Winter Moths

by Jim Wilson

The many moths coming to the window screens on warm spring and summer nights have always been a source of interest to me. After many years I finally purchased Peterson’s field guide to the Eastern Moths, then spent many enjoyable, if frustrating, hours trying to identify as many species as possible.

For some time I had mentioned my interest in late-winter-flying moths to a few of my naturalist friends; then, to my surprise, Ron Tozer of Algonquin Park, mentioned he had read an article on that very subject in Natural History (2/94 ) written by a Berndt Heinrich. After digesting this informative article, I decided to write something, along with some of my personal observations, that would relate to our Muskoka-Haliburton area.

I discovered there are more than fifty species of North American moths that are active throughout the winter months. Those that I had identified are Noctuid or Owlet moths; one, at the Brown’s Maple Syrup operation in Huntsville, Ontario, where many individuals of Three-Spotted Sallow ( Eupsilia tristigmata) sought the sweetness from the sap pails suspended on the trees. Sallows are dull-yellowish to brown; (“sallow “ is an old-fashioned adjective we used to describe an unhealthy, skin colouring). Another tan and brown,winter moth, the Wanton Pinion ( Lithophane petulca)has come to our house as late in the year as May 5th (1992).

These moths emerge, totally developed within the pupae, after spending the winter months under layers of leaves and snow. In March and April when bare patches of ground appear and about the time the woodcock does his mating dance , these insects can be observed by painting nearby tree trunks with a brushful or two of a paste containing ripe fruit, molasses , sugar and a can of beer. Indeed, maple syrup is an excellent attractor to be used in place of sugar! In this manner they can be observed closely in order to see their soft, subtle colours as they gorge themselves. Snowfall may appear a few times after this, however by late April- early May with the bats and spring bird migrants back, winter moth activity approaches its end.There is no coincidence then, that the arrival of birds and bats and the disappearance of the winter moths occurs at this time.

Winter moths spend most of their time in a torpor. When they do warm up and fly, they lose energy quickly. During the warm periods of our long winter the sweet sap from birches or maple wounds created by squirrels provides their necessary high energy food.

Winter moths lay eggs before tree buds open permitting their larvae to feed on the very first new leaves.Warblers, just returned, eat some of these eggs and larvae, but others continue their development-cycle finishing the pupal stage before all birds return. Now, the pupae drop to the ground, bury themselves, and go into their long summer sleep (estivation) until the following late winter arrives.

Early in their evolutionary history moths escaped birds by becoming nocturnal. In Eocine times, ( 45,000,000 years ago), bats evolved so that moths became vulnerable again at night. The late Kenneth Roeder of Tufts University conducted experiments showing some moths, in turn, have evolved ear structures that let them hear bats’ sonar usually in time to evade the bat.

Some moths are cryptically coloured to blend into bark or other perch sites in order to look like leaves, sticks, or bird droppings. Birds hunt by sight and may fail to detect a resting moth. Not too long ago Alan Kamil, of the University of Nebraska, demonstrated blue jays can detect even the most well-camouflaged moths.

Other moths have developed different defences for their protection. Underwings startle predators if touched or disturbed while at rest.This gives a second chance of escape by dropping out of sight to the ground. I find Underwings are easy to identify on screens; the colourful hindwing is covered by the forewing if you were observing from outside of the house. Fortunately, the human observer inside, and away from mosquitos,the colourful hindwing pattern is clearly visible through the screen or windowglass.

Eye patterns of large moth wings are for frightening predators, while many day-flying tiger moths are brightly colourful to advertise they are poisonous.

Escaping predation by adapting to extremes in temperature is a method used by many animals. This can happen during the extreme heat of an Arizonan desert, while , at the opposite extreme, w inter moths escape predators when those same predators are hibernating or away in the distant American tropics. Their problems consist of obtaining sufficient food for the adults in winter, food for the larvae before the predators return , and“the cold”, which is their greatest challenge. Adult moths have to be able to become active at a few minutes notice on a wintery day.They fly best on a “warm” spring day where nighttime temperatures are above freezing. “Shivering” their wings on a cool day, while standing still, will raise the temperature sufficiently inside the thorax for the muscles to perform flight.

To keep the thorax temperature high for the proper function of the muscles of flight, moths have developed dense “fur” made of greatly elongated scales similar to the wing scales of all Lepidoptera. The winter moths keep the tympanic air sacs used by their ancestors for detecting bat sonar, uncertain if they still function as originally intended; nevertheless these air sacs thermally insulate the thorax from the abdomen, thus conserving necessary heat.

The circulatory system of winter moths has evolved to reduce heat leaving the thorax muscles from going to the head or the abdomen. Blood leaving gives up its heat to blood entering, so winter moths don’t use the abdomen as a radiator as summer moths do.The thoracic temperature of the winter moth is surprisingly similar to that of summer ( 86 -95 degrees F)

Some winter moths are brightly coloured. One found on our screens on July 11/91, The Herald (Scoliopteryx libatrix) , has bright orange patches on the forewings.The moths predilection for cold weather activity seems to have evolved fairly recently because the colouration of some species still carries the imprint of a long history of bird predation. These camouflage colours are unnecessary to winter moths as they spend their winter hiding in the relative warmth of leaves, not out on exposed surfaces where they would be vulnerable to attack. As well, the temperature under snow-covered leaves is well above freezing.

I hope you will enjoy, as I will, seeing your first winter moth frolicing on the warm breezes of one of our first spring-like days!