Western Tussock Moth

Western tussock moth caterpillars can defoliate a tree in record time and they are wreaking havoc in many regions.

There are over 2,500 species of tussock moth (Lymantriinae) worldwide. The family name Lymantriinae comes to us from the Latin word for ‘defiler’. Cousin to the gypsy moth of East Coast nightmares. the western tussock moth is making itself felt west of the Rockies.

Two western tussock moth larvae (Melchoir) CC BY-SA 3.0

Western tussock moth caterpillars feed on leaves and young fruit. What makes these pests so much of a threat is how much and how fast they eat. Western tussock moth larva, or caterpillars, will feed heavily on apple, apricot, cherry, citrus, pistachio, plum, and prune trees, as well as many ornamentals, such as oak, ceanothus, and willow.

​Western tussock moths (Orgyia vetusta, formally known as Hemerocampa vetusta) are normally present in this area, being native to the west coast of North America, but chilly winter weather and natural predators usually keep their numbers limited. Warmer winter weather allows these moths have a boom population, and 2018 is one of those years.

Currently, local parks and walking trails are finding trees, sidewalks, and park equipment covered with these caterpillars. Caterpillars are falling out of trees onto cars and people, there are so many of them. Newly hatched caterpillars have a habit of using a silk thread to balloon themselves to a new location. Walking into a cloud of ballooning caterpillars isn’t something most people enjoy.

Western tussock moth description

These pests are easy to identify. A Western tussock moth caterpillar is 1-1/2 to 2 inches long and features white tufts (called pencils) that stick up along the back, with two black tufts on its head, and one on its read end. They also have paler tufts along their entire length and sides. You will also see bright red and orange spots. The adult female moth is unique in that she does not fly. She is short-winged (brachypterous) and stays with the cocoon. She is a pale gray color, but you probably won’t see her. The males find her because of chemical scents, called pheromones, that she releases. The male moth does have wings. He also has prominent, fringed antennae. He may be dark or light brown, depending on the local environment, with black and white markings. Larva go through several stages, called instars. Until they reach their full size, they are black, with black hair pencils on their first four abdominal segments.

​Western tussock moth lifecycle

Adult moths do not feed. Each female moth lays hundreds of eggs. Eggs are laid directly on the cocoon. The female moth covers the eggs with a protective coating and then covers that with a camouflaging layer of hairs (setae) from her own body. The eggs overwinter in this protected state and then hatch in spring through mid-summera. These caterpillars feed intensely for 40 to 60 days before pupating. While the caterpillars of some tussock moth species spread an irritant on the setae (hairs) that cover their bodies, the western tussock moth does not fall in that group, but they can be a pain, nonetheless.

Male western tussock moth by Guersk [Background squares are 0.2 x 0.2"] CC BY-SA 3.0

Western tussock moth controls

Local governments use several methods to control heavy infestations of western tussock moth caterpillars:

• They are spraying playground equipment with pressurized water and then sweeping up piles of caterpillars.
• They release specific beneficial insects to prey on the pests.
• They spray insecticides.

Hopefully, your garden and landscape will be spared a visit from the western tussock moth. If you are not so lucky, you can spray Bacillus thuringiensis (Bt) to deter these pests. If a severe infestation does occur, you may have to prune out heavily affected limbs.