Infestations of Lymantria dispar increase during drought

Localized rainfall patterns can lead to infestations of Lymantria dispar, formerly gypsy moth, caterpillars in Michigan.

Lymantria dispar, formerly gypsy moth, a common menace to trees across Michigan twenty years ago, is once again becoming a threat in many areas. From 1985 through 1999, thousands of acres of Michigan forest were stripped of foliage by Lymantria dispar caterpillars and there seemed to be little hope of stopping the yearly battle with this invasive pest. Help came in the form of a non-native fungal pathogen, Entomophaga maimaiga, introduced into Michigan in the 90s, which caused Lymantria dispar populations to drop dramatically by the early 2000s. Levels dropped so low that many people thought it was gone for good. The truth is that Lymantria dispar never went away.

When Lymantria dispar populations surged in 1985 most of us had never seen large areas of forests stripped of their leaves. It brought panic to both rural and urban areas as it spread across Michigan. The public feared that the caterpillars would kill their trees. Fortunately, most trees, like oak, black cherry, poplar, and maple did survive. Evergreens did not fare as well because pines and spruces stripped of their leaves are less likely to survive. Caterpillars consuming more than 60 percent of the foliage caused trees to use energy reserves to produce a second set of leaves. Weakened trees can be further be impacted if the summer proved to be hot and dry. The problem is compounded if this happens a couple years in a row.

During the Lymantria dispar outbreaks in the 80s and 90s, the Michigan Department of Agriculture working with Michigan State University, and the USDA managed an aerial spray program designed to reduce the impact. Using a soil-borne bacterium, Bacillus thuringiensis (Bt), many acres of woodlands were aerially sprayed across Michigan. Bt is a biological control that kills caterpillars by breaking down their gut tissue, causing them to starve. It does not eliminate the insect but helps to keep their numbers low. Leaving small populations of this exotic insect allowed natural controls of pathogens, predators and parasites to develop in Michigan. This was a slow process but nature began to seek a balance. The greatest impact came when a fungal pathogen was introduced into Michigan from the East Coast – Entomophaga maimaiga – which caused populations to die off in large numbers, leaving tree trunks covered in dead caterpillars, hanging straight down on tree trunks.

The success of this pathogen made it rare to find large Lymantria dispar outbreaks across the state by the early 2000s. However, rare outbreaks did not mean its days of damaging trees were over.

The effectiveness of the Entomophaga fungus drops off during periods of drought. Dry conditions mean fewer caterpillars dying from the fungus, and an increase in egg laying by moths later that summer. Localized pockets of Lymantria dispars that build during drought cycles may cover a couple of acres or only a few trees. A drought in 2007 led to an outbreak at a metro park in Washtenaw, where 5 acres of oaks were completely defoliated. Later that summer, the same park saw a massive die-off of Lymantria dispar caterpillars as consistent rainfall increased levels of the fungus.

Over the last 20 years, this cycle has repeated across much of Michigan. Michigan State University Extension’s, state Lawn and Garden Hotline at 888-678-3464 received many calls concerning Lymantria dispar in 2018. Drought conditions in 2016 and 2017 created new outbreaks in Kent, Washtenaw, Livingston, Oakland and Macomb Counties. A number of these outbreaks were in cities.

Water is the key to helping Entomophaga maimaiga remain effective, keeping moisture available to the tree at the right time. Lack of water leads to outbreaks. Once trees are defoliated, water is critical for new leaf development and without it, trees may fail to produce a second set of leaves and die.

People often call and ask if they should fertilize stripped trees in the summer. The answer is no, the immediate need is for water. Valuable trees that experience a significant defoliation need water either from nature or from irrigation. One inch of water per week directly under the canopy of the tree can help the plant.

Be observant this spring. If you had Lymantria dispar the previous summer, remove as many egg masses as possible before they hatch this spring. Knock them into soapy water. Do not scrape them onto the ground. For information on managing Lymantria dispar populations, contact the MSU Extension Lawn and Garden Hotline at 888 678-3464.

This story first ran in the Michigan Farmer Magazine.

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