Why Do Moths Swarm at Night in Cold Weather?

Updated on November 12, 2017
Bob Bamberg profile image

Bob has been in the pet supply business and writing about pets, livestock and wildlife in a career that spans three decades.

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Winter moths (Operophtera brumata) are native to temperate regions of Europe, the Near East, and the U.S.

The adults are only active for a brief period during the early winter, but their life's journey is an interesting one. The adults don't do any damage, by the way, but their progeny sure do. The damage to trees and shrubs occurs in the spring, after their eggs hatch.

Originating in Europe, they showed up in Nova Scotia sometime prior to 1950. The moth was introduced separately to Western Canada around 1970 and is now also found in Washington State and Oregon. Winter moth larvae in these states can have a negative impact on the blueberry crop as well as deciduous trees and perennials.

Descendants of the original "pilgrim" winter moths that arrived in Nova Scotia found their way to my little corner of the world, Southeastern Massachusetts, in 2003. For some reason, we were the first in the Northeast United States to get hit and remain the most infested.

The winter moth is now established throughout Rhode Island and has been picked up in traps in southeastern NH, coastal Maine, southeastern CT, and out on Long Island, New York.

So here it is, late Fall and into Winter, and swarms of moths are dive-bombing your headlights, festooning the side of your house, and plastered all over your lit doors and windows. Some are crawling...those are the girls...while the guys chaotically flutter around, drunk on the pheromones the girls are releasing.

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It Lives Unmolested By Predators

When you put a little thought into it, you'll realize that the winter moth really is a marvel among nature's many wonders.

Perhaps the greatest of their natural adaptations is that they emerge from their cocoon, which is buried in the soil, from late November through early January, which is a cold weather period in temperate zones.

They have no natural enemies lurking about because they're only active during periods of darkness, when birds that would prey upon them are roosting for the night.

And bats, which would have had a field day with them, have long since returned to the caves and other venues in which they hibernate.

Of course, any insects that would have preyed upon them aren't around this late in the season, so they're one of the few insects that can truly relax and enjoy life.

But, they don't have any time for that. In fact, they don’t even have time to pause for a meal...and it goes downhill from there.

Living On Borrowed Time

The moths you see flying around are the males. The females have tiny little wings that don’t enable flight; so you’ll usually find them legging it up tree trunks or the sides of your house, but they can be anywhere.

The females emit a strong pheromone that attracts hoards of males. Lights attract hoards of males, too, which results in mass suicides as the hapless males dive into headlights of oncoming vehicles.

But those that can resist the lights will follow their noses to receptive females and mating will occur. Assured that his DNA will continue on, the male then dies.

After entertaining her male suitor, the female deposits eggs loosely on bark, in bark crevices, under bark scales, on buds, on lichen, or elsewhere.

Then, with her only mission in life fulfilled, she, too, passes on.

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They Do Their Damage In Springtime

The eggs over-winter somewhat safely and hatch from late March through early April, just before bud break on the trees and plants that host them.

What hatches is the larva, a tiny green inchworm that crawls inside, and feeds upon, the bud.

This severely impacts fruits such as apples and blueberries, and destroys leaves before they've had a chance to develop.

The tiny larvae grow, and as unmolested buds burst into tiny leaves, they feed upon them. This often results in the defoliation of the tree or bush.

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A healthy tree can survive a defoliation and will produce another crop of leaves, but the tree is weakened somewhat.

In what amounts to probably the only fun these little guys get to have during their short time in the sun, the green larvae can be seen suspended in mid-air, hanging from silken threads.

Breezes transport them towards the canopy of the tree, and a fresh food supply, in what’s known as “ballooning.”

They often drop to perennials on the ground and feed upon them; and in certain situations where topography and winds cooperate, can balloon into areas that hadn’t seen them before and aren’t expecting them.

Those folks are in for a big surprise in about six months.

They Spend Their Summers Pupating Underground

Mother Nature makes an adjustment to their biological timer and the larvae cease feeding by late May, dropping to the ground and burrowing in to pupate.

They then spin the cocoon that will entomb them until early winter, thus beginning their metamorphosis into the adult winter moth you'll curse in about six months.

Winter moth cocoon (pupa)
Winter moth cocoon (pupa) | Source

All of a sudden, at dusk some night around Thanksgiving, the vanguard of the winter moth orgy-goers will emerge and the reproductive process will begin anew.

They'll have a brief window of opportunity, just a few days, to sow the seeds of next spring’s crop of winter moth larvae.

How To Control The Moth's Cold-Weather Life Stages

There are biological and chemical controls that can be used in the fall and spring against the winter moths and their larvae.

In late November and December, after the eggs have been deposited on trees, you can apply a dormant oil or all-season oil spray to the trunk and branches, as high up as possible.

The products are popular with homeowners because they're not insecticides, but highly refined petroleum that smothers the eggs and destroys them.

Dormant oil or all season oil are generally available in ready-to-use sprays or liquid concentrates which you mix with water.

The mixture can then be applied with a pump sprayer or a hose-end sprayer, which can deliver the oil to a height of 20 feet or so.

How To Control The Moth's Warm-Weather Life Stages

When the larvae emerge in the spring, they can be controlled by many of the insecticides that are presently on the market.

For those who are not a fan of chemical pesticides, there are a couple of natural products you can use instead.

One is BT (Bacillus Thuringiensis [kurstaki]), a biological agent that preys upon the larval form of certain insects, including the winter moth larvae.

It is harmless to all other life forms and is available from various manufacturers as ready-to-use or concentrates.

Spinosad...pronounced spin-õ-sid...is defined by Bonide as a biorational compound. That being a new term to me, I looked it up.

There is no legal or regulatory definition for the word, but the U.S. Environmental Protection Agency (EPA) considers biorational pesticides to have different modes of action than traditional pesticides.

They generally offer greater selectivity and considerably lower risks to humans, wildlife and the environment.

Since Spinosad is cleared for organic gardening, it would also make a good choice for folks who are concerned about using conventional pesticides.

Both of these solutions should be applied when the larvae are still small but actively feeding.

Questions & Answers

    © 2012 Bob Bamberg

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      • Highland Terrier profile image

        Highland Terrier 

        6 years ago from Dublin, Ireland

        Your very practical about nature which given its the only game in town is a very good thing.

        Thank you.

      • Bob Bamberg profile imageAUTHOR

        Bob Bamberg 

        6 years ago from Southeastern Massachusetts

        Hello Highland Terrier, nice to see you again! Yes, nature can be both of those which, I think, comes as a surprise to most people, who think of nature in terms of rainbows, butterflies and snowflakes.

        But, everything in nature has its purpose and place. The winter moth's eggs provide some winter food for overwintering birds.

        When I was a kid, robins used to migrate (or were thought to) but actually large flocks of them spend their winters in the deep woods, searching tree bark for the eggs and pupae of insects who deposit them there.

        That's one of the reasons why insects lay prodigious volumes of eggs...because a lot of them don't survive their sub adult stages.

        Nature's design prevents the moths from being a drain on resources because they die within a few days, without having to feed, and their carcasses provide food for birds that feed upon them, cleaning up the landscape to boot.

        And, I don't think nature's design took into account civilization, with it's "common sense," sensitivities, nor its manicured lawns and landscaped properties.

        As far as the winter moth larvae are concerned, they cull the unhealthy trees, creating housing for cavity dwelling birds and for insects, and also opening up space to sunlight and ultimately various other life forms. The larvae also are a food source for returning birds and newly emerging insects.

        But to homeowners trying to protect their trees and perennials, and to growers trying to protect their berries, they sure make nature one cruel b*****d or b**ch.

        Thank you for stopping by and commenting. Regards, Bob

      • Highland Terrier profile image

        Highland Terrier 

        6 years ago from Dublin, Ireland

        I said it before and I say it again, nature is one vicious cruel b*****d or b***ch. The poor moths not to mention the poor old trees. Really you have to wonder what is the point of it all. Where is the rhyme or reason to the poor moths existence.

        Very well written and explained, clear and understandable.

        Thank you.

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