~ By Heather Faubert, URI Cooperative Extension ~
Useful download: Winter Moth Control Measures
When winter moth eggs hatch depends on temperature and other factors. I don’t expect eggs to hatch until after red maples start blooming and when McIntosh apple buds begin to crack open and expose a little bit of green tissue
Winter moth eggs are orange now, but turn blue a couple of days before hatching.
For landscape trees it’s not important to control winter moth when eggs start hatching, but for apple, pear, and blueberry growers it’s very important. Once eggs hatch, winter moth caterpillars wriggle into swollen buds and begin feeding. For apple & pear trees and blueberry bushes, swollen buds are primarily flower buds. Caterpillars crawl inside flower buds and begin feeding. Once caterpillars are inside buds they are protected from insecticide sprays until close to bloom and by this time many flowers may have been damaged or destroyed, destroying the crop. Landscape trees, on the other hand, can withstand early winter moth feeding damage. To save landscape trees from being defoliated, insecticides can be applied after trees leaf-out, but while caterpillars are still small and before excessive feeding damage has occurred.
Dormant oil can be applied before eggs hatch, but this may not be very effective if unsprayed trees are nearby or if you cannot get complete coverage with an oil spray. Winter moth eggs are often located in bark nooks and crannies, so complete oil coverage is very difficult. When applying oil, temperature must be above freezing and remain above freezing for 24 hours after application or plant damage can occur.
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 product names are Delegate (for commercial growers), Entrust (for organic growers), and Captain Jack’s Deadbug Brew (for backyard growers). Adding a dormant oil may be useful for the first spray of any of the listed insecticides.
Once buds 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.
There is a lot of interest in gypsy moths this year, but gypsy moth eggs won’t hatch until late April or early May. This time of year gypsy moth egg masses can be found and scraped off trees. This may be beneficial in some situations, but not many.
April 3, 2017 – Update
For those of you planning to spray a dormant oil to help control winter moth eggs, this week is a good week to apply a 2-3% dormant oil solution. Dormant oil can suffocate overwinter eggs and is most effect when applied close to when eggs hatch. Dormant oil should not be applied if temperatures are expected to go below 40 degrees for 48 hours after application.
Those of you wanting to protect apples, pears, and blueberries from winter moth caterpillars should plan on applying an insecticide soon – once winter moths start hatching in your area. Since winter moth caterpillars do not feed as they crawl into buds, a contact insecticide is needed. Unfortunately, a Bt insecticide such as DiPel or Biobit is not effective against newly hatched caterpillars since Bt must be ingested to be effective. Insecticide choices to control hatching caterpillars for fruit growers include spinosad, Imidan, and malathion. Spinosad product are Delegate (for commercial growers), Entrust (for organic growers), and Captain Jack’s Deadbug Brew (for backyard growers). Adding a dormant oil may increase effectiveness of these insecticides.
It depends on the weather whether or not one insecticide application can sufficiently control winter moth caterpillars. Rain reduces effectiveness of insecticides and cool temperatures extend how many days it takes for all overwintering winter moth eggs to hatch. My team of egg monitors will tell me when eggs start to turn blue, when they start to hatch, and when eggs have completed hatching. Eggs are being monitored in Cumberland, Smithfield, North Scituate, Warwick, North Kingstown, South Kingstown, Charlestown, Jamestown, Tiverton, Little Compton, and Westerly, RI, Franklin, MA, Hanson, MA, and Pawcatuck. CT.
Once winter moth caterpillars are inside buds they are protected from insecticides, so to protect this year’s fruit crop it is important to apply an insecticide to fruit buds when eggs start hatching.
Once buds open, B.t. kurstaki products (Bacillus thuringiensis kurstaki) such as DiPel and Biobit work well controlling winter moth caterpillars. For landscape trees, caterpillars can be controlled with B.t. kurstaki (Dipel Pro, Javelin, and others) or spinosad (Conserve) after leaves emerge. 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. Most other insecticides will kill bees in addition to winter moth caterpillars so do not apply insecticides to blooming plants or near blooming plants.
April 27, 2017 – Update
Just a quick update on winter moth. It’s a good time to check your blueberries and see whether or not you find winter moth caterpillars. Look on the top of blueberry buds for insect frass (droppings). Consider spraying Bt or a spinosad product (Captain Jack’s Deadbug Brew, Delegate, or Conserve) if you find much damage. Don’t use spinosad if buds are close to opening. Bt is safe to use on open flowers since it does not harm bees.
Gypsy moth egg masses started hatching on April 27th. Eggs were still hatching as of May 3rd. I expect it takes at least a week for all eggs to hatch. Once eggs hatch, caterpillars may remain on the mass several days before dispersing. Gypsy moth caterpillars disperse by ballooning – sending out a silken thread and then carried by a breeze. It may take several days for caterpillars to find a suitable host. Small caterpillars will feed on oaks, aspen, apple, blueberry, speckled alder, basswood, gray and river birch, and willow. Less desired but still attacked are maple, black, yellow, and paper birch, cherry, cottonwood, elm, black gum, hickory, hornbeam, larch and sassafras. This list of plants is from the excellent, new, RI DEM gypsy moth website at http://dem.ri.gov/programs/forestry/gypsy-moths/
Once caterpillars are over an inch long they can feed on pines, spruce, and hemlock.
URI Cooperative Extension