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The Spongy Moth: Caterpillar Identification Guide (With Photos)

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Spongy Moth Caterpillar Identification

Although Lymantria dispar's common name "gypsy moth" has been discarded in favor of the new common name "spongy moth," it is still a serious pest of deciduous forest trees. L. dispar is an invasive species that can destroy entire forests. It is critically important to control their spread. If you have found caterpillars that you think might be spongy moths, this identification guide can help.

The photos in this guide show you exactly what these destructive caterpillars look like. Spongy moth caterpillars have a very distinctive and consistent pattern consisting of red and blue raised bumps. It only takes a little close-up examining to tell if the caterpillar you have found is indeed the destructive moth. Let's start by going over some basics.

Common Name

L. dispar caterpillars get their former common name "spongy moth" from the spongy egg masses are often transported by firewood, building materials, or even car tires.

Latin/Scientific Name

Lymantria dispar is composed of two Latin-derived words. Lymantria means "destroyer," and dispar is derived from the Latin for "unequal," referring to the differing characteristics between the adult male and female spongy moths.

Sexual Dimorphism

Males are brown and fly during the day, like butterflies; females are cream and black and don't fly far, or at all, from where they hatch out from the cocoon. Males come to to them as they sit where they have hatched out, and they lay eggs in that same spot.

Range

Spongy moths can be found throughout much of the world. It was introduced into North America in 1869 from Europe, and quickly became one of the country's first, and most destructive, invasive species.

Mature Spongy Moth Caterpillar

Mature Spongy Moth Caterpillar

Identification of the Mature Spongy Moth Caterpillar

There are several unique features that identify this species, but there is one absolutely surefire way to tell if your caterpillar is L. dispar. These caterpillars have a very distinctive pattern of raised bumps that no other caterpillar has:

Look for two rows of blue bumps in the front and two rows of red bumps in the back.

If the caterpillar you have found has this pattern of raised, spiny bumps, then there is no question that you found a spongy moth caterpillar.

Full-grown spongy moth caterpillar showing red and blue bumps.

Full-grown spongy moth caterpillar showing red and blue bumps.

Immature Lymantria dispar Moth Caterpillar

Immature Lymantria dispar Moth Caterpillar

Immature or Baby Caterpillar Identification

Immature spongy moth caterpillars are a little more difficult to identify, but they have one distinguishing characteristic that you can use:

Immature spongy moth caterpillars have two furry bumps, one on either side of their head, that give them the appearance of having a "collar."

If you have found a caterpillar that is tiny, furry, and has noticeable collar-like lumps on either side of the head, then you very likely have found one of these caterpillars. Look around—there are probably many more!

Baby Caterpillars Often Dangle From a Thread

This behavior is shared by many other caterpillars, so this identifying factor alone does not mean you have a big problem. However, immature L. dispar caterpillars have been known to descend from their food plant on a silk thread and hang there for a bit. This may be to escape a predator like an ant or a beetle, but more likely it's due to a particular method these caterpillars have for spreading to new territory.

The baby caterpillars are so light and furry that a good breeze can carry them for long distances—sometimes even for many miles. One study suggested that immature spongy moth caterpillars used this method to spread all the way across Lake Michigan—a distance of 50 miles—during a storm!

Gypsy moth caterpillars have the appearance of a "face" caused by markings on the head capsule.

Gypsy moth caterpillars have the appearance of a "face" caused by markings on the head capsule.

Moth Caterpillars Almost Always Occur in Groups

It's very unusual to have only one spongy moth caterpillar on your trees. They typically show up in large numbers, and even if you have found only one, a little looking will turn up many more. For every one that you can find, there are dozens or even hundreds more up in your trees, eating and growing.

Time lapse showing the spread of the L. dispar caterpillar in the United States

Time lapse showing the spread of the L. dispar caterpillar in the United States

What Do They Eat?

The answer to this is "almost everything." If it has leaves, L. dispar caterpillars will use it as a host plant. That said, there are several species that the caterpillars are notorious for attacking, and many of them are mainstays in forests around the world.

Trees that are most often attacked include alder, apple, aspen, beech, birch, boxelder, crabapple, hawthorn, hazelnut, larch, linden, mountain ash, and just about every kind of oak, their favorite food.

A rash caused by L. dispar caterpillars

A rash caused by L. dispar caterpillars

Are These Caterpillars Toxic?

The quick answer is no, but some people are very sensitive to the stiff hairs on spongy moth caterpillars. For some, the reaction is as bad as poison ivy, which produces itchy blisters that can persist for weeks.

Most people, however, do not have this reaction, even though a prickly dermatitis can result in contact with the caterpillars. Other species, like the brown tail moth, are similar in appearance but are legitimately venomous and can create a dangerous allergic reaction in some people.

If You Think You Have L. dispar Caterpillars

The first thing to do is to get on the phone and contact your state's Department of Agriculture! This species can be a serious threat to forests and green spaces—not just your backyard garden. If you report these pests, you may be helping to save your town's parks. Here are some local methods of control, courtesy of planetnatural.com.

How to Get Rid of L. dispar Caterpillars

  • Set up a trap that can be used to monitor the moth population and prevent male moths from homing in on females.
  • A sticky trap like the Tanglefoot Pest Barrier can be placed around tree trunks to stop caterpillars from traveling up to where the leaves are.
  • Azadirachtin, the key insecticidal ingredient found in neem oil, may help repel the caterpillars.
  • Diatomaceous earth is a non-toxic, naturally occurring substance that controls caterpillars of all kinds. Use very sparingly to avoid killing all of the good insects in your yard or garden!
L. dispar caterpillars are susceptible to many pathogens, including viruses and fungi

L. dispar caterpillars are susceptible to many pathogens, including viruses and fungi

Resources

  • https://www.uaex.edu/environment-nature/ar-invasives/invasive-insects/docs/dcnr_005626.pdf
  • https://news.wttw.com/2021/07/21/gypsy-moth-s-end-marks-start-movement-scrub-slurs-insect-names
  • https://nature.mdc.mo.gov/discover-nature/field-guide/gypsy-moth
  • http://www.bartlett.com/resources/tree-host-preferences-of-gypsy-moth.pdf
  • https://www2.illinois.gov/sites/agr/Insects/Pests/Pages/gypsy-moth.aspx
  • https://pubmed.ncbi.nlm.nih.gov/22842865/
  • https://www.planetnatural.com/pest-problem-solver/tree-pests/gypsy-moth-control/

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Comments

Liz Westwood from UK on June 17, 2020:

This is an extremely well-illustrated, interesting and informative article. I have learnt a lot about Gypsy moths from reading it.

Miebakagh Fiberesima from Port Harcourt, Rivers State, NIGERIA. on June 17, 2020:

GreenMind, I start to learn all this when I was a boy. But here I am refining my knowledge and understanding. Thanks for sharing.