What a difficult spring it has been! The weather and winter moth have not been behaving according to plan. The more normal weather forecasted for the next several days will expand fruit buds and most likely make it possible to reach winter moth caterpillars that are now inside buds or maybe still hatching. The cold conditions really slowed down egg hatch making everything very confusing. And then there has been a lot of rain, and a lot of snow!
Once blueberry and apple buds are at the tight cluster bud stage (see attached photos), it is possible to reach caterpillars with an insecticide. Since caterpillars are now feeding you can use a Bt product such as DiPel which kills only caterpillars. You can also use a spinosad (Entrust, Captain Jack’s Deadbug Brew), Imidan (commercial growers only), Malathion, Sevin, and other insecticides.
For landscape trees it is still too early to spray foliage since trees haven’t budded out yet.