About half of winter moth eggs in Tiverton and Little Compton have turned blue, indicating eggs have started to hatch in the cooler areas of RI. Winter moth eggs have been hatching elsewhere in RI since about March 24th. In Kingston where eggs started turning blue on March 23, most monitored eggs hatched, but eggs on the north side of the tree are still orange. The cool weather over the past week has slowed down egg development and is stretching out the time when eggs hatch. Usually all eggs hatch in about 8 days. This year it is taking longer and with predicted cold weather I’m not sure when all eggs will hatch.
Egg development can continue as long as temperatures are above 38 degrees, so even though it will be cold, winter moth eggs can continue to hatch – though cold temperatures surely slow them down. So what to do? Apple, pear and blueberry growers that have not sprayed yet might consider applying an insecticide Saturday, provided there is a period of dry weather. (Do not add oil to a spray on Saturday because below freezing temperatures are predicted within 48 hours). I know it doesn’t sound very feasible to spray on Saturday – Maybe what makes the most sense is to wait until warmer weather returns on April 7.
My egg checkers and I will check eggs again early next week and let you know if winter moth eggs have completely hatched or not. It does not help to spray after all caterpillars are safely inside buds.
For landscape trees, wait until leaves emerge to control winter moth – it’s much easier than trying to kill caterpillars before they enter the buds!
From March 4th message about insecticides:
Winter moth caterpillars are pretty easy to kill, provided they are not inside closed buds. Insecticide choices for when caterpillars start to hatch for fruit growers include, but are not limited to, spinosad, Imidan, Sevin, Malathion and synthetic pyrethroids such as Asana. Spinosad products are Delegate, mostly for commercial growers, Entrust for organic growers, and Captain Jack’s Deadbug Brew is formulated for small-scale growing. Adding a dormant oil may be useful for the first spray of any of the listed insecticides. Do not add oil when below freezing temperatures are expected within 48 hours of spraying.
Once buds are open, B.t. kurstaki products (Bacillus thuringiensis kurstaki) such as DiPel and Biobit work well. For landscape trees, winter moth caterpillars can be controlled once trees leaf out with spinosad (Conserve), B.t. kurstaki (Dipel Pro, Javelin, and others), as well as synthetic pyrethroids such as bifenthrin. B.t. products are a good choice because they kill only caterpillars, but B.t. products break down in sunlight after 3-5 days so may need to be reapplied more frequently.