HISTORY & CURRENT OHIO STATUS
The gypsy moth is one of the most destructive insect pests threatening the forests and ornamental plants of Ohio. Native to Europe, Asia, and North Africa, the gypsy moth became established in North America in 1869 when brought to Massachusetts for an unsuccessful attempt to cross it with the silkworm. A few of the insects escaped, and the gypsy moth has gradually spread throughout the northeastern states ever since.
“The slow, natural spread of the gypsy moth continues to move south into the unglaciated hill country from northeastern Ohio and into northwestern Ohio from Michigan.”
The gypsy moth was first detected and eradicated from Ohio in 1914 in a suburb near Cleveland. Since that time, there have been over forty eradication projects in the state. The present program was started in 1971 as a detection/eradication program. This program included an intensive trapping survey of the entire state. In 1987, as the gypsy moth continued its march westward into Ohio, the northeastern part of the state became quarantined to prevent the artificial spread of this pest. The Ohio Department of Agriculture (ODA) is charged with the regulatory aspect of this quarantine. The quarantine helps to ensure that Ohio products are able to be marketed without restraints.
As gypsy moth populations began to expand into Northeastern Ohio, it became unfeasible to attempt eradication in this area. The gypsy moth populations were no longer isolated populations that could be eradicated, but rather a somewhat contiguous infestation that was continuously fed by populations from adjoining states. In this area, ODA began a suppression program to minimize the impacts on Ohio’s resources and people. The first suppression projects and the first noticeable defoliation occurred in 1990.
The slow, natural spread of the gypsy moth continues to move south into the unglaciated hill country from northeastern Ohio and into northwestern Ohio from Michigan. Northwest Ohio’s first significant defoliation occurred in the Toledo area in 1996. Ohio must now battle the gypsy moth on two infestation fronts. All counties not in the quarantined area are monitored, and hotspots discovered more than 50 miles from any known infestation are eradicated. Counties currently quarantined include: Ashtabula, Belmont, Carroll, Columbiana, Coshocton, Cuyahoga, Geauga, Harrison, Holmes, Jefferson, Lake, Lucas, Mahoning, Monroe, Portage, Summit, Stark, Trumbull, and Tuscarawas.
THE GYPSY MOTH FUNGUS
There is good news to report in the fight against the gypsy moth. A new weapon has emerged. A fungus, Entomophaga maimaiga (Em), has emerged as a tool that can be used against this voracious feeder. This highly virulent and host-specific fungal pathogen of gypsy moth larvae, is known as one of the most important causes of mortality in Japanese gypsy moth populations. The fungus was probably imported from Japan to areas near Boston, Massachusetts around 1910. This attempt to establish the fungus seemed to fail since extensive surveys did not reveal the pathogen. Em was not observed in North America until June, 1989 when dead caterpillars found clinging to trees in the northeastern U.S. revealed its presence. Ohio first documented the fungus in Trumbull county in 1993.
“If these moisture conditions are present and the temperature is between 14 and 26 degrees Celsius, infection can occur. You might say “when it rains it spores!”
The life cycle of Em closely parallels that of the gypsy moth. The fungus over winters in the soil in the form of dormant resting spores. As springtime temperatures and moisture levels reach proper levels (usually about 2 weeks prior to gypsy moth egg hatch), the resting spores germinate and begin forcibly releasing fragile, short lived conidia (active, infectious spores). Caterpillars are infected by coming in contact with soil borne resting spores or the germinating conidia. An enzyme helps the fungus penetrate the larva’s body. Disease develops in the caterpillar, resulting in death within 7-10 days. After death, fungal hyphae form in the caterpillar’s body, producing conidia (outside the larval body) and/or resting spores (inside the larval body). Conidia produced at this time can infect other caterpillars. The process can be repeated as long as weather conditions are favorable, and usually ceases about 2 weeks after gypsy moth pupation (mid-July). Resting spores from dead larvae are eventually leached back to the soil. Entomophaga killed caterpillars typically hang from tree trunks from their prolegs in a head down position. They also have a “rubbery” texture and appear dry. Some dead larvae will fall from tree trunks in 9-10 days, while some will remain attached throughout the autumn and winter.
Adequate moisture is key to the biology and pathogenicity of Em. This water dependence can be seen in two of the fungus’ important processes. In the spring, resting spores germinate best 1-2 days after precipitation because high humidity (approaching 100%) is required for conidial development and discharge. Also, conidia production from dead larvae usually occurs on days when there is rainfall because free water is needed for conidial germination. If these moisture conditions are present and the temperature is between 14 and 26 degrees Celsius, infection can occur. You might say “when it rains it spores!”
This relationship of Em to moisture levels is exhibited in the recent history of the fungus. In 1989 near record rainfall occurred throughout the northeastern U.S. during May and June. This was when the fungus was first discovered killing gypsy moth caterpillars in North America. Near record precipitation also fell in Ohio during the Spring of 1996, and an amazingly rapid spread of Em coincided. Since 1993, the Ohio Department of Natural Resources (ODNR) and ODA carefully monitored a few local populations of Em (many were experimental introductions of the fungus). After the summer of 1996, it was clear that Em had spread into most counties harboring a significant gypsy moth population. Research indicates the typical spread would be closer to 1 kilometer per year.
“The relationship between Em and gypsy moth appears to be very exclusive.”
E. maimaiga shows promise as a gypsy moth management tool for many reasons. Host-specificity is a prime example. Other species of Entomophaga native to the northeastern U.S. can not successfully infect gypsy moth caterpillars. Similarly, Em does not often infect other species of Lepidoptera (butterflies and moths). The relationship between Em and gypsy moth appears to be very exclusive. The list of possible non-target organisms for E. maimaiga is very short. First, Em is only likely to affect Lepidopteran larvae which are active during roughly the same time period as gypsy moth larvae. Research has shown that only a few species of heavily contoured or densely hairy caterpillars can be infected with Em, even using extreme laboratory methods. It is very rare to find a naturally infected caterpillar (other than gypsy moth) in the field. This type of host specific attribute is a big plus for any potential management tool. The fungus also works at low, as well as high, gypsy moth population levels. This sets it apart from natural controls such as the nucleopolyhedrosis virus (NPV), a viral disease of gypsy moth which kills caterpillars under stress from high population densities and diminishing food supplies. Research also shows indications of a strong synergistic relationship between Em and NPV. Improving the effectiveness of other natural controls is another positive attribute for Em as a gypsy moth control tool. The toughness of the resting spores also increases the effectiveness of Em as a management tactic. Resting spores can survive 2-3 years without the gypsy moth host.
Some are touting E. maimaiga as the ‘cure-all’ for forest and tree health concerns caused by the gypsy moth. This will probably not be the case. This fungus is one of the most promising gypsy moth management tools to surface since our struggle with the insect began, however, other management techniques will still be needed in many instances. Em will probably not affect all gypsy moth populations the same way in a given year. In some areas gypsy moth populations will totally collapse, some areas will show a population reduction, while others show little impact on the gypsy moth population. Many areas at the leading edge of an infestation do not even harbor the fungus. It usually takes 2-4 years for the fungus to establish itself naturally in a gypsy moth population. The initial outbreak will have already occurred and the most severe tree mortality often results from these first defoliation events. In addition, defoliation of trees will still be evident, even in areas where caterpillars are infected with Em. Often, infected caterpillars will not die until the 4th or 5th instar, by which time they will have already caused significant defoliation. Although gypsy moth larval mortality rates due to Em are often 75-100%, some caterpillars are not infected and continue to feed and eventually reproduce. Also, forested ecosystems offer plenty of shade and leaf litter to help maintain the high moisture levels required by Em. The survival and effectiveness of the fungus in ore open fields or in grassy suburbia is unknown.
OHIO RESEARCH PROJECT
Since 1993, ODNR and ODA have established 18 experimental E. maimaiga plots in cooperation with California University of Pennsylvania (CUP). The research plots were largely located on public lands (for instance, state forests), to insure future access to the plots and to maintain some management consistency (no pesticide treatment and retention of test trees). The purpose of this ongoing research project is threefold: to gather information on our ability to artificially introduce Em into a low level population of gypsy moth; the fungus’ ability to control the number of gypsy moths in an area; and, the manner of fungal spread through a given area.
“It seems crucial to introduce the fungus when the gypsy moth populations are low, so that it can build its levels and provide control prior to the first (and most damaging) outbreak.”
Even during the drought of 1991 and in very low density gypsy moth populations, the fungus was established at all but one test site. During dry years the fungus seems to establish, but does not kill as many caterpillars as in a moist year. Success was also seen in terms of gypsy moth population control. Em was introduced into one site with egg mass counts of 800 per acre. This level of infestation would normally result in noticeable defoliation. This population of gypsy moth was reduced to a level where caterpillars were difficult to find and no noticeable defoliation occurred. It is important to remember, however, that this test plot was monitored during the near record moisture levels in the Spring of 1996. Em killed gypsy moth larvae could be found in almost every infested county during this same time period. It would be misleading to assume that the mortality rate would be this high during low to normal precipitation periods. The plots should be monitored for a few more years to develop a clearer picture of the relationship dynamics between the gypsy moth and the fungus. The wet spring of 1996 clouded the data regarding fungal spread. Prior to 1996, the data showed a spread of slightly more than 1 kilometer per year, which is typical of other research findings. After the wet period, however, the fungus appeared to spread more rapidly and over longer distances. Now that it is found in so many places, it is difficult to actually trace the spread from the point of introduction.
The project is showing evidence that Em can successfully be introduced into low density gypsy moth populations. Once established, E maimaiga can reduce the number of gypsy moth larvae and limit population expansion. It seems crucial to introduce the fungus when gypsy moth populations are low, so that it can build its levels and provide control prior to the first (and most damaging) outbreak. It appears promising that Em can help us manage gypsy moth by keeping populations below damaging levels, effectively limiting defoliation.
GYPSY MOTH MANAGEMENT PRACTICES
“It is important to note, if you are shipping nursery stock, there is a zero tolerance for shipping any lifestage of the gypsy moth with the stock.” T he first thing to realize with gypsy moth is that once it becomes established in an area, it is there to stay. We must learn to live with this pest as well as learn to manage it. Management practices will be determined by an individual’s objectives. To manage any insect population, an Integrated Pest Management (IPM) approach needs to be followed, regardless of particular objectives. First and foremost, the population needs to be monitored. Learn to identify the lifestages and habits of the gypsy moth. Burlap banding of trees is a useful detection tool, especially in low level populations. If the gypsy moth is present, what is the incidence of parasites, predators, and pathogens? You may not be able to rely solely on these natural controls, but they will be important in determining the timing and method of control.
If your business ships nursery stock or wood products, it is important to note there is a zero tolerance for transporting any lifestage of the gypsy moth. Therefore, it is important that a detection and control program be implemented. Check to see what other pests are present and try to synchronize your control efforts. Be aware of populations in your general vicinity that may be causing defoliation. Blow-ins from nearby populations can infest nursery stock within one season. Most importantly, do not rely on natural controls when there is zero tolerance.
PREDICTIONS
“In 1997, we may see another drastic decrease in defoliation due to the large outbreak of Em in the spring of 1996.”
Eventually, E. maimaiga may cause the gypsy moth to behave more like a native insect, and less like an unquenchable exotic force. This change in behavior would result in less forest damage with fewer pesticide treatments. But, remember the gypsy moth is unpredictable. There are many environmental factors which influence the population dynamics of the gypsy moth, many of which are not yet fully understood. Adding the moisture dependent Em to the mix increases the importance of the weather factor. The graph on the next page shows the defoliation trends in Ohio. The first detectable defoliation occurred in 1990 at just over 100 acres. In six years, it has increased to almost 50,000 acres. There was a population crash after the bitterly cold winter of 1994 that caused defoliation to decrease to 100 acres. The following year, defoliation increased to over 34,000 acres. This increase may be attributed in part to the gypsy moth’s incredible viability, which enabled it to come back after the cold spell much faster than its predators and parasites.
As gypsy moth expands into new territories in Ohio, particularly the oak-hickory dominated forests found in the southeastern unglaciated part of the state, we may see defoliation numbers expand. Dry years may see heavier defoliation due to less activity from Em. Northwestern Ohio should see a generally slower rate of spread due to the large areas of tilled farm lands and non-contiguous forests. In 1997, we may see another drastic decrease in defoliation due to the large outbreak of Em in the spring of 1996. It will be necessary to learn more about this fungus so that it can be added as a reliable tool in our IPM arsenal.