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Gypsy Moth Mayhem

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Gypsy Moth Mayhem
The gypsy moth is by far the most destructive pest of forest and shade trees in Maryland. The caterpillars eat the leaves of oaks and other hardwoods in May and June. Heavy populations of caterpillars will eat most or all leaves in a tree. Large outbreaks have affected hundreds of thousands of acres statewide.

The gypsy moth caterpillar often is confused with the eastern tent caterpillar, which also appears each spring. Though very visible, the eastern tent caterpillar is not as serious a threat to trees. For help identifying these caterpillars see .

Maryland is currently experiencing the worst gypsy moth outbreak in a dozen years. Wooded areas of Cecil, Harford, Baltimore, Anne Arundel, Howard, Prince George’s, Montgomery, Carroll, Frederick, Washington, Allegany and Garrett Counties are having problems with defoliation by the gypsy moth. This is despite the spraying of over 50,000 acres of public and privately owned land in May by the Forest Pest Management Program.

Many areas with current gypsy moth problems but which were not sprayed have turned out to be either areas known to have low – moderate population levels going into Spring 2007 or where no egg masses were observed prior to Spring 2006. Apparently dry weather the last two seasons has resulted in more caterpillars surviving to the large stage than usual, causing populations to surge and defoliation to increase. May 2007 was one of the driest Mays on record. Dry weather generally benefits the gypsy moth because it discourages the spread of the virus and fungal diseases that can infect a large percent of caterpillars.

Many residents and homeowners are seeking advice regarding combating the heavy populations on their properties. As of the middle of June, the gypsy moth caterpillars in almost all areas (except in Allegany and Garrett Counties) are nearly full grown and it is not cost effective to hire an applicator to spray with insecticides at this late stage. Homeowners may still wish to directly treat trees and shrubs that can be reached with a pump sprayer or to kill caterpillars by brushing them with a wire brush into a bucket of soapy water to drown. Employing burlap skirts or sticky tapes will accomplish little unless the caterpillars are removed and destroyed daily. However, these and other control techniques designed for the homeowner will be almost futile when there are high numbers of caterpillars in their own and surrounding yards.

If the Oak trees have been stripped of their leaves, the caterpillars will seek other food sources, including ornamental trees and bushes on your property. These can be protected by spraying with fast acting insecticides labeled for that use.

The best actions to take are those which preserve the health of your trees. Trees that have lost 60% or more of their leaves to the gypsy moth are at the greatest risk. These trees may re-foliate in another 3 weeks or so, and will be in a weak and vulnerable condition. Trees can benefit greatly from being watered with a slow trickle of water over several hours. Water flow should be slow enough so that there is not run-off, but that allows the water to pool and soak into the ground. Homeowners are encouraged to also seek the advice of an arborist or licensed Tree care expert.

Surveys of egg mass counts are conducted in each area as part of the selection process for proposing areas to treat in the following year (spray 2008). The number of areas treated will depend on the amount of available funding from the U.S. Forest Service and State and local sources. [Maryland Department of Agriculture, ]

More than 5,000 acres of trees in Frederick County have suffered from gypsy moth infestation. The moths prefer hardwood and softwood trees but oak, poplars, birch, alders, beech, apple and sugar maple are also welcoming. The insect places eggs on the bark of trees and other protected spaces in or around the tree during the winter allowing them to hatch in the spring. The larvae have instant food access on the young leaves until July when the feeding season is complete. The light brown male moth is slender and has a wingspread of about 37mm. It is quite unlike the larger, light colored, heavy-bodied female.
A homeowner can actually do several things to help reduce gypsy moth damage around their property.
Remove Nesting Sites- Anything that a gypsy moth caterpillar can crawl under that is placed on or near a favored gypsy moth food plant can become a nesting site. Some common backyard gypsy moth "hot spots" are tree houses, trespass signs, stacked fuel wood and lumber, junk piles, old tires, rock piles, picnic tables, and outbuildings.
Eliminating these preferred hiding spots can help slow gypsy moth buildups. Even beneficial items such as bird boxes and feeders, which should be provided to encourage the presence of birds, are excellent resting sites. Examine and remove any resting caterpillars from them frequently.
Destroy Visible Egg Masses- Each female moth or egg mass you destroy eliminates an average of 500 caterpillars from next year's gypsy moth population. Female moths are flightless and crawl from their pupation sites to mate and lay eggs. Look for the white females and buff-colored, nickel-to-quarter size egg masses.
Carry a spoon and a jar or tin can half full of strong detergent solution with you as you search the tree trunks, loose bark flaps, and other dry, sheltered sites. Scrape any moths and egg masses you find into the solution and let it stand a day or two before discarding. Do not merely scrape them onto the ground. This will not kill the eggs, and winter snows will actually blanket and protect them from natural enemies. Be sure to wear gloves to prevent a minor rash that the hairs covering the masses cause.
Burlap Skirt- The most widely used mechanical device is the burlap skirt around trees. Burlap skirts are actually traps designed to provide sheltered resting sites for caterpillars. Because you install them at chest level, the resting caterpillars are easy to collect and destroy with your spoon and detergent.
The burlap skirt is simply a 30 -45 cm wide band of burlap cut a little longer than the distance around the tree. Wrap the band around the tree, overlapping the ends, and tie securely around the middle with baler twine. Fold the top half of the band down over the bottom half to form a loose skirt. Make sure that the skirt flares out and that the bottom edge hangs loosely. Band all trees and check every day or two by lifting the skirt to collect hiding caterpillars.
Combining Forces to Get Better Bang for the Buck: Because of inadequate funding for spraying infested areas, wood lot owners with larger acreages are urged to join together to achieve economies of scale from companies who will spray to control these invasive insects. They hope that this joint effort might enable them to get a group rate of $50 an acre for spraying the pesticide Dimlin from the air. The Frederick Forestry Board and Frederick DNR Forester, Mike Kay, are reaching out to area landowners to educate them about the threat posed to mature forests from the moths. Frederick County commissioners and area residents are discussing ways to create a county fund for spraying costs. Combining sources of funding is difficult with each source having separate regulations that determine which chemicals are permitted.
As Forestry Board members are quick to point out, spraying for gypsy moths is much cheaper than losing mature forests and planting new trees. For example, one project that installed larger caliper trees at the South Frederick Arboretum cost almost $9,500 to plant an acre. In addition, dead defoliated trees in our forests create conditions for forest fire that can impact large areas of diverse forests, wildlife and residential developments. Their loss also has significant negative impacts on air and water quality in the area.
Woodlot owners have suggested to members of the Frederick Board of County Commissioners that they consider investing in treating forest acreage for gypsy moth suppression. One resident suggested that the County use funding collected under the Forest Resource Ordinance from land developers for gypsy moth spraying. The Board plans to discuss gypsy moth spraying during January at a regular work session, tentatively set for 8:30 am on January 29th.

For additional information, contact Mike Kay at or 301.473.8417.

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