While it might be fun to imagine tiny worms wearing hardhats and utility belts, there's nothing cute about carpenterworms.
Carpenterworms (Prionoxystus robiniae) are the larval form of a common moth, and they love to burrow into apricot and pear trees. They can also be found in many ornamental trees, such as maple, oak, birch, cottonwood, ash, and willow. Once these pests are inside your trees, they can be difficult to evict.
As you can see, this is a robust caterpillar. They can be 1/2 an inch in diameter and 2 to 3 inches long. They have a dark, brownish head and a yellowish white body that is covered with fine hairs. They have sharp, hooked legs on the middle section (thorax) and distinct fleshy legs on the abdomen
Damage caused by carpenterworms
These wood-boring insects live in galleries, feeding on sapwood. Knowing the signs of infestation can help you get a handle on this pest before the damage becomes irreversible. The galleries created by carpenterworm feeding tend to be vertical, except for the entrance. These entrances are often found in branch crotches and in bark crevices. Tunnels are 1/2 an inch in diameter and 6 to 10 inches long. This tunneling creates points of entry for many other pests and diseases. Adult female carpenterworms seem to prefer areas that are already infested for egg laying, which can result in multiple galleries in the same area of the same tree. All that feeding and tunneling can weaken branches, making them more likely to break in strong winds, or when supporting heavy crops. Branches can also become girdled by carpenterworm feeding and tunneling, and die.
Adult carpenterworms are large, mottled grey moths that can have a 3-inch wingspan. Their coloration blends with tree bark and lichen. This camouflage makes them difficult to see. If you are able to catch one and spread out its wings, you wing be able to see if you have a male, with orange hind wings, or a female, with off-white hind wings.
Because adult female carpenter moths cannot fly very far, they tend to lay their eggs near the gallery where they were feeding. Three to six eggs are laid in the crevices near an existing gallery entrance. Upon hatching, the larvae immediate start boring into the sapwood, leaving small, rectangular entrance holes.
As they feed, the larvae will occasionally push sawdust and frass (bug poop) out of the ever-widening entry. The larvae will feed on the sapwood and hardwood until they reach maturity, molting 8 to 31 times over the next 2 to 4 years. Finally, mature pupae wriggle their pudgy selves to the entry hole and create a protective pupal case, which will block the hole until adult moths emerge. In California, this usually occurs May through July. As soon as adults start flying, they mate and the cycle continues.
Signs of carpenterworm infestation
The first sign of carpenterworm infestation is stained areas on the trunk. These stains are a combination of sap, sawdust, and frass. You may also see pupal cases sticking 2/3rds of the way out of the tree. Since the stained areas and branch dieback may also be caused by clearwing moths/currant borers, flatheaded borers, bark beetles, and longhorned borers, it is important to identify the pest before trying to control the problem.
Healthy trees are better able to protect themselves, so start by planting trees in the right location, at the proper depth, with regular fertilization and irrigation.
Because these caterpillars are already protected by the tree, insecticides do not work. There are a couple of specific nematodes, Steinernema feltiae or S. carpocapsae, that have been very successful at controlling carpenterworm larvae. Before you place your order, however, make sure that these are exactly the type of beneficial nematodes you are buying. Any other variety will be ineffective against carpenterworms. And be sure to follow the package directions exactly, or you will have wasted your money.
Small infestations can sometimes be controlled by poking long, sharp, flexible wires into the galleries and skewering the caterpillars. This is tricky because you really can’t see if you killed them or not. The only way to really know is to clear all the frass and other debris away from the area and mark the spot with some paint. Then, check the area every week for signs of frass. If frass and sawdust are seen, you missed and the caterpillar is still alive and busy feeding and burrowing.
Heavy infestations are dangerous and should be left to a professional arborist. This is because tree branches that are compromised this badly are very likely to fall on you. Since none of us are exempt from the laws of physics, and heavy branches can paralyze or kill you, stay away from them, and call an expert.
This winter, take a few minutes each week to inspect your trees for signs of frass and sawdust, or pupal cases, and cut those cute little, hardhat wearing pests off at the knees.
You can grow a surprising amount of food in your own yard. Ask me how!