Are your crabapple and maple trees looking like Swiss cheese? Tiny green winter moth caterpillars are the likely culprit.
Winter moth caterpillars feed on a wide variety of plants, but their favorites include maples (including Japanese maples), oaks, blueberries, apples, linden, ash, and horse chestnut. They also feed on roses and some perennials. If you have had damage on trees in recent years, it’s important to treat early to prevent weakening the trees. Another benefit of early treatment is that eggs can be treated with dormant oil spray, which is a less-toxic alternative to some of the insecticides used to treat caterpillars once they have hatched. Treatment can begin as soon as the temperature is above 45 degrees, and likely to stay above freezing for a couple of days.
Winter moth damage usually starts while buds are still closed, in late March-mid April. This time is particularly critical for blueberries and other fruit trees, because damaged flower buds mean no fruit. When leaves emerge, the caterpillars continue feeding, and damage to leaves continues through May. By late May, when most people notice that their maple leaves are full of holes, it is too late to treat the caterpillars, which are on their way to becoming adult moths.
If you are noticing damage in late spring, put a note in your calendar to seek treatment early next spring, before the next generation of eggs hatches out.