Forest Tent Caterpillars and Your Trees and Shrubs

Reading time: 5 minutes

Forest tent caterpillars are a North American species known to cause mass devastation in forest and wooded areas. They are indigenous to the continent, so you can expect to see them around your trees.

Understanding what these destructive pests can do to your trees is essential to maintaining tree health. Read on to learn more about these creatures and how to care for your trees if you ever find them.

Forest Tent Caterpillar on a green leaf
Photo by Greg Hume

What Are Forest Tent Caterpillars?

Forest tent Caterpillars (Malacosoma disstria) are a species native to North America that become Forest tent caterpillar moths at the end of their four-stage lifecycle. There are 26 different species of tent caterpillars, with Forest tents being one of the most common. Starting as an egg, the caterpillar hatches in springtime and begins foraging for food shortly after.

Once they’ve hatched, they build a silken mat that covers large areas of trees. Used for rest, they tend only to leave this spot to feed on leaves. Because these pests are very social and caterpillars from one egg mass will usually stay together, they have the potential to become massive colonies.

The Lifecycle of a Forest Tent Caterpillar

As mentioned previously, the Forest tent caterpillar has four lifecycle stages. First, eggs are laid in large quantities on the twigs and branches of trees and hatch in the springtime once winter is over. Then caterpillars emerge, forming communal nests on branches and trunks. After this, they’ll go through several moults and enter the pupal stage, developing into adult moths. These adult moths mate, lay eggs, and the cycle begins again.

How Do They Feed?

Known as defoliators, Forest tent caterpillars feed on the leaves of the trees where they live. Other similar species of caterpillar build silken tents, but Forest tents create communal webs between branches and tree trunks. They can explore the tree from these webs and venture onto the foliage to feed.

The Forest tent caterpillar diet includes the leaves and needles of trees like the quaking aspen, balsam poplar, oak, ashes, birches, and more. Because they consume vast amounts of foliage, these caterpillars can defoliate entire tree canopies during outbreaks. The most significant feeding happens during the four to six weeks the caterpillar is in the larvae stage before they become pupae.

Tree Damage and Impact on the Ecosystem

The defoliation caused by forest tent caterpillars can weaken trees, making them susceptible to infections and reducing their growth and reproductive capacity. A healthy tree may be able to regrow its leaves within three weeks, but severe defoliation over consecutive years can lead to tree death, particularly in young or stressed trees.

Effects on Surrounding Plants

When an outbreak is severe, the impact of Forest tent caterpillars can extend beyond individual trees and affect the structure and function of entire forest ecosystems. The ecosystem's delicate balance is changed as the caterpillars consume foliage and leaf coverage reduces.

This then impacts light penetration, soil moisture, and nutrient cycling, which affects other organisms that rely on the tree and forest ecosystems. Once they’ve decimated the leaf coverage, Forest tent caterpillars move onto the shrubs and other vegetation surrounding the trees and beyond.

Signs of Tree Damage

Many trees will survive feeding from the caterpillars because they can regrow their leaves during the same growing season. Still, after consecutive defoliation, your tree cannot care for itself. The resulting signs of damage include visibly thinning leaves and the destruction of leaf canopies.

The speed of growth is the first thing you’re likely to notice, but you can also see as leaves begin to disappear. The leaf loss impacts the aesthetics of your trees, so your property values may be affected.

Treatment Options

Infestations of Forest tent caterpillars can be treated chemically and non-chemically. The best option will depend on the infestation's seriousness and where it occurs.

Chemical Treatments for Forest Tent Caterpillars

There are several insecticides that you can use, like Bacillus thuringiensis var. Kurstaki targets caterpillars and is considered relatively safe for non-target organisms that might be nearby.

Chemical treatments should be used sparingly to account for any issues they might cause within the local ecosystem. Our certified arborists will be able to advise you on how best to chemically treat Forest tent caterpillar infestations and carry out any treatments required.

Natural Treatment Options

There are many natural treatments you can turn to as well. If you have the time, you can remove egg masses from trees before they hatch in the spring. To control populations, you may also brush the caterpillars off your trees or house.

Another excellent natural treatment is introducing natural predators and helpful birds or other enemies. Bird species, like warblers and cuckoos, feed on the caterpillars. Other predators include spiders, wasps, and insects like beetles and ants.

Parasitoid wasps and flies also lay their eggs inside the caterpillars, eventually killing them. This isn’t a treatment you choose, but the natural flow helps cull the caterpillars and maintain the population balance.

Integrated Pest Management (IPM)

Integrated Pest Management is a combination of several different strategies that work together to manage pest infestations, including the Forest tent caterpillar. These strategies aim to minimize the environmental impact of pests. Green Drop prioritizes using IPM because it uses science and evidence-based practices. This often makes it the safest, most practical option for you and your trees.

For Forest tent caterpillar infestation cases, IPM could include simultaneously using pesticides, monitoring population size, and encouraging natural predators through habitat management. Tree health will also be promoted through methods like selective pruning. Like most similar issues, early detection and quick intervention are vital in preventing severe damage and outbreaks.

If you spot a Forest tent caterpillar in June, and they are around one or two inches long, likely, they’re no longer feeding, but you still have options for getting rid of them and reducing the risk of them coming back in years to come.

Green Drop Can Help!

Forest tent caterpillars can pose a significant threat to trees and forest ecosystems due to their feeding and breeding habits, and understanding their impact is essential in learning how to treat them.

The expert arborists at Green Drop understand any issues that could threaten your trees, so get in touch today for a free estimate and get back to enjoying your trees all summer.

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