The winter moth, also known as Operophtera brumata, poses one of the biggest threats to your fruit trees and shrubs, including crab apples, apples, blueberries, and cherries — not to mention other deciduous trees like oak and maple. Winter moths not only make homes inside these trees during the winter, but they also munch on young leaves and buds in the spring and can severely impact your next harvest. Even nearby perennials may be at risk, as these pests have been known to fall down from the trees and snack on ground plants as well.
Although the winter moth is most common on the east coast of the United States, it’s made its way west and can now be found on both sides of the country. So how can you prevent these pesky insects from feasting on your trees this year?
Understanding the Pest Process
In order to tackle this problem, you have to understand the winter moth's life cycle. The process begins when adult moths mate and lay eggs in the late autumn, typically around Thanksgiving. The moths, which display tan and white coloring on their wings, can lay up to 150 eggs at a time, often around the stems, under the bark, and within any cracks and crevices of the host tree or shrub.
While the adult moths die after reproducing, their eggs remain on the stems until the spring, when they hatch and the larvae begin to feed. These larvae, which resemble small green inch worms, are difficult to see with the naked eye and hatch at about 55 degrees Fahrenheit. To feed, they must tunnel into the emerging buds of your trees and shrubs. Much like the popular children’s book about a completely famished larva, a "very hungry caterpillar" will eat the entire bud of the plant and continue to feast until it’s had its fill. Then, the larvae will fall into the soil, pupate, and go through the process of becoming moths. Once they emerge from their cocoons, they reproduce and die.
Since the trees need these buds to flower and eventually bear fruit, an infestation of these caterpillars can kill your entire crop and can severely weaken the host tree. If left untreated, the tree can even die.
Treating an Infestation
Unfortunately, since the larvae are so small, they’re often difficult to see on the leaves. In most cases, you may notice the damage before you notice the actual insects — and by then, it might already be too late. In early to mid-April, you'll want to take the time to inspect the leaves of your deciduous trees and shrubs and fruit trees for any tiny green visitors.
Although many trees will be able to grow a new set of leaves, the extra work required to do so is extremely taxing on them. Plus, infestations can damage entire branches and even kill trees, especially if it's coming on the heels of other environmental stresses like drought or unusual temperature fluctuations. As with anything, the key to saving your trees and eliminating these pests is to treat the problem as early as you can.
In the Fall and Winter
Prevent adult female winter moths from laying eggs in your trees and shrubs by wrapping the trunk in a pest barrier like a sticky band. If the moths aren’t able to get up the tree, they won’t be able to lay their eggs in the branches. However, they may still attempt to lay their eggs below the bands. If this is the case, you'll want to apply horticultural oil below the band to take care of the eggs. Apply the same oil to the tree's branches to take care of the eggs laid by the moths that were able to make it up the tree. Typically, horticultural oil is used once temperatures reach about 45 degrees Fahrenheit.
In the Early Spring
If you're unable to suffocate the eggs early on and they end up hatching, apply Bacillus thuringiensis (BT) to the leaves to kill the caterpillars as they feast (typically in the mid to late spring). You can also use neem oil or spinosad, which attacks the pests' nervous systems.
Once the caterpillars have finished dining on your trees and shrubs, they form cocoons and go through the process of becoming moths. At this point, they’re no longer a direct threat to the tree, and there’s not much you can do to get rid of them, as the damage will have already been done. However, if you’ve had an infestation this year, make it a point to take proactive steps toward avoiding another one in coming years. Winter moths will emerge in the late fall, so make sure that your trees are wrapped and you have horticultural oil on hand to smother any eggs that might pop up.
What If You Have a Large Infestation on Your Hands?
Large infestations require professional help. In these instances, an arborist is your best bet at ridding the tree of winter moth larvae and saving it from severe damage. Arborists have the knowledge and tools required to take care of these problems and ensure the health of the tree.