A web-exclusive manual to battle one of the most destructive pests attacking hardwood trees in our area

As gardeners and lovers of nature we tend to think of winter as our ‘time off’, an almost dormant season for humans. No planting, no weeding, no watering. And yet our eyes are always on the job and observe something most of us did not notice last winter: the small beige patches splotched on the bark of trees. These are gypsy moth egg masses; look closely and suddenly you will see them everywhere.

Each of these masses can contain over 500 eggs. Some trees have dozens of them; down by the roots, on the trunks, more on the underside of branches. The caterpillars that hatch from these egg masses in late April and early May feed on tree leaves and can defoliate even large trees. This year, Cornwall experienced extensive damage to its fruit and ornamental trees and especially to the oaks and birches that turned our normally green hilltop views brown. And we got lucky; the rain that followed helped some of the trees grow new leaves.

But other trees, such as hemlock, white pines, spruce, aren’t programmed for re-growth once defoliated. “There was a stand of maybe 100 hemlock where Cream Hill meets Music Mountain Road and they were killed this year by the gypsy moths” arborist Skip Kosciusko relayed. “The Housatonic basin is the worst. I’ve never seen such a density of egg masses – scary to think what will happen to the tree canopy along the river.” According to Skip, a tree that suffers from defoliation two years in a row is very likely to die.

Winter is an excellent time to begin an offensive against gypsy moths and not difficult to accomplish. But it does require us to snap out of our winter resting state and back into action! Start now by carefully scraping egg masses off of the tree bark and into a can or zip lock bag. A small putty knife will work well to do this as will an ordinary table knife.  It is important not to leave the eggs on the ground as they will still hatch and crawl up nearby trees.  Once collected, the eggs should then be left in soapy water for a day or two before disposing. You can take a small knife and zip lock with you on hikes; saving trees as you walk.

Eggs can also be burned off using a propane or butane burner and a flat stick that can be used to flatten the masses as you burn, allowing the flame to reach the interior eggs. This technique is best done when snow is on the ground to prevent forest fire. For this method search YouTube for “destroy and kill gypsy moths”.

March – April:  Before they hatch, you can continue removing egg masses and also start an easy-to-do banding method with either duct tape or burlap that will work to capture the caterpillars that hatch from the out of reach eggs. Best again to search within YouTube using the following keywords to find some of the many tutorials: “tree wrapping” along with “gypsy moths” and “Burlap banding.”

Horticultural oil can be used as a topical spray when temperatures are above 40 degrees. Some arborists began this remedy in the Fall; the oil penetrates the masses and kills the eggs. The oil is safe for trees and animals including humans.

A less appealing method uses a product called Tempo which can work as a chemical barrier at the bottom of the trees but it is not universally recommended as it will kill any insect – good or bad – that comes into contact with it.

April – May:  Gypsy moth eggs usually hatch between late April and mid-May and their caterpillar stage lasts about 7 weeks. Once the caterpillars hatch there are several tactics that can be used simultaneously. The most obvious is to pluck the caterpillars off the leaves and branches as soon as you can and put them into soapy water to kill them. Wear gloves as the caterpillars can cause a skin reaction for some people. If you installed the burlap traps you can collect numerous caterpillars each day – don’t forget to bring a can or bowl with soapy water to collect them. It seems that when the caterpillars are young they will come down the trunk in the early afternoon to escape predators so collect them from noon to 6pm. When they are older – in late June – they will come down the tree at dawn and go up at dusk and feed on the leaves at night. An early morning check will yield the most caterpillars.

In early May, trees that need extra care (perhaps because they were defoliated the prior year and are especially vulnerable) could be helped by a chemical option applied by a professional arborist. Fred Scoville injects trees with a chemical solution that is absorbed into the leaf canopy; any insect that eats the leaf will die as a result.  The result lasts all year and will reduce populations of Japanese beetle, wooly adelgid in addition to gypsy moths. “Birds don’t eat live gypsy moth caterpillars much less dead ones” Fred asserts “but I won’t use this treatment on fruit trees as the chemical does get into the fruit.”

May or early June is also the time when you can spray tree leaves with a biological control, Bacillus thuringiensis kurstaki (Btk) which is available in bulk online at Arbico Organics. The upside of this bacteria is that it infects and kills young gypsy moth caterpillars that eat it but not the birds that eat these caterpillars. The downside is that it can also harm other butterfly and moth caterpillers, the vast majority of which are either harmless or beneficial. So use with care!

The goal of all of the actions outlined is to reduce the number of caterpillars before they cocoon and turn into moths, continuing the cycle of laying eggs and proliferating. If each of us takes action starting now, even if that is just in our yards and property, we will certainly see the benefit in keeping our trees alive.

Dee Salomon

📷: CT DEEP  (1 & 2) and Cara Weigold (3 & 4)

A young Gypsy Moth caterpillar in late May 2021

Gypsy Moth caterpillar damage in mid-June 2021