How to Control Gypsy Moth Caterpillars
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How to Control Gypsy Moth Caterpillars

How to control gypsy moth caterpillars and prevent or reduce damage to your trees.

Gypsy moth caterpillar infestations can ruin your backyard shade trees for the season. These caterpillars are found mostly in the northeastern United States, Michigan, Washington, and southern Canada. The caterpillar problem gets worse as the population increases over a number of years to the point where they can defoliate entire forests in one summer season.

How Caterpillars Stress Trees

Tree damage is caused by the insect larvae, or caterpillars, which emerge from their eggs beginning in early spring and continuing through mid-May. The larvae move to the leaves of trees and begin to eat, mostly during the night. During daylight hours, larvae generally seek shade from the sun, but feeding can occur in daytime in heavy infestations. Gypsy moth larvae grow by molting, five molts for males and six for females. Feeding occurs during the period between each molt an feeding increases as the caterpillar grows larger. Feeding continues until mid-June or early July when the caterpillar enters the pupal stage emerging, finally, as a moth. After the females lay their eggs from July to September, depending on location, moths of both sexes die.


Gypsy Moth Lifecycle

An infestation of caterpillars isn't necessarily fatal to trees, but it can contribute to their decline. Trees can replace leaf loss, but the tree uses a significant amount of energy to grow a second set of leaves. If a tree suffers this stress over several seasons combined with heat and drought, they can eventually die.


Male Gypsy Moth

Female Gypsy Moth with eggs

Caterpillar Control

Females will mate and lay eggs throughout the fall and die off as winter arrives. Before she dies, the average female will lay thousands of eggs. Her egg masses will be found on the bottom side of tree branches and bark of trees. Female moths will also lay eggs on homes and other structures. This behavior seems to be one of the main reasons why gypsy moths have found their way into most residential neighborhoods. Ornamental trees seem to be a favorite as their numerous branches provide ample hiding places for these egg masses. The average size of these masses will be 500-1000 eggs.

Gypsy Moth Defoliation

There are some simple control methods to help your trees during times of heavy infestation:

Manual Control

• Eggs are laid in branches and trunks of trees and they are light brown or buff colored spots slightly larger than a quarter.

• In the off season, examine your trees, and scrape off and destroy all egg masses you can reach to reduce the number of caterpillars that hatch.

• Destroy the eggs by burning them or drown them in soapy water and discard.

• Watch for small caterpillars in late spring. A garden hose has enough water pressure to knock them off leaves and tree trunks and kill them, especially when they are very small.

• Fold a piece of burlap in half lengthwise and wrap it around the tree trunk. The caterpillars feed at night and crawl into the burlap fold to escape the heat during the day. Collect and destroy caterpillars each afternoon. If you're not too squeamish, you can squash them or hand pick and drown them in a bucket of soapy water. Insecticidal soap sprays also kill them when they are small.


• Trap male moths by hanging pheromone traps on the trees. These traps act as decoys and prevent male moths from mating with females.

• In severe infestations, you can use a biological insecticide called Bacillus thuringiensis kurtsaki or Btk. This natural organic insecticide kills gypsy moth caterpillars, but is only effective when they caterpillars are young.

• The ideal time to apply Btk is when the bridal wreath spirea shrub is in bloom. For it to work, it must be ingested by the caterpillars when they're very young, just as they start to feed. After they take the Btk bacteria in, they get sick, stop feeding and eventually die. Btk is harmless to people, pets, spiders, birds and bees.

• Btk is not effective when the caterpillars are larger. It also should not be applied when there is rain forecast within 8 hours.

Btk breaks down quickly in the environment in about three to seven days. Sunlight will help to break the product down. For the most effective use of Btk, it is usually applied more than once in infested areas. This is because the caterpillars must be in an early larval stage in order for the Btk to work. Unfortunately, not all caterpillars hatch at the same time. Because the product breaks down quickly, it will not be available to caterpillars that hatch after the first spraying, unless several applications are made.


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Comments (4)

Interesting and well illustrated article.thanks for posting

I actually had a terrible infestation while living in N.Y. My house and surrounding trees had to be sprayed to control the problem.

I've had the problem. Excellent advice on how to control the problem. Well written an illustrated, as usual.

Wu Jianing

I'm interstred to it .but I really understand it.