David J. Shetlar, Ph.D.
Extension Landscape Entomologist
Department of Entomology
Daniel A. Herms, Ph.D.
Department of Entomology
The gypsy moth is a pest that was imported into the United States in 1869 in an experiment to produce an improved silk producer. Once it escaped, it established in the New England states and has since defoliated forests, killed trees, and created great nuisances in urban areas.
It has slowly spread across Pennsylvania, New York, and down across Maryland and Virginia. It is also established in Michigan. In Ohio, it was first known along the eastern forest areas. Recently, it has moved into the northwestern counties from Michigan.
The gypsy moth survives the winter as eggs laid in masses of up to 1,000 eggs or more. Eggs hatch in April or early May into tiny (about 1/4 inch long), black, hairy caterpillars.
Newly hatched caterpillars climb into tree canopies and begin feeding. If their first tree is not to their liking, they will produce a silken thread, which carries them like a balloon on wind currents to more suitable hosts. The caterpillars continue feeding throughout the rest of the spring, undergoing five to six molts. The first three caterpillar instars have black heads and generally black bodies. By the fourth instar, the caterpillars are about one inch long, have mottled brown heads, bodies covered with black and brown hairs, and a series of five pairs of blue spots followed by five pairs of red spots on the tops of the bodies.
The greatest feeding damage is done by older caterpillars during the last two weeks of June, sometimes making it appear as if trees are stripped of leaves practically over night. After they have completed feeding in late June or early July, caterpillars enter the pupal stage from which adult moths emerge after 10 to 14 days.
Adult moths do not feed. The brownish male moth flies about during the day in search of females with which to mate. Whitish females do not fly, but attract males to them by means of a chemical 'perfume', or pheromone. Egg masses deposited during mid- to late July will hatch the following spring, completing the life cycle.
The reaction to gypsy moth invasions and tree defoliation ranges from complete horror to a total lack of interest. However, almost everyone has some level of concern when gypsy moth caterpillars cause noticeable defoliation of trees, drop their frass (feces) on everything under the trees, or when the hairy caterpillars begin to crawl over everything in sight. At this point, people are willing to spray almost anything that they think will eliminate these caterpillars from their daily routines.
While most pesticides registered for caterpillar control will kill the gypsy moth, there are also concerns about the effects of these pesticide sprays on the environment as well as the human residents.
While the actual "risk" from registered pesticides is usually misunderstood and overemphasized, the fact that most people "prefer" to not come into contact with pesticides is understandable. So, how can people avoid coming into contact with synthetic pesticides, yet control the gypsy moth? One answer is to use products derived from a naturally occurring, soil dwelling bacterium, called Bacillus thuringiensis, or "BT" for short.
In 1915, B. thuringiensis was discovered from diseased flour moth caterpillars, but few experiments or attempts to use this bacterium occurred until the 1950s. By 1961, the first BT product was registered for control of caterpillar pests, mainly in agricultural crops. Since that first registration, dramatic discoveries have occurred in identifying BT strains. Now we have BT products that affect caterpillars, leaf beetles, and mosquito larvae.
The actual mode of action of BT is simple. The bacterium produces a crystal protein toxin that kills the cells lining the insect gut. When ingested, the bacterial cell wall is digested which releases this toxin. Since insects have guts that are only one cell layer thick, this toxin literally "eats" a hole in the gut, causing an infection in the body cavity.
The interesting thing about BT, is that only certain insects digest and are affected by the protein toxins. In most insects, as well as people, birds, fish, and other animals, the BT proteins have virtually no measurable effect.
Insecticides made from BT and its toxins are usually called microbial or biological insecticides.
With the development of fermentation technology, the BT bacterium can be grown in artificial media and gallons can be produced economically.
As stated above, neither BT bacteria nor the protein toxins have any effect on people, pets (unless you are raising caterpillars as pets!), livestock, or honey bees. In fact, there is a BT product that is specifically registered to control the honey bee infesting caterpillar, the wax moth.
Unfortunately, there is considerable misunderstanding about the effects of BT sprays. Indeed, BT sprays will kill most of the young caterpillars that may ingest it as they feed on treated plant leaves. Both damaging caterpillars, such as the gypsy moth as well as harmless butterflies and moths, can be killed. However, by avoiding sensitive areas that may serve as habitat for desirable or endangered butterflies and moths, unwanted effects can be avoided.
In general, BT and its toxins are destroyed within three to five days by sunlight and other microbes. Caterpillars killed by BT stop feeding, drop to the ground, and decay harmlessly. The BT applied in a spray does not multiply or accumulate in the environment.
Because of the way that caterpillars grow, only the young caterpillars have gut linings thin enough for the BT toxin to punch holes in it. For gypsy moth, once the larvae have gotten larger than 5/8-inch long, they are rather difficult to kill with BT. Therefore, the best time to apply BT is after all the eggs have hatched and while the caterpillars are no larger than third instars. The third instar is the caterpillar stage that has molted two times after hatching. Third instar larvae are usually 1/2- to 5/8-inch long and still have black heads.
Most federal, state, and local governments use BT products to control gypsy moth on public and private lands. While there are many insecticides registered for control of gypsy moth, most governmental agencies use BT as the "least toxic" method of control.
Occasionally, federal and state agencies will also use another biological pesticide based on the nucleopolyhedrosis virus (NPV). NPV is even more specific for gypsy moth caterpillars, but it is very expensive to produce and use, and is not commercially available.
Yes! BT products for caterpillar control are available from your local garden center. However, you will have to read the labels carefully because the BT package looks just like regular insecticides. Common over-the-counter products are: Dipel , Thuricide , and Caterpillar Attack .
Be sure to read the label for the active ingredients. "Bacillus thuringiensis, var. kurstaki" should be on the label. If the variety is "israelensis," "San Diego," or "tenebrionis," then you have a BT that is active against fly larvae or beetle larvae, not gypsy moth larvae.
Follow the label instructions carefully, and don't waste your time or money if the caterpillars are over 5/8- inch long!
If you need to dispose of the extra BT you may have purchased, check out the label. Many other garden and ornamental plant infesting caterpillars are on the label. It is excellent for control of cabbageworms, fall webworms, and bagworms - as long as you spray these caterpillars when they are very small!
This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and Ohio State University Extension assume no liability resulting from the use of these recommendations.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868