What To Do About Gypsy Moths


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A close-up of a male gypsy moth showing its beautiful feathered antennae.

They’re everywhere! Bouncing off windows, fluttering in doorways, clustering in swarms and dancing wildly around any human stuck in their flight path. And we thought the caterpillars were bad.

According to the Associated Press, this may be the worst infestation of gypsy moths since the 1980’s. Some attribute this to the unseasonably dry springs of 2014 and 2015, which prevented the growth of the fungus Entomophaga maimaga, lethal to gypsy moth larvae.

Now that they’re here, what can we do about them? Precious acres of trees are being defoliated, so much so that the Boston branch of the National Weather Service recently tweeted that it’s possible to observe the caterpillar-induced defoliation from space. How will our trees survive?

According to the US Department of Agriculture Forest Service, “The effects of defoliation depend primarily on the amount of foliage that is removed, the condition of the tree at the time it is defoliated, the number of consecutive defoliations, available soil moisture, and the species of host.

If less than 50 percent of their crown is defoliated, most hardwoods will experience only a slight reduction (or loss) in radial growth. If more than 50 percent of their crown is defoliated, most hardwoods will refoliate or produce a second flush of foliage by midsummer. Healthy trees can usually withstand one or two consecutive defoliations of greater than 50 percent. Trees that have been weakened by previous defoliation or been subjected to other stresses such as drought are frequently killed after a single defoliation of more than 50 percent.”

Least-toxic Control of Gypsy Moths

Gypsy moths have been a periodic problem in the U.S. since 1969, when it escaped from cultures maintained by a scientist trying to establish a “native” silk industry. It is identified as a destructive pest, causing individuals, and local and state governments to intervene.

The best way to attack a gypsy moth problem is to attempt non-chemical pest management strategies first. There are inherent risks associated with all pesticides, and a number of steps that can be taken as alternatives to chemical insecticides.

Gypsy moth caterpillars prefer to eat oak, birch, apple, willow, linden, hawthorn, and sweet gum trees. Older caterpillars may also attack crabapple, cherry, beech, hickory, walnut, hemlock, and pine trees. They avoid tulip, poplar, sycamore, eastern red cedar, American holly, ash, and black locust trees.

PREVENTION — Trees should be maintained in a healthy and vigorous condition. Remove debris that may provide shelter gypsy moth larvae.

MONITORING — Look for and destroy any egg masses in late summer, and again the following spring.

CONTROL

  • Destroy egg masses in the winter and spring before they hatch. Egg masses are light tan to pale yellow and are laid in June or July. They overwinter and hatch around April 1 the following spring. An egg mass can be as large as a 50 cent piece, is usually oval and flat, and has a felt-like texture. They are found in sheltered locations, such as under tree limbs, tree trunks, tree wounds, corners around windows and doors, house eaves, gutters, fences, and woodpiles. Egg masses should be destroyed whenever possible by scraping them into a container. To kil them, scrape the eggs into a container and douse them with boiling water, being careful to avoid skin contact. Do not merely scrape the eggs onto the ground. They can survive temperatures 20-30 degrees below zero Fahrenheit. 
  • Sticky barrier bands keep caterpillars from climbing up the tree trunks. A barrier can be made with sticky gum products that are applied directly on the tree bark, such as Tree Tanglefoot, Roxo Bug Glue, and Bug Gum Mastic Barrier. Sticky tape products are also available – the best type for rough bark is an aluminum foil called Repel’m II or III. Barriers are most effective when placed around the trunk by the first week in April. The barrier should be checked at least once a week, and removed when caterpillars are not seen for several days (probably around the end of June). To capture larger gypsy moth caterpillars in trees, place a burlap hiding band above the tape or sticky trap in early May.
  • Barriers should be at least two inches wide and not be easily torn. The adhesive on the underside of the band must securely adhere to the tree bark surface and be pliable enough to fit snugly into cracks and crevices. The sticky material on the outré surface of the band must retain its tackiness for the entire period of gypsy moth activity, without runoff because of warm temperatures or rain. Repair or replace any barrier that becomes separated from the bark; periodically check the barrier bands for dirt, trapped insects and other debris. Remove large insects, silken mats and debris to keep the barriers sticky, or, if it is easier, replace the barriers themselves.
  • Place burlap hiding bands around tree trunks to reduce the number of wandering caterpillars and detect very low population levels. To make a burlap hiding band, wrap and tie a strip of burlap about 12” wide around the trunk of the tree about 5’ off the ground. Fold the upper burlap portion down over the tie. The larger caterpillars will come down the trunk to hide during the daytime and can be captured and killed in soapy water. You must destroy the caterpillars daily from May through June for this method to be effective.
  • If populations of gypsy moths get high enough that an insecticide is necessary, Bacillus thuringiensis (Bt) is an option. This biological pesticide is a bacterium that attacks the gypsy moth caterpillar. It is very effective if applied properly and can be applied by commercial applicators. It should be used only after all caterpillars have hatched, but before they get to be about 1 inch in length, and good leaf coverage is essential. Two applications are necessary, five to seven days apart. Bt is harmless to predators of worm pests and allows the populations of natural predators to grow, resulting in less of a need to spray and less chemicals needed when spraying occurs.
  • As with any pesticide application, use extreme caution when applying Bt. It is non-specific and causes fatal disease in many species of butterflies and moths that are non-target and even beneficial or rare organisms. Use it only as a last resort when all other control tactics have failed.
  • Natural predators help to control gypsy moth populations, such as birds, spiders, beetles, flies, and wasps.

What Not to Do

According to Mass Audubon, here are things NOT TO DO for gypsy moth prevention:

Do not use chemical pesticides. Despite extensive control programs using various insecticides - first DDT, now mainly carbaryl (Sevin) - the gypsy moth has steadily increased its range. Although these substances do kill the larvae and thereby protect the foliage in the year of application, the insects are never totally eliminated. Furthermore, insecticides also kill the insect predators and parasites of gypsy moths and interfere with other natural controls such as the virus that kills the caterpillars at high population densities. Applications of carbaryl or other pesticides may actually prolong or exacerbate outbreaks.

Gathering and destroying the caterpillars by hand is a waste of time and effort. You will lose because you will be greatly outnumbered by larvae.

Traps to catch and eliminate the gypsy moth chiefly benefit the seller. Traps are sometimes used by scientists to count numbers of larvae and predict outbreaks. Disparlure, a synthetic chemical that mimics the sex attractant of female gypsy moths, is used to lure male moths into traps. This will not control outbreaks, however, because there is no hope of trapping enough males to prevent females from mating.

Carol Bedrosian is the publisher of Spirit of Change.

Sources:
Beyond Pesticides. “Least-toxic Control of Gypsy Moths.”
Mass Audubon. “Gypsy Moths.”
US Department of Agriculture Forest Service. “Forest Insect & Disease Leaflet 162, Gypsy Moth.”

See also:
Pesticide Reduction: A Goal For American Agriculture
We Must Save The Honeybees And Here’s How You Can Help

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