(WWJ) - They haven't been much of a problem in recent years, but experts are warning that gypsy moth populations are on the rise in Michigan.
The Department of Natural Resources says some of Michigan's trees are likely to suffer attacks from the leaf-munching moths in the coming weeks.
The invasive pest caused widespread damage in the state from the mid-1980s to the early 1990s. They feast during the caterpillar stage, and officials say lots of caterpillars are hatching this spring.
Leaf loss has already been reported in Barry, Ionia and Washtenaw counties and the problem is expected to spread. Heavy defoliation likely will become visible within the next few weeks in localized outbreak areas and persist through mid-July.
"Gypsy moths rarely kill trees in Michigan," James Wieferich, DNR forest health specialist, said in a statement. "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 two-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 DNR says the best way to protect trees is to promote their overall 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. At that time, the moths were new to the state, and the population grew rapidly without natural control from parasites, predators and pathogens. In 1991, a fungal pathogen found to be killing gypsy moth caterpillars in the northeastern states was deployed in Michigan.
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.
