

  In the Sphingidae, we found high levels of infection in Manduca sexta , the tobacco hornworm, and we're interested in looking at that further. There was no infection in other sphingid species tested. In the Noctuoidea, the only high levels of infection that we found was in the Lymantriidae, the family that includes gypsy moth. Based on these results, it seemed that E. maimaiga   is quite specific to the family that includes gypsy moth, although it might cause low levels of infection in a number of other species.

  In the field, the only cadavers we found on trees were gypsy moths. We went to locations where there was an active epizootic of E. maimaiga occurring in gypsy moth populations and we collected more than 1500 insects of 53 different species. Of those 1500 individuals, we found 1 individual of lasiocampid and 1 individual from the Noctuidae that were infected.

  We would like to continue our studies by investigating further the lack of agreement between the lab infection rates and the field infection rates. Also, we're interested in working further on the epizootiology and potential use of resting spores for control as well as the potential for in vitro  and in vivo  production of these spores.

  Based on our results to date, E. maimaiga  isn't the silver bullet that will end gypsy moth outbreaks in North America, but it is a very important natural enemy regulating gypsy moth populations and is capable of creating epizootics in both low density and high density populations. We think that it will have the long-term effect of decreasing high level gypsy moth populations and perhaps will decrease outbreak frequency.

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Last modified December 22, 1998
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