Winter moths

burly males and wingless females

In late fall and early winter, you may notice moths fluttering in your headlights. Or perhaps you see them plastered on a windowpane when there’s a light on inside the house. Sometimes dozens of them gather near a porch light shining in the darkness.

It’s a peculiar time of year for cold-blooded creatures to be active, but around the world there are a number of moth species that fly at this season.

The general term for these moths is ‘winter moth’ and sometimes that term is also applied to particular species. When more than one species has the common name of winter moth, it can be confusing. For instance, there is a winter moth in Europe that has invaded North America and become a nasty pest, because the larvae defoliate deciduous trees, sometimes seriously.

That, however, is not OUR winter moth, which is also called the Bruce spanworm, and it is native here. It also defoliates deciduous trees, and a severe infestation that lasts several years can reduce tree growth or even kill the affected trees. Our winter moth is very hard to distinguish from the European invader, but it seems that the invader is not (yet) found here.

Winter moths belong to the family Geometridae, also known as inchworms or loopers, because of how the caterpillars hump their way along. The adults of the local species that you see at your lights are males; only the males have functional wings. The wingspan is only about an inch or a bit more. The wings have a fine fringe around the edges, and the general color is brownish-gray. Close inspection reveals a delicate pattern of brownish wavy lines across the wings. Females are flightless and inconspicuous, often resting in bark crevices.

Physiological studies of our local winter moth or Bruce spanworm have shown that their wings and flight muscles are unusually large for an insect of that size, and those muscles are adapted to work at lower temperatures than is true for most moths. Because of this combination of features, the males can fly at temperatures as low as the freezing point and sometimes even a little lower.

Females of our local species fill their body cavity with 100-200 eggs; eggs comprise an average of 63% of the female body weight. There is a trade-off between fecundity and flying ability, and females have specialized in a way that leads to higher fecundity. If females were to re-evolve the ability to fly, they would need to allocate much more body tissue to muscles and wings. To fly as well as males, over a similar range of temperatures, females would have to reduce their fecundity (number of eggs per female) by about 82% compared to non-flying females.

Bruce-spanworms-mating-2
Bruce spanworms mating. Photo by Bob Armstrong

Late fall and early winter is the mating season for winter moths. Females send out airborne chemical signals to attract males—although the males seem to be easily diverted to lights, where there are no females. (There’s a puzzle!) Females lay their eggs in bark crevices or under lichens or moss on a tree. Development of the young caterpillar within the egg proceeds well at temperatures not far above freezing, but development is improved if the egg is chilled first. The eggs hatch in spring and the caterpillars feed on buds and developing leaves. They can move about from leaf to leaf, or even tree to tree by ballooning through the air on fine silk threads. In June, or thereabout, the larvae pupate in the soil, and transform into the adult form.

Birds prey on the caterpillars, and bark-searching birds probably prey on the females too. The pupae in the soil are attacked by an assortment of ground beetles and probably shrews. But the adults are active at a season when predation by most birds, bats, other insects, or spiders is probably much reduced.

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