Department of Natural Resources
June 12, 2019
|Gypsy moths are an invasive species, a term for non-native pests that can cause harm to native species and ecosystems. In its caterpillar life stage, the insect caused widespread defoliation in Michigan from the mid-1980s to the early 1990s. A large population in 2018 has led to more caterpillars hatching this spring.
“Gypsy moths rarely kill trees in Michigan,” said James Wieferich, DNR forest health specialist. “Only stressed trees suffering from problems like drought, old age or root damage are at high risk. In most cases, gypsy moth caterpillars are more of a nuisance in residential areas than in the woods.”
Defoliation most often occurs in the season following a drought-heavy year, such as 2016, 2017 and the summer of 2018. Many forest pests tend to target trees that are weakened – perhaps from drought – or otherwise not in the best of health.
The leaf-eating caterpillars are hairy, up to 2 inches long and have a pattern of blue and dark red spots. Male moths are dark buff in color and fly; females are white with black, wavy markings and do not fly.
The No. 1 thing people can do to reduce the effects of pests like gypsy moth is promoting tree health.
“Water trees regularly and avoid damaging the roots and bark,” Wieferich said. “That goes a long way in helping trees fend off the effects of defoliation.” Periodically removing dead and dying trees in woodlots also helps keep remaining trees strong.
Mature forests normally can withstand heavy gypsy moth defoliation with little impact. Defoliated trees will begin to develop new leaves in July to replace those that were eaten. Even heavily defoliated trees will recover without serious long-term effects. However, consecutive years of mass defoliation will start to take a toll, even on the healthiest of mature trees.
Gypsy moths were first discovered in Michigan in 1954. By the 1980s and 1990s, large gypsy moth populations cycled through Michigan, defoliating up to a million acres in some years, said Scott Lint, DNR forest health specialist. At that time, the moths were new to the state, and the population grew rapidly without natural control from parasites, predators and pathogens, which are organisms such as bacteria and viruses that can cause disease.
In 1991, a fungal pathogen found to be killing gypsy moth caterpillars in the northeastern states was deployed in Michigan. This fungus, Entomophaga maimaiga, proved an effective biological control, remaining in the soil from year to year and infecting gypsy moth caterpillars that come in contact with the soil or with other infected caterpillars. Moist soils help to activate the fungus and spread it among gypsy moth populations.
Nucleopolyhedrosis virus (NPV) is a naturally occurring virus specific to gypsy moths. It can spread quickly during major gypsy moth outbreaks, causing a population crash. This virus also was used in Michigan in the 1990s with positive effects.
To determine whether NPV is at work in a certain location, look for dead caterpillars attached to tree trunks in an upside-down “V” position. Caterpillars affected by the E. maimaiga fungus also remain attached to trunks but hang straight down.
These natural enemies of the gypsy moth are now well-established across Michigan and actively are reducing populations. Wet spring weather in many areas has helped these organisms to develop and spread quickly. To date, these pathogens have limited the size of outbreaks, which rarely last more than a few years.
To address a gypsy moth infestation in a handful of individual trees, homeowners can purchase a spray containing Bacillus thuringiensis var. kurstaki (Btk), a bacterium that naturally occurs in the soil but can be lethal to certain caterpillars and moths. Be sure to follow label directions exactly. The best time to spray is when caterpillars are small, usually through mid-June. When caterpillars are massing, spraying tree trunks with a mixture of dish soap and water or scraping caterpillars into a bucket of soap and water also are effective.
For more information about the DNR’s Forest Health Program or to view last year’s Forest Health Highlights report for Michigan, visit Michigan.gov/ForestHealth.