Gypsy moths in the United States facts for kids
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The gypsy moth (Lymantria dispar) is an insect that was brought to the United States in 1868. A French scientist named Étienne Léopold Trouvelot living in Medford, Massachusetts, brought them over. He wanted to breed a stronger type of silk-spinning caterpillar because the ones already here often got sick.
However, some of the gypsy moths escaped from his lab. They found good places to live and started to breed. Now, the gypsy moth is a big problem for hardwood trees in the Eastern United States.
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How the Gypsy Moth Spread
The first time many gypsy moths appeared in the US was in 1889, near the East Coast of the United States. In 1923, people tried to stop the moth from spreading west. They created a special barrier zone from Canada to Long Island, covering a huge area of about 27,300 square kilometers. But this barrier didn't work by 1939.
By 1987, the gypsy moth had settled across the Northeastern United States, southern Quebec, and Ontario. The insect has now spread to Michigan, Minnesota, Virginia, West Virginia, Illinois, and Wisconsin. Sometimes, small groups of moths have appeared in places like Utah, Oregon, Washington, California, and British Columbia. But people have worked hard to get rid of them in those areas.
As of 2021, the Washington State Department of Agriculture is trying again to get rid of two types of gypsy moths, L. d. d. and L. d. asiatica. They are using a special spray called Bacillus thuringiensis kurstaki (Btk) to stop them from settling there.
What Happens When Moths Attack Trees
Since 1980, gypsy moths have eaten the leaves off more than one million acres (about 4,000 square kilometers) of forest every year. In 1981, they stripped leaves from a massive 12.9 million acres (52,200 square kilometers)!
In neighborhoods with lots of trees, when there are many moths, their larvae (caterpillars) can crawl everywhere, even into homes. As they eat, they leave behind tiny pieces of leaves and their waste, called frass. During big outbreaks, the sound of the caterpillars chewing and the frass dropping can be so loud it sounds like light rain.
Usually, gypsy moth numbers stay low. But sometimes, they increase a lot. This can cause trees to lose some or all of their leaves. A report from 2011 said that the gypsy moth is one of the most damaging insects in the Eastern United States. It and other leaf-eating pests cause about $868 million in damage each year in the US.
What Trees Do Gypsy Moths Eat?
Gypsy moth caterpillars really like oak trees. But they can eat the leaves of many different kinds of trees and bushes, both hardwood and conifer (like pine trees).
In the eastern US, they prefer oaks, aspen, apple, sweetgum, speckled alder, basswood, gray birch, paper birch, poplar, willow, and hawthorns.
They usually avoid ash trees, tulip-tree, cucumber tree, American sycamore, butternut, black walnut, catalpa, flowering dogwood, balsam fir, cedar, American holly, mountain laurel, and rhododendron bushes. However, if there are a huge number of moths, older caterpillars will eat these too. Older caterpillars also eat some softwoods that younger ones avoid, such as cottonwood, hemlock, Atlantic white cypress, and different types of pine and spruce trees.
Gypsy Moth Rash
Some people have reported getting a rash, similar to a poison ivy rash, if they touch the hairs of the gypsy moth caterpillar. This can happen from direct contact or even if tiny hairs are carried by the wind onto skin or clothes. Rashes from gypsy moths were first noted in the early 1980s during a large outbreak in the Northeastern United States. In coastal Maine and Cape Cod, Massachusetts, a similar caterpillar-caused rash is more often from the browntail moth (Euproctis chrysorrhoea).
How Defoliation Affects Trees
When gypsy moths eat all the leaves (defoliation), it affects trees differently. It depends on the tree type, how many leaves are eaten, the tree's health, how many times it loses leaves, and how much water is in the soil.
If a tree loses less than half its leaves, most hardwood trees will only grow a little bit slower. If it loses more than half, most hardwoods will grow a second set of leaves by midsummer. Healthy trees can usually survive one or two big defoliations in a row. But trees that are already weak or stressed (like from a drought) can die after losing just half their leaves once.
When trees grow new leaves, they use up their stored energy. This can make them weak. You might see dead twigs and branches at the top of the tree, or new sprouts growing from old buds on the trunk. Weakened trees grow about 30 to 50 percent less. They are also more likely to get sick from diseases or other insects. For example, the Armillaria fungus can attack their roots, and the two-lined chestnut borer can attack their trunk and branches. Trees affected by these pests often die two or three years later.
Even though pines and hemlocks are not the moths' favorite food, they can lose many leaves during outbreaks. These trees are more likely to die than hardwoods. If all their leaves are eaten, about half of pine trees and 90 percent of older hemlocks can die. This is because conifers (like pines) don't store energy in their roots, except for the larch tree.
Natural Ways to Control Gypsy Moth Populations
Natural enemies are very important when gypsy moth numbers are low. These predators include wasps, flies, ground beetles, ants, many types of spider, and several kinds of birds. Birds like chickadees, blue jays, nuthatches, towhees, and robins eat them.
About 15 types of small woodland mammals also eat gypsy moths. These include the white-footed mouse, shrews, chipmunks, squirrels, and raccoons. Small mammals are the biggest predators when gypsy moth numbers are low. They are very important in stopping big outbreaks. When there are many gypsy moths, other animals like Calosoma (ground beetles from Europe), cuckoos, and birds that flock together (like starlings, grackles, and red-winged blackbirds) come to eat them.
Biological Control of Gypsy Moths
Diseases caused by bacteria, fungi, or viruses also help reduce gypsy moth populations. This is especially true when there are many moths and they are stressed because there aren't enough leaves to eat.
A disease called wilt disease, caused by a special nucleopolyhedrovirus (LdNPV), is the most damaging natural disease for gypsy moths. It causes a huge drop in moth numbers by killing both the larvae (caterpillars) and pupae. Caterpillars with wilt disease look shiny and hang upside down in a "V" shape. This virus is the most common reason for deaths in large moth populations.
Since the 1980s, a fungus called Entomophaga maimaiga has also greatly affected gypsy moth populations in North America.
Weather can also impact how well gypsy moths survive and grow. Very cold temperatures (like -29°C for 48 to 72 hours) can kill exposed eggs. If it freezes and thaws many times in late winter and early spring, it can stop eggs from hatching. Cold, rainy weather makes it harder for new caterpillars to move and eat, and slows their growth.
Here are some predators that help control gypsy moth larvae:
- Deer mice – These are seen as the most important predators when gypsy moth numbers are low. How many deer mice there are can decide if moth populations will have a big outbreak. The number of deer mice is greatly affected by how many acorns (or similar nuts) were available the year before.
- Tachinid flies – These flies lay their eggs on gypsy moths. While they can become very common during an outbreak, they don't seem to have a big effect on how the moth population changes.
- Braconid wasps – These wasps also lay their eggs on gypsy moths, but they play a smaller role in controlling their numbers.
It's hard to tell how effective releasing or increasing gypsy moth predators or parasites is. This is because how many moths are affected changes with how many moth larvae there are, what other hosts the parasite uses, and the weather. Scientists in both America and Europe are still studying biological ways to control the species. For example, Baculoviridae viruses show promise for control.
Managing Gypsy Moth Problems
There are several ways to manage gypsy moths. These include watching how many moths there are, keeping trees healthy, catching and killing caterpillars, removing egg masses, and using sprays to kill larvae and protect leaves.
To catch caterpillars, you can tie a dark cloth strip, about 12 inches wide (like burlap or old blue jeans), around a tree at eye height. Tie a string around the middle of the cloth to make a fold. In the middle of the morning or later, check the cloth bands for caterpillars and kill them.
The egg masses are oval-shaped, about 3⁄4-inch long, and look like tan felt. They are present from late July until May, when they hatch. You can scrape the egg masses off the tree and burn them or throw them in the trash. If they stay on the ground, the eggs can still hatch. These methods might be too much work or too expensive for forest managers to use in large forest areas.
The gypsy moth currently lives in less than one-third of the places it could spread to in North America. A lot of effort goes into stopping it from spreading further. Every year, over 100,000 special traps with pheromones (a chemical scent) are placed in areas that don't have moths yet. These traps help find new outbreaks that sometimes happen when people accidentally move moth eggs or other life stages to new places (for example, egg masses on RVs). If traps catch moths for several years in a row, it means a population is starting. These new groups are usually removed by spraying a special bacterial pesticide called Bacillus thuringiensis ('Bt').
In 2008, officials in California quarantined a rural 5 square mile area in Ventura County near Ojai. This was to stop a newly found gypsy moth group from spreading.
Using Pesticides
Deciding to use pesticides depends on many things. These include how many egg masses you can see, how many of the trees are the moths' favorite food, if branches are dying, and how close the area is to heavily infested woods. When there are many gypsy moth larvae, pesticides can be the most effective way to kill them.
Pesticides come in two main types: microbial or biological, and chemical. Microbial and biological pesticides contain living things that the pest must eat. These include bacteria, viruses, and other organisms. Biologicals can also include man-made versions of natural substances. These pesticides are sprayed before the larvae grow past their third stage. As larvae get older, they become more resistant to microbial pesticides.
A naturally occurring virus called Nucleopolyhedrovirus (NPV) has been turned into a microbial pesticide. It's sold under the name Gypchek and is used in programs sponsored by the USDA Forest Service. NPV and Gypchek only affect the gypsy moth.
Bacillus thuringiensis (Bt) is both microbial and biological. It's the most commonly used pesticide and also works against other pests like the western spruce budworm and tent caterpillar. When an insect eats Bt, it becomes paralyzed, stops eating, and dies from starvation or disease.
Chemical pesticides work by contact or by being eaten. The timing of when you spray them is less important than for microbial pesticides. Chemical pesticides can affect other living things that you don't mean to harm and might be risky for human health.
| Active ingredient | Common names | Notes |
|---|---|---|
| Bacillus thuringiensis | Foray | Can be sprayed from the air or ground. Also sold under other names. Harms other moth and butterfly larvae. Safe to use near water. |
| Acephate | Orthene | Can be sprayed from the air or ground. Also sold under other names. Harms bees and some gypsy moth parasites. Often used on single trees from the ground. |
| Carbaryl | Sevin | Can be sprayed from the air or ground. Also sold under other names. Harms bees and gypsy moth parasites. Was once the most used chemical in control programs. |
| Diflubenzuron | Dimilin | A special pesticide that only trained people can apply. |
The most common chemical pesticides approved by the US Environmental Protection Agency (EPA) for gypsy moths contain carbaryl, diflubenzuron, or acephate. Other chemicals like Malathion, methoxychlor, phosmet, trichlorfon, and synthetic pyrethroids (permethrin) are approved but not used often.
Studies by Peter G. Kevan and others from the University of Guelph between 1975 and 1995 showed that spraying insecticides from the air could seriously reduce pollination of blueberry and other crops. This happened because the sprays killed wild bees that were not the target. Diflubenzuron is a special pesticide that stops insects from growing properly by interfering with their molting process. It doesn't affect adult insects. Water creatures like crabs and young insects that molt can be sensitive to this pesticide.
Stopping Mating
Mating disruption is a way to manage gypsy moths that works well at different levels of infestation. It can be used alone or with other methods, like regular pesticides. This method uses man-made chemicals that are just like the natural sex pheromones (scents) of the gypsy moth. These chemicals confuse the moths and stop them from mating.
Female gypsy moths release a special scent called disparlure to attract male moths for mating. By spreading many small sources that constantly release this scent, male moths get confused. They spend most of their time trying to find these "fake" scent sources. This makes it much harder for them to find real female moths, wasting their time and effort. This greatly reduces how many adult moths mate.
Mating disruption has been used successfully and safely to control gypsy moths in many eastern US states, from Wisconsin to North Carolina. This is part of the federal Slow the Spread (STS) program. Because mating disruption uses the moth's own natural scent to stop mating, these products only affect gypsy moths. They don't harm other animals or helpful insects like natural predators and pollinators. So, they can be used instead of or along with other control methods. This helps reduce the bad effects of wide-range insecticides on other species and prevents insects from becoming resistant to pesticides.
Only two mating disruption products are approved by the USDA Forest Services for the STS program: Hercon Disrupt and ISCA Technologies's SPLAT GM. These products contain the gypsy moth sex pheromone. They have been sprayed from the air over millions of acres since the early 1990s. This has successfully slowed the spread of the gypsy moth from the northeastern United States to the rest of the continent. SPLAT GM is also sold in handheld dispensers for smaller areas (2.5 acres or more). It can be used as another option or with other ways to control gypsy moths.
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