The gypsy moth caterpillar has a major outbreak every 7 to 10 years and early signs are that this is going to be a bad year for them. Many of the control methods need to be started when the caterpillar is still small and before they do much damage to trees. If you wait longer, it will be much harder to control them.
There is a lot of talk about the gypsy moths on the internet, but many people confuse it with other look-a-like caterpillars. It is import to ID them correctly, because different caterpillars require different control methods. If you don’t know which one you have, you might be wasting your time.
The internet is also full of DIY solutions that just don’t work. In this post, I’ll help you ID your caterpillars and show you how to control the gypsy moth.
How to Identify the Gypsy Moth Caterpillar
The gypsy moth starts out as eggs which are laid late in the year and overwinter attached to trees and other man made structures. The egg sack is unique and looks like a fuzzy patch, about the size of a quarter or slightly larger.
In spring the larvae hatch out just as hardwood trees are budding. At this stage they feed during the day and spend all of their time in the tree tops. They are black and very hairy.
As they grow, they develop more identifiable characteristics. Their head is a tan-yellow color with two distinct eye spots. Behind the head, they have 5 pairs of blue spots, followed by 6 pairs of red spots. They do not build web tents like the tent caterpillars.
At about 1″ in size, they change their behavior, and start feeding at night. In the morning, they’ll climb down the tree and hide near the base of it and in the evening they climb back up the tree. This is an important fact that can be used to control this caterpillar.
The gypsy moth prefers oak, aspen, willow and birch trees, but it will also attack maple and beech.
At some point they pupate and eventually hatch out into a moth. The female is a whitish color and about 2 inches wide. The male is much smaller, a brown color and has large feather-like antennae which he uses to find a female. The male flies but the female doesn’t. After mating the cycle starts all over again.
Now lets have a look at some common look-a-likes.
Eastern Tent Caterpillar
The eastern tent caterpillar hatches out at the same time as the gypsy moth. The caterpillar has a distinct whitish strip running down its back. It also has a row of oval blue spots on each side. This caterpillar feeds at night and hides during the day in a web nest that it builds in the crotch of tree branches. This tent is very characteristic of this caterpillar and unlike the gypsy moth caterpillar, once its in the tree, it stays there. The adult moth is a brown color.
This caterpillar prefers to feed on fruiting trees like wild cherry, apple, crabapple, and hawthorn.
Forest Tent Caterpillar
The forest tent caterpillar is similar to the ones described above, but a close look will allow you to identify it. First of all, it does not build tents, even though it is called a tent caterpillar. It may make some silk mats where it rests and molts, but these are very clearly not the tents of the eastern tent caterpillar. It has cream colored spots along its back that look like foot prints showing a clear toe and heal. Blue lines run down each side. Like the previous two caterpillars, it is active in spring and the adult moth is a tan color.
The fall webworm is easily identified and does not look like the others, but its webbed tent can be confused with the eastern tent caterpillar. It makes tents at the end of branches as opposed to the crotch of branches, and they are not nearly as substantial as the eastern tent caterpillar. Unlike the other caterpillars, this one is active during the day and roams the trees in late summer.
Controlling the Gypsy Moth
If you really have gypsy moth caterpillars, control is a multi-step process.
Remove Egg Sacks
In winter or early spring, look for the egg masses. Many times they are on trees, a few feet above ground level. If you find some, scrape them off with a putty knife and soak them in water containing bleach or ammonia. Don’t try to squish them by stepping on them – you won’t kill the eggs.
Spring Tree Banding
The goal of the second attack phase is to prevent the caterpillars from crawling up the tree. This method works best for small caterpillars as they first climb the tree, but it can also be used for larger ones because they come down the tree in the morning and go back up at night. You can use several commercial banding products for this including Tanglefoot.
A simple alternative is to use duct tape but the tree needs to be dry for this to work. Wrap duct tape around the tree with the sticky side down. Make sure it sticks to the tree and push it into cracks if needed. You want to prevent caterpillars from crawling under it. Go around a second time to widen the strip. Without cutting the tape, flip it over so the sticky side is facing out and make two more wraps around the tree, covering the first two. This provides a sticky surface that will catch a lot of caterpillars and once the tape is covered with them, replace with new tape.
If you have a heavy infestation, add another ring of tape about 10″ below the first. Any caterpillar that makes it across the first ring is almost certain to get caught by the second ring.
As the caterpillars get larger they are better able to crawl over the tape or Tanglefoot, so a different approach is used. Take a strip of burlap about 2 ft (60 cm) wide and wrap it around the tree. Tuck in the top edge as if you are wrapping a beach towel around yourself. Now tie a string around the center to hold the burlap in place. Next, take the top part of the burlap and fold it down over the string. This forms a type of pocket in the burlap and caterpillars will hide there during the day to stay out of the sun. Have a look in the burlap every afternoon, and scrap caterpillars into soapy water to kill them.
Note: I tried this in 2020, and it was not very effective.
Spray With Btk (Bacillus thuringiensis var. kurstaki)
Another option is to spray the tree with a biological pesticide containing Btk (Bacillus thuringiensis var. kurstaki) This is a specific strain of Bt bacteria that will kill gypsy moth caterpillars. Other types of Bt won’t work. Spray the leaves where the caterpillar is feeding and they will be dead in about a week.
This will only harm caterpillars of moths and butterflies that feed on the sprayed tree. It will not affect birds that eat these caterpillars and sunlight destroys the leftover Btk in 3 to 5 days. This product needs to be applied in early spring before the caterpillars are 1/2″ long. It is not effective on older caterpillars.
Chemical insecticides can also be used, but consider the damage they do to beneficial insects and only use insecticides labeled for gypsy moths. Neem can be an effective spray for controlling the smaller caterpillars, but this is a contact spray, which means you need to spray the caterpillar, not the tree.
Controls That Don’t Work for Gypsy Moths
As gypsy moths become more of a problem, the internet is becoming congested with all kinds of DIY solutions. The following don’t work.
- home made moth traps – they catch an insignificant number of moths
- pheromone traps – catch moths which can be an indicator of problems the following spring, but they are not effective of controlling the caterpillars.
- bug zappers
Control of Tent Caterpillars
Here is a simple way to control the eastern tent caterpillar and the fall webworm.