Growing populations of gypsy moth caterpillars raise concern for landscapes and woodlots

Spring hatch of gypsy moth caterpillar begins in Michigan.

Hatching gypsy moth taken May 2, 2018, near Ann Arbor, Michigan. Photo by L. J. Vogel.
Hatching gypsy moth taken May 2, 2018, near Ann Arbor, Michigan. Photo by L. J. Vogel.

Caterpillars are eating my trees! Last summer, residents from many areas across the southern half of the Lower Peninsula contacted the Michigan State University Extension Lawn and Garden Hotline concerned that gypsy moth caterpillars were damaging their plants. For many of these folks, it was the first time in almost 20 years they had seen large populations of gypsy moth caterpillars feeding on trees. By mid-summer, numerous white female moths could be seen laying their eggs in buff-colored egg masses on the under sides of limbs and along tree trunks. Egg masses can contain up to 400 – 600 eggs. The egg mass stage lasts about eight months until tiny caterpillars emerge in early spring. This growing population has the potential to defoliate trees and woodlots in localized areas in Lower Michigan.

Caterpillar hatch was observed this spring, the first week of May, as seen in the photo taken near Ann Arbor, Michigan. The tiny caterpillar often go unnoticed until they have developed by late spring into large caterpillars, 1.5 to 2 inches in length. Gypsy moths feeds on a broad range of trees. Oaks, poplars, willow, birch, white pine, blue spruce and many fruit trees are all at risk for defoliation when gypsy moth populations are high. Defoliation of trees and forests was a common occurrence from 1985 to 1999 as this exotic moth spread across Michigan, without the natural controls that keep native species of caterpillars in balance. A few parasites, predators and a viral pathogen were known to attack the gypsy moth larvae, but did not keep populations of gypsy moth down for long.

A soil-borne pathogen was discovered in 1989, killing gypsy moth caterpillars along the East Coast. Entomophaga maimaiga, a fungal pathogen, turns caterpillars into dried out shells of fungal spores. The fungus was brought into Michigan in 1991 by scientists at MSU and the USDA Forest Service. Within a few years, populations of gypsy moth declined significantly, providing a natural control of the pest. Though gypsy moth continues to feed on trees every year, it often goes unnoticed. Over the last 20 years, it has been rare to see trees totally defoliated.

The one Achilles heel of Entomophaga maimaiga is rainfall. It needs moist conditions to grow and infect caterpillars. During periods of drought as we had in the summers of 2007 and 2012, there was a noticeable increase in gypsy moth caterpillars feeding on trees. By late summer of these droughty years, larger numbers of egg masses could be found in landscapes and woodlots. For the next couple of years, localized damage from caterpillars feeding on leaves was very evident. One park near Dexter, Michigan, in 2009 had about five acres of defoliated oak trees. Thanks to spring and early summer rains of that same year, the Entomophaga fungus spread and infected hundreds of thousands of caterpillars in the park. Dead and dying caterpillars covered the bark of many of these trees by mid-summer. Caterpillars dying from the fungus hang straight down on trunks of trees.

So why is this occurring again this spring?

Summer droughts in 2016 and 2017 reduced the effect of E. maimaiga on caterpillars. This meant larger number of egg masses across much of southern Michigan.

As caterpillars hatch this spring, monitor their feeding. Pay close attention to pines and spruces for caterpillar feeding. Conifers can suffer greatly from heavy feeding from gypsy moth.

One method of reducing populations is to band trees. A sticky substance, Tanglefoot, is spread on tree wrap, which is placed around tree trunks. This entangles caterpillars as they climb up trunks to feed on leaves. This may not be as effective in wooded areas where caterpillars can easily crawl from branches of nearby trees, rendering the banding of one tree useless. For more information on control measures, visit the websites below.

Of all our natural tools to manage gypsy moth, Entomaphaga maimaiga can have the greatest impact on small and large populations. Since its introduction into Michigan, the fungus has spread across the state. Resting spores of the fungal pathogen can survive for years in soil litter, becoming active during rainy periods in spring and early summer.

The long-term extended forecast for much of Michigan calls for a year with above-normal temperatures and above-normal rainfall. Rainfall in late spring would provide the perfect conditions for growth of the Entomaphaga fungus with the potential to greatly reduce populations of gypsy moth.

For more information, contact the MSU Extension Lawn and Garden Hotline at 888-678-3464 or visit the following websites:

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