Only a few years ago, Winter Moths were confined – mostly – to southeastern New England; but more recently has made its way north into our back yard here on the seacoast. These meddlesome pests manifest themselves in the form of tiny, green caterpillars that tunnel into the buds of a variety of trees – from oaks, maples and basswood to elms, ash, mountain ashes and fruit trees. Even the largest trees can fall victim to the Winter Moth quickly – and its lasting effects on its targets can be immense.
Especially in dryer springs, their presence can last even longer and continued infestation can completely kill almost any tree. On average, Winter Moths can devour a healthy tree in as little time as three years. Any plant where Winter Moths are detected needs to be treated immediately. Even catching infestation a little bit late is too late. Simply put, the window to treat Winter Moths is small and action needs to be taken as soon as possible.
What to do
The best course of Winter Moth treatment is to use pesticides – particularly pesticides with a bacterium called Bacillus Thuringienesis of ‘Bt’ for short. When a tree is sprayed with this kind of pesticide, the caterpillars will die once they attempt to consume the leaves. It’s particularly effective because Winter Moths feed the most in early stage development – preventing them from fully growing and causing lasting damage.
The only issue with this sort of pesticide is that it’s not the sort of treatment you can simply apply and leave it be. It only lasts 3 to 5 days, which means if you’re trying to protect a certain plant, a shot or two of protection might not be enough to get the job done. In windier areas like we have along the New Hampshire coast, caterpillars can get blown from tree to tree, so multiple applications will be necessary over the course of an entire season.
A more natural, biological treatment is a particular fly that has historically limited the spread of moths. These flies simply lay their eggs on tree leaves and when the Winter Moth eats them, the eggs gestate and hatch inside the caterpillar – eating them from the inside. This treatment has had wide spread success in Europe and is just beginning to come into vogue in the United States. The flip side to this form of treatment is that it can take many years for them to take effect – years that many populations of indigenous trees may not have.
Regardless of what course of action you choose to take, your immediate response needs to involve a professional arborist who can make an accurate read and diagnosis.