Last month, the Eco-Pros offered advice on controlling winter moth. An alert reader in Wellesley, MA brought to our attention an outreach effort in that town advising against spraying. Our Eco-Pros respond.
A response to last month’s Eco-Answers from the Pros about managing winter moth referenced a letter from the Wellesley (MA) Department of Public Works which recommended foregoing spraying for winter moth larvae in the spring within Wellesley, where the parasitic fly Cyznis albicans had been released in 2008. Since the release of the fly, the numbers of winter moth have dropped considerably in an around the release site. This is an encouraging development, especially if releases of the fly might be as successful in other locales.
The reason for foregoing treatment in Wellesley wasn’t given in the letter. It could be that treatment for winter moth might also somewhat affect the growing population of predator flies. Or, it may simply not be necessary to spray insecticides to ensure reasonable tree health as the predatory flies are reducing the winter moth numbers enough so that the damage to trees is dropping below an acceptable threshold.
A couple of interesting aspects of the biology involved: C. albicans is active relatively early in the season, with egg laying occurring just after winter-moth egg hatch. The fly lays eggs on newly emerged leaves, and the eggs are ingested by the young winter moth larvae as they chomp their way through your landscape plants. When the fly eggs eventually hatch within the winter moth pupae, they feed on and eventually destroy the pupae.
Also, the fly is not the only predator of winter moth. Other soil-dwelling organisms also prey on winter moth, but, alone, their numbers aren’t sufficient to keep up with the huge numbers. C. albicans reduces the winter moth population to the point where the rest of the foodweb can contribute to effective control.
Most landscape trees can recover from some amount of damage from winter moth. Minor to moderate damage – or even severe damage if it is not chronic – is usually not life threatening to most trees. Extensive damage over a number of years can result in more severe and lasting impacts to trees.
But, for trees and shrubs which are managed for harvestable fruit – apples, pears, blueberries, etc. – treating for winter moth is essential, even in locales in which C. albicans may have been released. Newly hatched winter-moth larvae can wriggle between the scales of swelling flower buds early in the season before any leaves have emerged. Larvae can tunnel through and destroy the buds, thus reducing – or in some cases, eliminating – fruit set for the season. In most years, an appropriate insecticide needs to be in place as larvae are emerging to protect the season’s crop, and treatments need to continue for a number of weeks.
Dormant-oil sprays for winter moth will have no effect on C. albicans, but oil, by itself, generally won’t provide acceptable control of winter moth for fruit-bearing plants. Given that C. albicans eggs are not yet available to the earlier instars of winter moth, applications of an appropriate insecticide (such as spinosad or Bt) just after winter-moth egg hatch will also have no effect on the flies.
Nick Novick, Small Planet Landscaping, Ashland, MA
To read the original responses to the winter moth question, go to: https://www.ecolandscaping.org/11/pests-pest-management/eco-answers-from-the-pros-winter-moth/