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Parasite and Pathogen

I spend my summers in wonderful forest settings, where I delightfully survive the heat. No living organism's presence or absence has as far reaching effects as a tree's. As a member of a species that exploits trees and forests daily, it becomes alarming to watch another species consume a tree's life force as thoroughly as the gypsy moth, Lymantria dispar. Last fall, we had high egg mass counts along the eastern slope of High Point Ridge. This spring, that slope had a gypsy moth infestation that seemed destined to defoliate the entire slope, until a fungus eliminated the caterpillars.

Two strains of gypsy moth have been introduced into North America. A European strain was accidentally introduced into Massachusetts in the late 1860's. In 1991, an Asian strain was introduced from eastern Russian ports to Oregon and Washington, and in 1993, another Asian strain was introduced to North Carolina from a ship returning military cargo from Germany. The Asian introductions were eradicated, but are being monitored to determine whether follow-up treatments will be required. The Asian strain is of particular concern because it has the potential to spread much faster than the European strain. Even though they are the same species, they have different behavioral characteristics. Females of the Asian strain have been documented to fly up to eighteen miles before depositing an egg mass, while females of the European strain do not fly. In addition the Asian strain feeds on a wider variety of trees and shrubs, potentially causing more damage.

A gypsy moth's life cycle involves four stages: egg, larva, pupa, and adult. Usually coinciding with budding of most hardwood trees, especially oaks, the eggs begin to hatch and small larval caterpillars emerge. A natural dispersion occurs during emergence, when larvae balloon on silken threads as they hang from their host trees. As the larvae feed on tree leaves, the males proceed through five instars (stages between molts) and females, six. Most extensive and destructive feeding occurs in later instars. Pupation lasts from seven to 14 days, with the male moths emerging first. Males immediately fly in zigzag patterns, searching for female pheromones. Males follow the chemical signal and mate with the egg-laden female. Female moths lay buff-colored egg masses on the branches and trunks of trees, which is how the species overwinters.

Like most introductions, this insect quickly found favorite foods. In this case, the caterpillars acclimated to eating leaves from a variety of North American trees and shrubs, including black, chestnut, pin, red, scarlet, white, swamp, and willow oaks; alder; bigtooth aspen; gray and paper birches; boxelder; hawthorn; hazelnut; larch (tamarack); American basswood; American mountain ash; willow; and witch-hazel. What the gypsy moths did not find were predators.

Attempts to control gypsy moths involve insecticide treatments such as Bacillus thuringiensis var. kurstaki, diflubenzuron (Dimilin), nucleopolyhedrosis virus (Gypchek), and MIMIC, as well as noninsecticidal treatments such as mass trapping, mating disruption, sterile insect release, and introduction of the gypsy moth fungal pathogen, Entomophaga maimaiga. Mitigating the adverse effects of gypsy moth treatments is a priority.

All of the insecticide treatments have the fundamental problem of lacking specificity to gypsy moths and should not be considered more than temporary aids. Nucleopolyhedrosis virus only becomes a factor during high intensity outbreaks. Mass trapping, mating disruption, and sterile insect release are viable solutions in isolated cases, but become labor intensive and less effective in extensive infestations. Specific information on insecticide treatments, compiled by the Department of Agriculture, Forest Service, and Animal and Plant Health Inspection Service in the Draft Environmental Impact Statement can be found here. What is not mentioned in the impact statement is the fungal pathogen, Entomophaga maimaiga.

Entomophaga maimaiga is native to Japan and a natural pathogen to gypsy moth caterpillars. This fungus was first introduced as a biological control in sites around Boston in 1910 and 1911 with no great effect. The project was considered a failure and abandoned. In 1989, researchers identified this pathogen while studying collapsing gypsy moth caterpillar populations in New England, eastern New York, and northeastern Pennsylvania. It remains unclear whether the fungus resulted from the original release or whether a new accidental introduction took place. Since 1989, with the aid of manual distribution beginning in 1991, the fungus has spread quickly.

Entomophaga maimaiga winters as resting spores (azygospores), which have an obligate dormant period. Azygospores germinate in spring, primarily a day or two after significant rain, and develop pear-shaped germ conidia, a second type of spore that can cause infections in gypsy moth caterpillars from egg hatch until two weeks before pupation. Enzymes assist the spore in penetrating into the caterpillar's hemolymph, which is the fluid in the main body cavity of arthropods analogous to the blood and lymph found in other life forms. There it reproduces in a protoplast (a fungal cell without its cell wall) using nutrients in the blood. Prior to larval death, the fungus invades the vital organs. Taking less than a week to kill the larva, the fungus produces hyphal bodies in the hemolymph, leading to the production of conidiophores that grow out of the caterpillar's integument and release tremendous amounts of conidia and/or azygospores, depending on their host's developmental stage and environmental conditions, especially humidity and precipitation.

In terms we can all understand, the fungi's resting spores infect and kill gypsy moth caterpillars, given appropriate humidity and precipitation. Another spore body forms in the cadavers and can release either tremendous amounts of conidia spores, disseminated by the wind and infecting surrounding gypsy moth populations, or resting spores, which remain dormant until the next spring. Interestingly, research is beginning to correlate fungus infection in early instars to conidia production and caterpillar behavior to move towards canopies of trees, and fungus infection in late instars to azygospore production and caterpillar behavior to move towards trunks and bases of trees. In years with adequate rain and appropriate humidity, the fungus can devastate gypsy moth caterpillar populations. Fungus-caused mortality rates seem to depend on climatic conditions and remain proportionately high, in a good growth year, for all levels of gypsy moth infestations -- a major distinction from the viral control agent. Once introduced, the fungus minimizes the extremes of gypsy moth infestations and increases the number of years between major defoliations. Field research to date has reinforced this fungus's host specificity to gypsy moth caterpillars in natural conditions. Researchers have also demonstrated artificial spread of the fungal spores as a viable control method.

I must thank Ralph E. Webb for providing a list of 95 percent of the current literature on gypsy moths and Entomophaga maimaiga, including "Quantitative Analysis of a Pathogen-Induced Premature Collapse of a 'Leading Edge' Gypsy Moth Population in Virginia," by R.E. Webb, G.B. White, K.W. Thorpe, and S.E. Talley (Journal of Entomological Science 34[1]: 84-100 [January 1999]), which I used along with various parts of 24 or so papers coauthored by Ann E. Hajek, a professor in the Department of Entomology at Cornell University. More than 65 papers have been published on this pathogen-host relationship.

Logically, the ideal control agent for an introduced pest such as gypsy moth is specific to the species, has no negative immediate or residual effects on humans or the environment, and only needs to be applied once. Of all the control agents presently tested and in use to control gypsy moth infestations, none are ideal; only the fungus comes close. We know large-scale gypsy moth infestations are economically, ecologically, and emotionally damaging. We demand control of this pest for these reasons and look to the government and private firms to provide it. State and local governments and property owners determine what if any controls are to be used on a yearly basis.

Last year, Virginia Outdoor Foundation (VOF, which owns the Bull Run Mountain Natural Area) had a choice, like all property owners, on whether to participate in Prince William County's free spray program for 2001. VOF sought scientific advice from the Division of Natural Heritage's steward managers and field ecologists. When VOF asked for FoBR's opinion on whether to participate, we were able to offer an informed opinion, based on egg mass counts, knowledge of the benefits and costs of spraying, and the probability of the fungus contributing to control. We already knew we were experiencing a moister spring than in previous years. After the final analysis, VOF decided not to participate.

This year we will all be looking at new egg mass counts and a different overall plan of attack in both Fauquier and Prince William counties. MIMIC may be the chemical of choice statewide. MIMIC kills instars at any developmental stage, lasts longer, and provides pilots with more effective spray days than Bacillus thuringiensis. Unfortunately, its specificity ends at Order Lepidoptera and will possibly have negative impacts on native moth and butterfly species. Truthfully, no one knows the best approach. Each property, woodlot, and tree has different stories to tell about this moth. VOF demonstrated an effective approach to the problem by analyzing all of the information available and then making the choice that was best for their property at that time.

As citizens, we must be responsible stewards of our property. Few ecological problems fly into all of our back yards. Take a moment from your day to day this fall, count egg masses, investigate your county's control policy this spring, and start watching the population trends of this moth?fungus?tree relationship in your special place of the world. Every year will reveal new findings and require you to make a choice, but that's all it is: You can always make a different choice next year, if you just stay informed. After all, life's possibilities are endless, but life's truths are fleeting.