Winter Moths - Act Now to Save Your Trees

December 3rd update: Male winter moths are seen flying after sunset. Our birch tree is trapping far more males than females. This indicates that after four years of tree banding our local infestation of females is lessened, but the pheromones the trapped females release are attracting males from a larger distance. In effect, we are doing the neighborhood a favor. It is not necessary to band every tree every year.

November 15, 2017: Winter moths, previously written up (Nov 2014 and May 2016) as an invasive species, will soon be starting to emerge from their pupal state, triggered by the first few frosts. If you start seeing large numbers of moths flying around in the evening, it’s time. And if you intend to protect your trees, act now. Garden supply stores, including Russell's Garden Center (Wayland), carry Tree Tanglefoot in 15-ounce containers. One container should be enough for a few trees. The product can also be ordered on line in that size or five-pound pails.

White birch tree with plastic wrap covered with Tree Tanglefoot. Females
get stuck at the bottom edge. Males, attracted to their pheromones, get
stuck higher up. Click on photo to enlarge.

The procedure for using this sticky stuff is to wrap the trunk of the tree two or three feet off the ground with clinging plastic wrap. Optionally, secure this with a band of duct tape on the top and bottom edges of the plastic wrap (not touching the tree). Then, open a container of the product and use a 1.0 to 1.5 inch wide spatula to smear the stuff on the plastic. Do not apply directly to tree. Wearing disposable plastic gloves while working with this stuff is a good idea, as it is hard to wash off bare hands. The wrap should be left on until late December.

Garden supply stores also sell a roll of a paper product that has the sticky stuff pre-applied. All you have to do is unroll it around the tree trunk. Negatives with this version of the product are that it's expensive, moths may be able to pass under the band if the tree bark has deep fissures, and the sticky band is so narrow that it can become completely filled with moths within days. This means you will have to remove it and repeat the process, else the next wave of females just climbs over the dead bodies of the early ones.    

Female winter moths climbing up a birch tree trunk.

The reason this works is that winter moths have an interesting dimorphism. Males have strong flight muscles, with an ability to pre-warm these muscles through shivering before cold weather flight. In contrast, females have only vestigial wings. Sacrificing flight capacity allows more than fifty percent of their adult body weight to be given over to eggs. Mating is achieved after the females climb up tree trunks and then release scent pheromones into the air. Males fly to them. Both sexes get stuck in the sticky stuff.  

Winter moths, native to northern Europe, reached Canada in the 1930s. The introduction was accidental, the problem monumental.  The "winter" part of the name refers to an evolutionary strategy used to avoid predation. Most insect eaters are active during warmer months. By not emerging until after late November frosts, there perils are avoided. This plague appeared in eastern Massachusetts around 1990 and to date has slowly spread to affect land east of Interstate I-495 and down into Cape Cod, but not farther west. Yet.

Female winter moth, with vestigial wings.

The trees to protect are birch, maple, and any type of fruit trees. Winter moth eggs hatched in early April. The tiny, tiny hatchlings climb inside beginning-to-open leaf and flower buds and nibble from the inside. By early June the full-sized, green, inch-worm-like caterpillars will descend to the ground where they will transform into pupae, not to emerge as adults until next winter.

Help is on the way. Canada successfully introduced parasitic flies and wasps from Europe. Both prey specifically on winter moth caterpillars, with eggs hatching inside, and larvae consuming the caterpillars from the inside out. The U.S. Department of Agriculture has been experimenting with this approach. The net result is a downgrade from traumatic damage to minor damage, with occasional bad years. However, until these bio-controls are introduced locally, best advice is to sticky-band your favorite trees in the fall as being much less expensive than spraying in early spring.