Gypsy moth management
What is being done to control the gypsy moth?
To protect natural resources, state and federal agencies have been involved in gypsy moth management for many years using a wide range of integrated pest management tools. Some of the tools implemented to reduce impacts caused by gypsy moth include natural (biological) and chemical control, mating disruption, and selected forest management practices.
Predators: White-footed mice feed on gypsy moth pupae and are probably the most reliable predator in the region. However, the mice do not occur everywhere the moth is found, and there are not enough mice to effectively control gypsy moth populations. Sapsuckers are one of the few bird species that will eat the hairy gypsy moth caterpillars, but again there are not enough of them to control the population.
Entomophaga maimaiga: In years of wet spring weather, a fungal pathogen, Entomophaga maimaiga, has dramatically reduced the number of gypsy moths in the eastern states. The fungus was introduced into the northeastern United States in the early 1900s to help control gypsy moth, but it was not found in moth populations until the late 1990s. Since then it has spread naturally, following gypsy moth populations as they move west. However, the fungus requires high rainfall and humidity to germinate and infect gypsy moth larvae, making its effectiveness in a given year subject to local weather conditions.
Nucleopolyhedrosis virus (NPV): NPV is a naturally occurring virus specific to gypsy moths. It spreads like the common cold and is very effective at reducing high-density gypsy moth populations. In generally infested areas, it is the organism that is most likely to cause a major outbreak of gypsy moths to crash, creating cyclic outbreaks every 8–12 years. The cycle is similar to outbreaks of native defoliators, such as forest tent caterpillar. However, NPV is not effective in low-density populations and is not likely to be seen in Minnesota until many years after the gypsy moth has become permanently established.
- Gypchk is a product made from the nucleopolyhedrosis (NPV) gypsy moth virus. It is specific to gypsy moths, so it is safe to use in areas where sensitive butterfly species occur. However, the product is difficult to produce and is available only in very small quantities. There is barely enough Gypchk available today to treat gypsy moth in areas where sensitive moths and butterflies, such as Karner blue butterfly, also occur.
Egg parasites: Tiny nonstinging wasps, Ooencyrtus kuvanae, deposit their eggs into gypsy moth egg masses. As the O. kuvanae larvae develop, they consume the gypsy moth egg, killing it. These wasps are also a nonnative species that were introduced to control the gypsy moth. The wasp typically attacks 10 to 40 percent of all gypsy moth eggs. It can produce three generations during the spring and summer, making it a significant parasite of the gypsy moth. The MDA has been establishing O. kuvanae in eastern tent caterpillar populations (another host species) in order to increase its numbers to help control gypsy moth when it arrives here.
While there are many chemicals labeled for use on gypsy moths, only four chemicals are allowed under the Federal Environmental Impact Statement (FEIS) that guides all publicly funded gypsy moth treatments. These four chemicals are Btk, Dimilin, Gypchk, and Mimic. Many other chemicals can be used to effectively control gypsy moths, but because the potential to harm other species is considered too great, they are not recommended for widespread use.
Btk (Bacillus thuringiensis var. kurstaki) is a naturally occurring soil bacterium. When eaten, Btk kills only certain butterfly and moth caterpillars. It does not harm humans or animals other than gypsy moths. Because it can impact some sensitive species, like the Karner blue butterfly, it cannot be used everywhere. Btk is applied as a spray in the early stages of the caterpillars' growth cycle and can be effective in eradicating small isolated infestations or in reducing large defoliating populations of gypsy moths. Due to its relative environmental safety, Btk is the product most widely used by government agencies.
Dimilin is a synthetic hormone that inhibits molting in immature insects, thereby killing them. It is very effective. Because it also affects aquatic species, it is rarely used by government agencies. However, it is the product most widely used by private landowners.
Mimic is an insect growth regulator. Tebufenozide, the active ingredient, mimics the action of a natural insect hormone that induces premature molts of the gypsy moth caterpillar, resulting in death. Mimic impacts most lepidopteron larvae, but does not impact other orders of insects. However, until recently it was not covered by the FEIS, so had not seen widespread use. With the new FEIS published in 2006, it may become more common in private land applications.
Disparlure and Disrupt II: Mating disruption can be very effective in controlling low-density gypsy moth infestations. Disparlure is a synthetic reproduction of the female moth sex attractant, or pheromone, used to lure males. Disparlure is used in gypsy moth survey traps and to disrupt mating when applied over large areas. A small amount of Disparlure is put inside tiny permeable plastic flakes in a product called Disrupt II. When the flakes are spread over an infested area they stick to leaves and other surfaces, emitting the pheromone for up to three months. This confuses male gypsy moths and limits their ability to find a mate, thus reducing reproduction. While Disrupt II effectively controls low-density populations, it does not work in high-density populations where female moths are abundant and easy to find.
Urban tree care: Preferred host trees under stress are the most at risk of mortality following severe defoliation. Proper site selection, planting, and tree care are essential to optimize the trees' own natural defense mechanisms. A vigorously growing tree can withstand some defoliation and most weather extremes. Root disturbance or trunk injuries can lead to stress that may leave a tree prone to additional damage or death.
To keep trees healthy:
- protect the critical root zone
- avoid all work that may result in wounding of oaks during April–October
- maintain proper watering and tree care
- consult an arborist as needed
Forest management: There are two broad strategies to consider in forest stand management. When and where you apply these strategies depends on your land use objectives, stand composition, and site-specific conditions. The combination will determine which practices are feasible for your stand. When in doubt on the appropriate strategy, be sure to consult a professional forester.
Managing for stand diversity is the best means of limiting any insect defoliation. Encourage a mix of tree species, forest types, ages, and sizes. Managing for tree health and vitality is the best means of limiting tree mortality associated with defoliation (no matter how diverse your stand, some defoliation still may occur). Thin overly dense stands to reduce competition. Where consistent with management objectives, harvest and regenerate oak and aspen stands growing beyond their normal rotation age. Remove suppressed trees likely to die anyway and create growing space for seed and crop trees. Maintain oak as an important component of the stand, but encourage other species where possible.