Redhumped caterpillar chewing on redbud tree leaves.

Redhumped caterpillars often appear in large numbers after a year with a mild winter. If you notice a tree with chewed or “skeletonized” leaves in late summer or fall, you might have this pest.

Redhumped Caterpillar Identification

Caterpillars are 1 1/2 inches long with a red hump behind the head. The body is yellow with red and white stripes, and the legs have prickly looking spines on them. You may also see clusters of round, yellow eggs on the underside of leaves.

Plants Affected

Redhhumped caterpillars are common in the Central Valley of California. They eat the leaves of ornamental trees like Western redbud, cottonwood, birch, willow, liquiadamber. They also eat the leaves of  several fruit and nut trees like cherry, almond, walnut, prune, plum and pear.


Caterpillar parasitized by beneficial wasps.

The caterpillars usually appear in large numbers and can completely defoliate a tree. Since the damage happens in late summer and fall, you don’t need to do anything. However, in the case of walnut trees you will want to take action. Walnut trees sunburn easily which makes them more susceptible to other pests.

Caterpillar Control

If your tree has a small infestation, prune off the branches that have caterpillars. For bigger problems, use the organic pesticide Bt (Bacillus thuringiensis). This microbial formula affects caterpillars only (gives them a deathly stomach ache) and won’t harm bees. Just remember that caterpillars are the larvae of butterflies and moths, so only spray near redhumped caterpillar larvae.

If you do nothing about the caterpillars, they will drop to the ground and form a pupae. The pupae stays in the soil until spring and then hatch into a nondescript looking moths. A cold winter will kill off the overwintering pupae.

Interestingly, if you see a white formation on a caterpillar, it’s been parasitized and killed by a beneficial wasp. These tiny wasps don’t sting people and are voracious predators of many pests in the garden.

Where to Purchase Bt

Bt can be found in many local garden stores. Or you can purchase it online. Doing this helps support Anne’s blog!