Heavy Gypsy Moth Activity Anticipated Again in 2006

 EN026 (4/06)

Heavy Gypsy Moth Activity Anticipated Again in 2006

By Kirby C. Stafford, III
Department of Entomology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P.O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8441 Fax: (203) 974-8502
Email: Kirby.Stafford@ct.gov

There was an outbreak of gypsy moth activity in parts of eastern Connecticut in 2005, with defoliation of white oaks and other trees, including some evergreens over an area of 64,273 acres (Table 1). Gypsy moth caterpillars were abundant from Guilford to Waterford and appeared centered in the East Haddam area. Aerial surveys in late summer and early fall by Deputy State Entomologist, Dr. Victoria Smith, and State Plant Inspector, Mr. Peter Trenchard, found that tree defoliation was most extensive in Middlesex County (32,985 acres), followed by New London County (19,062 acres). An estimated 10,896 acres or 17% of the total acreage with some defoliation was considered heavy (76-100% defoliation). Most heavy defoliation was observed in Middlesex County (6,544 acres) and New Haven County (3,325 acres). Very little gypsy moth activity was detected or reported in western Connecticut. The orange-striped oakworm, Anisota senatoria, was associated with defoliation of another 3,762 acres in Windham and New London counties.

Table 1. Forest defoliation in acres by county from the gypsy moth in 2005 based on aerial survey.

% Defoliation






Total acres












New Haven






New London
















The gypsy moth, Lymantria dispar, was first detected in Connecticut in Stonington in 1905. In 1981, 1.5 million acres were defoliated in Connecticut and the last large outbreak was in 1989 when our Experiment Station scientists discovered the entomopathogenic fungus Entomophaga maimagia was killing the caterpillars. This fungus has been the major agent suppressing gypsy moth activity since then. However, the fungus is not expected to prevent all outbreaks and hot spots will continue to occur. For example, there was 150 acres defoliated in Mansfield Hollow State Park in Windham County in 2000 and 2001. Only 626 acres were defoliated in Connecticut in 2004. Gypsy moth egg mass surveys are conducted at 1/16 acre plots distributed on a 7 mile spaced grid around the state. A large number of gypsy moth egg masses were detected in the southern central and eastern areas of the state in 2004 indicating potential high activity for 2005. Fall 2005 surveys have found a high number of egg masses (> 100 per plot) in the areas defoliated in summer 2005. Therefore, a high level of gypsy moth activity is anticipated in 2006 in the same areas of the state.

The heavy gypsy moth activity last year and that anticipated for this year shouldn’t mark a return to multiple years of widespread gypsy moth defoliations. Our scientists suspect that relatively dry weather conditions in 2005 were not ideal for the gypsy moth fungus, Entomophaga maimaiga. It may be that with a hot dry June in 2005, little precipitation was available at the right time for the fungus to infect gypsy moth larvae (caterpillars). Little fungal activity was observed in 2005. Since the resting spores of the fungus can survive for seven to ten years, the fungus should be capable of suppressing the gypsy moth in 2006 and future years if there is sufficient rainfall during May and June. Therefore, the impact of the fungus on the gypsy moths in 2006 will be dependent on weather conditions in May and June. Occasional caterpillar mortality from the nuclearployhedrosis (NPV) virus was observed in 2005. Levels of this second pathogen increase as the gypsy moth population increases and, in the past, the virus has mainly been important in ending gypsy moth outbreaks after caterpillar densities are high. Nevertheless, given the large number of egg masses present, there should be noticeable gypsy moth caterpillar activity in south central and southeastern Connecticut in 2006. Some tree mortality is possible in our forests with the combination of gypsy moth activity and a dry summer in 2005, a mild winter in 2006 (possible stress on trees not going into winter dormancy), and with the potential for some degree of defoliation by the moth in 2006.

There is only one generation of the gypsy moth each year. Caterpillars hatch from the buff-colored egg masses in late April or early May. An egg mass may contain 100 to more than 1000 eggs laid in several layers. A few days after hatching, the ¼ inch long caterpillars will ascend the tree and begin to feed on new leaves. These young caterpillars deposit silk trails as they crawl and, as they drop from branches on these threads, may be distributed on the wind. Larger caterpillars generally crawl up and down tree trunks and feed mainly at night. They seek cool, shaded protective sights during the day. However, under outbreak conditions with dense populations of caterpillars, they may feed continuously day and night and crawl at any time. The caterpillars will complete their feeding sometime around July 1 and seek a protected place to pupate and transform into a moth in about 10 to 14 days. Male moths are brown and can fly. The female moths are white and cannot fly. The female moth will lay a single egg mass and die. These eggs will pass through the winter and larvae will hatch the following late April or early May.

Given the potential impact of the gypsy moth caterpillars on shade trees and family activities around the home, some property owners may elect to treat for gypsy moth rather than wait and see what impact the fungus E. maimaiga and other natural enemies of the gypsy moth may have on caterpillar abundance and activity. One option is to remove and destroy egg masses found on tree trunks, decks, vehicles, outdoor furniture and other locations around the property. Egg masses can also be treated with a soybean oil product. The difficulty is that most egg masses will be located in inaccessible areas (i.e., high in the trees) and young caterpillars can be blown in from adjacent infested properties. There are a number of insecticides labeled for the control of gypsy moth on ornamental trees and shrubs. Timing of application is important and through coverage of the treated trees is necessary for good control. Proper treatment of most trees will require the services of a licensed arborist. An arborist is one who is qualified to perform arboriculture (tree services) and is licensed by the Department of Environmental Protection. The best results will be obtained after the larvae have hatched and settled down to feed, generally between mid-May and mid-June. A single application is generally sufficient to protect trees, but another application may be necessary if the entire tree was not treated or a property is adjacent to heavily infested woodlands. In the case of Bacillus thuringiensis var. kurstaki (B.t.k.), a bacterium that kills caterpillars of moths and butterflies, and insect growth regulators like diflubenzuron, they are most effective when applied to the early caterpillar stages, that is the first two larval stages or instars (prior to the second week of June or full leaf expansion).

The chemicals labeled for gypsy moth control on ornamental trees and shrubs are provided in Table 2. There are many individual brands or trade names for the insecticides; not all may be registered for gypsy moth. Some products are classified as a Restricted Use Pesticide (RUP) or a formulation for use only by a licensed applicator, mainly due to toxicity to aquatic invertebrate animals. Toxicological and other information for a particular chemical is available online from the U.S. Environmental Protection Agency (EPA) (www.epa.gov), the National Pesticide Information Center (NPIC) (http://npic.orst.edu/), and the Extension Toxicology Network (EXTOXNET) (http://ace.orst.edu/info/extoxnet/). The Pesticide Management Division, Connecticut Department of Environmental Protection, can provide information on laws and regulations governing the application of insecticides, certification of pesticide applicators and arborists, and which products are registered for use in the state (online -Kelly Registration Systems).

If there are questions or comments, please call Dr. Kirby Stafford at (203) 974-8485 or our insect inquiry office at (203) 974-8600.

Some material provided by CAES publications The Gypsy Moth by John F. Anderson [2-82] and The Fungus and the Gypsy Moth: A Tale of Two Foes and the Happy Outcome from Their Deadly Battles, by Ronald M. Weseloh; Frontiers of Plant Science (Adobe® Acrobat™ (.pdf) format); Volume 54, number 2, Spring 2002.

Table 2. Chemical and biological insecticides labeled for the control of the gypsy moth on ornamental trees and shrubs in Connecticut by general or restricted use. Chemicals or formulations listed as restricted use may only be used by a licensed applicator.

Chemical (active ingredient) Chemical type Comments
General use
Acephate Organophosphate
Azadirachtin Insect growth regulator (IGR) Insecticide ingredient found in the neem tree
Bacillus thuringiensis
var. kurstaki
Biological A bacterium that kills when ingested
Carbaryl Carbamate
Imidan (Phosmet) Organophosphate
Pyrethrins Pyrethrins Natural insecticide compounds from chrysanthemum flower
Pyrethrum (= pyrethrins) Pyrethrum
Spinosad New chemical class

spinosyn A & spinosyn D

Fermentation product

of a bacterium

Sodium aluminofluoride Inorganic fluoride
Soybean oil Oil For eggs only
Tebufenozide Insect growth regulator

General or restricted use

depending on product

Bifenthrin Pyrethroid Many products restricted use; some general use
Permethrin Pyrethroid Some products restricted use
Restricted use
Diflubenzuron Benzophenyl urea (an IGR)
Fluvalinate; tau-fluvalinate Pyrethroid
Lamda-cyhalothrin Pyrethroid
Tau-Fluvalinate Pyrethroid
Thiodicarb Carbamate

The list of active ingredients in products labeled for the control of gypsy moth is for informational use only and is based on searches of registry databases. Active ingredients and products may change over time. Mention of an insecticide does not constitute a claim of effectiveness or an endorsement by the Connecticut Agricultural Experiment Station. The product label is the legal document for use and homeowners and others applying an insecticide should read and follow the label directions.