Winter moth caterpillars continue to feed and grow. Right now in Kingston, RI, most caterpillars are 2nd and 3rd instars. When winter moths first hatch they are 1st instars, then they molt and become 2nd instars… Once winter moths are 5th instars they continue feeding to complete development and then drop to the soil to pupate. Winter moths caterpillars usually drop to the ground in late May. Fourth and 5th instar caterpillars are the largest, eat the most, and cause the most damage to foliage.
When looking for caterpillars, look for holes in leaves and then peel apart leaves stuck together. Often you’ll find a small caterpillar tucked inside. In apple trees, caterpillars may be in curled leaves or feeding on or in flower buds. It’s harder to see and certainly harder to photograph winter moth caterpillars in blueberries. Attached is a photo of a very infested blueberry bud. Dark frass (insect poop) and silk can often be seen from the outside of winter moth-infested blueberry buds. See photos.
Apple trees are starting to bloom and blueberries will be blooming soon too. During bloom the only insecticide that can safely be applied is a Bt insecticide such as Dipel. Bt insecticides kill only caterpillars and not other insects such as bees. The main problem with Bt insecticides is it lasts only 3-5 days so it will not control caterpillars that get blown onto sprayed trees more than 3-5 days after spraying.
Landscape trees that have leafed out can be sprayed with an insecticide now, but oaks and many other trees have not leafed out yet. It does not make sense to spray trees that have not leafed out yet.