Fuzzy fall visitors: Caterpillars that attract attention and could cause needless concern

Hickory tussock moth larvae and woolly worms are two common caterpillars found in the fall, but don’t cause much damage this late in the season.

Hickory tussock moth larva. Photo by John H. Ghent, USDA Forest Service, Bugwood.org
Hickory tussock moth larva. Photo by John H. Ghent, USDA Forest Service, Bugwood.org

Questions about two fall caterpillars are coming in to Michigan State University Extension educators. In many places in Michigan, people have seen what they think is a new, fuzzy caterpillar. From a distance, it is white with what looks like a black line down its back. This white, furry larva is the hickory tussock moth juvenile. People are concerned because they are used to seeing the woolly worm, which is black with a band of rust brown in the middle of its body. So who are these furry visitors? Each has stories told about them that are best termed fanciful or imaginative.

Hickory tussock moth larva

This caterpillar is this is the juvenile stage of the hickory tussock moth (Lophocampa Caryae). There is one generation a year in Michigan. The adult female moth lays her eggs in May or June, which are placed on a variety of trees besides the hickory from which they get their name. Other trees are walnut, ash, elm, maple, oak and a few others. The larvae feed on leaves for a number of weeks. Eggs are laid in groups and the larvae feed in groups, so it is not unexpected to find groups exiting the trees when they have grown as big as they are going to get.

When you are a white caterpillar, you attract a great deal of attention. The furry larva has a line of black down its back. Sometimes, the black line looks more like individual tufts than a line. There are also four small areas on the body where there is a very thin cluster of black hairs that are longer than the rest of the fuzz. These are called “pencils” and are found with one on each “corner” of the body, sort of like a black spot at each corner of a rectangle.

The hairs on the body of the larva can cause a rash if its body comes in contact with tender skin. Picking it up with your hand would be less aggravating than dropping it down your shirt, although neither is recommended. Since there are Internet sites that are calling the larvae toxic or poisonous, this is not what it seems. The only danger is skin irritation or a rash.

Woolly worm or woolly bear

The woolly worm or woolly bear (Pyrrharctia isabella) is the caterpillar that people look to for a prediction about the coming winter. The story goes that the more narrow the rust brown stripe in the center of the body is, the harsher the winter will be. Sadly, the truth is much less captivating. Larvae are born black and with each growth stage or instar, the central brown stripe gets wider. There appears to be great genetic diversity in regards to the amount or lack of brown, also. Some stay all brown or all black their entire lives.

This fuzzy caterpillar could also cause dermatitis if it comes in contact with tender skin, but be amazed at the woolly worm’s ability to withstand bitter winter temperatures. It often spends the winter under leaves or other plant debris and freezes solid. In the spring, this insect thaws and life resumes. Little woolly worms have a diverse diet in the late summer and fall, consisting of grasses and a few flowers like asters, clover and sunflowers. They might also take a nibble of maple or birch leaves, but there is such little damage it is inconsequential.

Woolly bear
Banded woolly bear larva. Photo by Whitney Cranshaw, Colorado State University, Bugwood.org

The good thing about fall caterpillars is that they are feeding on plants that are at the end of their growing season and all are shutting down for the winter. There is not a great deal of damage that can be done at this point and the larvae do not feed on buds for next year. Smart gardeners just consider them two fall color commentators to add insect interest at the end of the growing season.

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