Winter Moths

Most of us don’t notice moths in winter, which is what makes this species so unusual.

Winter moths (Operophtera brumata) can generate their own heat, so they are often seen much later in the season than other moth species. That being said, you will probably never see a female winter moth. More on that in a moment.

Male winter moth (entomart) Public Domain

There are actually three different moth species called winter moths in the U.S. In addition to our invasive Operophtera, there are also the native Bruce spanworms (O. bruceata) and linden loopers (Erannis tilaria). These native species feed on deciduous hardwoods.​

Bruce spanworm moth (Ronald S. Kelley) CC BY 3.0

Linden looper (Andy Reago, Chrissy McClarren) CC BY 2.0

Invasive winter moths feed on those trees, too, but can also be found causing trouble in apple, cherry, chestnut, crabapple, hazelnut, pear, and quince trees, as well as in raspberry and blueberry patches.

Winter moth identification

Male winter moths can be light brown, grayish-yellow, or slightly reddish, depending on where they occur. They often have dark brown bands on their fringed wings. The wingspan can be up to one inch wide. Antennae are stubby and covered with fine hairs. Female winter moths are brownish-gray. Adult moths are less than one-half inch long. Smooth green inchworms grow from one-tenth to three-quarters of an inch in length over six weeks. These caterpillars have narrow white racing stripes on either side of their body and look similar to fall and spring cankerworms.

Damage caused by winter moths

It’s not adult winter moths that cause problems. It is their bad-mannered children who defoliate trees. These heavy feeders can remove as much as 90% of the leaf cover, reducing trunk growth by up to 47%. This translates into reduced crop size and trees prone to pest damage and disease damage.
 
Freaky females

Winter moths take homebound to a whole new level. In a lifeform known for its ability to flutter from plant to plant or maniacally swarm patio lights, female winter moths do not fly. Instead of flying, they emerge from their pupal stage, walk to a tree, and start climbing. Female winter moths release pheromones to attract males as they climb upward. Each fertilized female winter moth will lay approximately 100 eggs near tree buds, in bark crevices, under lichen, and on branches.

Female winter moth (Adrian Tync) CC BY-SA 4.0

Those buds provide her babies with plenty of food, but that’s bad news for the tree. After burrowing into and feeding on new buds, winter moth caterpillars start feeding on leaves. When they get big enough, they spin silk strands in ‘balloons’ that carry them to neighboring trees. Caterpillars continue eating leaves until mid-spring when they abandon their tree and enter the soil where they will pupate. In late autumn and early winter, adult moths emerge, and the cycle continues.

Winter moth caterpillar (Donald Hobern) CC BY 2.0

Winter moth management

You can’t fight them if you don’t know they’re there. If you see significant leaf loss in any tree, take a closer look. If winter moths are identified, start handpicking the caterpillars and wrap your tree trunks with sticky barriers.

Imported parasitic flies (Cyzenis albicans) have proven effective at reducing winter moth populations in some studies conducted by several states, but I don’t think you can buy them online. I tried and couldn’t find any. These flies lay their eggs on leaves which are then eaten by winter moth caterpillars. The fly eggs hatch inside the caterpillars where they begin feeding. Dang. It’s a brutal world out there, isn’t it?

You can help protect your trees by making sure they have enough water. This will help them stay healthy and aid them in recovering from any defoliation. Winter sprays of dormant oils won’t cure the problem, but they may kill unhatched moth eggs. Also, foliar sprays of Bt kurtstaki (Bacillus thuringiensis kurtstaki) are somewhat effective at killing winter moth caterpillars. Unfortunately, they also kill the caterpillars of many native and beneficial butterflies and moths. And older caterpillars may develop a resistance to Bacillus.

Be sure to monitor your trees and shrubs regularly. Nipping problems in the proverbial bud can protect your buds and future crops