It is sometimes easy to tell which pest is breaking out around the county by the sheer number of phone calls that I get. This week it is the walnut caterpillar that is being found all over our beloved pecan trees.
Don’t let the name mislead you, the walnut caterpillar has a wide host range of woody plants and trees such as pecan, walnut and hickory. The larvae start off as a reddish-brown color and then become black with grayish lines and hairs as they grow and develop. These caterpillars can reach lengths of up to two inches when fully grown.
Damage caused by the younger reddish-brown larvae is typically localized to a few branches because the larvae feed in colonies. Larvae further along in their development (called ‘instars’) can cause more damage as they spread throughout the canopy and they venture away from each other. These caterpillars damage the trees by feeding on leaves, eventually consuming the entire leaf surface, leaving only the larger leaf veins.
The walnut caterpillar has been around here for ages as it is native to North America and is common throughout most of the eastern United States. The western edge of its range can be found as far west as Kansas, Minnesota, Oklahoma, and Texas. In Texas, the walnut caterpillar is found throughout areas east of the Pecos River.
The walnut caterpillar has a wide host range of both shrubs and trees. The primary host for walnut caterpillar includes trees in the Juglandaceae family such as butternut, hickory, pecan, and, of course, walnut trees.
Each year in mid to late spring, overwintering pupae emerge as adult moths. After mating, adult females lay eggs once in a mass of around 600 eggs. Egg masses are laid in a single layer on the underside of leaves on their preferred trees. Larvae hatch from eggs in roughly 9 days and pass through five larval stages (instars) over the course of about three weeks.
During the first four larval stages, the reddish-brown caterpillars can be found feeding in clusters. When molting from the fourth instar to the fifth instar, the caterpillars move to the trunk or a main branch and molt together. Larvae in the 5th instar are black with grayish markings and long hairs. Fifth instar larvae venture out to feed on their own. In Texas, the walnut caterpillar can complete two to three generations per year depending on the number of frost-free days between spring and fall.
In our part of the state, the walnut caterpillar rarely reaches levels that cause economic damage, but there have been occasional outbreaks that have caused economic loss to pecan crops. Normally, parasitic insects, predatory insects such as ladybugs, and arthropods like spiders keep populations from causing economic damage.
If you are really lucky and happen to find a caterpillar egg mass under the leaves of a tree, go ahead and remove them. If you already see the caterpillars feeding, they are easy to kill with most any insecticide so long as you can get it on them. I don’t have a simple method for homeowners to treat caterpillars feeding up high in a large tree.
Truth be told, your pecan tree has likely survived several attacks from these same caterpillars. Unless you are a commercial pecan grower whose livelihood depends on a good pecan crop, those caterpillars may bother you much more than they are bothering the tree!