Winter moths. Step one: It's the dead of winter but there is something you can do about them.

Here in Massachusetts our trees have been under assault from the winter moth caterpiller, which hatch in the early spring and start doing their damage before we even get the chance to enjoy the new spring foliage.

The winter moth, apply named for their habit of going on the wing and breeding in the early winter months, are an increasing problem, especially in southeastern Massachusetts and Norhern Rhode Island. Each year they march further south. and their number seem to be growing.

Each year I take a rough census of the moths that I see on the wing and congregating on trees and around porch lights, and this winter there were more than ever. This year, as last winter, there were early snows and single digit temps by mid December and the ground has had some snow cover since then, bringing their breeding season to an early end.

Last winters early snowstorm and long lasting snowcover, well into January, had little impact on the population. I expected the spring infestation to be milder than recent years due to the short breeding season, but last spring was as bad as ever.

Apparently, once the moths appear in mid November, they get to work very quickly breeding and laying eggs. The legnth of the winter moth season doesn't seem to have a significant impact on the populations because the most of the eggs are laid shortly after the moths appear. While I have observed male moths on the wing as late as late January,(and even into February one particularly mild winter a few years ago) there doesn't seem to be alot of congregating of females on the trees after mid December. The wingless females are less able to seek out shelter from the cold nights and are killed off earlier than the males who can find shelter and survive longer. The males just linger around a bit longer til the bitter cold penetrates deep into the leaves and debris on the ground where they hide.

But now, it's mid January, it's been cold and snowy enough to kill them all off so now is the time to take the first step against them.. That would be an application of dormant oil on the trunks and limbs of the trees.

Dormant oil coats the eggs and sufficates them. Dormant oil is the heaviest of the horticultural oils and is too heavy to apply to the trees during the growing season, but it is the most effective oil for killing eggs. Dormant oil is best applied during the dormant season, after leaves have dropped but before the buds begin to swell in the late winter, early spring. For most effectiveness, it should be applied as soon as possible after there has been a killing chill, to make certain that the egg laying season is done with. But it cannot be applied when it is below freezing. It should be applied when the temps are above freezing during the day and will stay above freezing for a night or two. The surface of the trees should be sufficiently warm enough that the oil solution will not freeze upon contact with the bark. The solution needs a day or two to dry thoroughly before temps go below freezing. The ideal temp range is above freezing but below fifty degrees.

You'll need to spray all the way to the top of the tree and the tips of the stems. While the moths may breed and lay eggs on the trunks, the females may climb further up the tree and lay their eggs on the limbs and stems. If you can't reach the very ends, spray as far as you can, you'll at least have some impact on their numbers.

I recommend two applications, one at the first opportunity after a killing chill, and a second later in the winter but before bud swell. In late winter,early spring, when temps begin to moderate and buds start to swell, I would switch to a lighter oil.  An  all season oil, or Neem oil would do nicely and can be used up until the leaves pop, at which point you'll move onto the next step.

Because the moths will lay eggs in protected places  where the oil cannot reach them, like in crevises in the bark or under lichen, horticultural oils may not be 100% effective but will have some impact in reducing the numbers of caterpillers that hatch.

Many of the spring flowering trees will blossom before the leaves come out and I don't like to spray oils on the blossoms. Dormant oils can clog the pores on leaves and cause scorching of the leaves in bright sunlight and warm temps. As the name implies, it is intended to be used when the plant is dormant, during the active growing season only use oils intended for use during the growing season.

For a dormant oil, I recommend Volk Oil by Ortho. You might not be able to find insecticides at box stores this time of year, but it should be in stock at your local garden center. Volke is a synthetic oil, but is not a toxin. If you want something more organic, then Neem oil would be a good alternative. It's stickier and thicker than other all season oils. The third alternative is Bonides All Season Oil, a synthetic, it's a lighter grade of oil. than dormant oils.
So that's what can be done during the dead of winter, but there are additional steps that can be taken in early spring to prempt the caterpillers before they can do too much damage.