Winter Moth Diagnostic Guide
Winter moth, Operophtera brumata, is a relatively unique species of Lepidopteran. The adult stage is active during the winter making it very noticeable. The larvae are voracious and are capable of destroying all buds on an entire tree. Successive years of heavy damage can kill trees and has been responsible for 40% mortality in some stands of red oaks in Nova Scotia. Larvae disperse themselves with long silken threads used to move with the wind.
Trees at Risk
Many species are affected by winter moth. Oaks, maples, crabapple, flowering plum, cherry, flowering pear, hawthorne, serviceberry and apple are preferred, but there are many others. In addition, winter moth will attack numerous shrub species including blueberry and deciduous azaleas.
Signs of Damage
- Thin canopy due to larvae feeding on buds.
- Partial leaf emergence.
- Complete defoliation if warm temperatures are delayed in spring.
- Branch death after successive years of infestation.
- Tree death if complete defoliation occurs in successive years, especially when coupled with abiotic stress.
- Leaves can be webbed together or folded over by the early instar caterpillars.
- Brown or tan male moths can be seen from November to January.
- Female is dark and wingless and rather immobile.
- Green caterpillar-like larvae are about 1” long when fully developed with white stripes down their sides and back.
- Small green eggs that turn orange or pink in late fall and early winter, these are very difficult to find.
- Moths (the adult stage) emerge from the soil usually in late November and can be active into January.
- After mating, the female deposits an egg cluster on tree trunks and branches, in bark crevices, under bark scales, under loose lichen, or elsewhere.
- Eggs hatch when temperatures are near 55 degrees.
- Larvae emerge and spin long silken strands which allow them to move with the wind and travel large distances. This is called “ballooning.”
- After landing on a host they feed until June and then fall to the ground to pupate until emerging again in November.
Although natural controls have been attempted, they have only provided marginal control on a region wide scale and should not be relied on to protect individual high value trees. Dormant oil applications are tricky because eggs are often laid in hard to reach bark crevices. Control is further hampered because larvae disperse themselves with long silken threads, which move them to previously controlled areas. Insecticide sprays can be used to treat for the caterpillar stage beginning at bud break in early spring. Do not use certain insecticides while the trees are in flower to avoid spraying bees that may be visiting these trees. Acelepryn™ or systemic treatments should be considered. Preliminary research has shown that systemic root flare injections of emamectin benzoate and systemic soil injections of Lepitect™ can provide acceptable levels of control.
Other Treatment Practices
- Emamectin benzoate by tree injection is a good option. Treat annually, but research is being done to test for multi-year control.
- Bt has been shown to be effective, with little environmental impact.
Winter moth can be difficult to control for three reasons:
- Eggs are laid in hard to reach areas such as bark crevices and under lichens. Therefore a dormant spray of horticultural oil will not fully control this insect.
- The larvae spin silky strands that allow them to disperse themselves and move between hosts. This means that even if a tree is treated with an insecticide, it could still potentially be infested.
- With larvae feeding in the actively growing buds, achieving adequate control with a canopy spray is challenging. Furthermore, often the host trees are prohibitively large or in areas where spraying is challenging. These situations are where systemic treatments can provide value.
A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org