The Carlisle Mosquito Online

Friday, November 21, 2008


Winter Moth alert – more trouble for our trees


The Winter Moth is tan with a wingspan of about an inch. (Photo from UMass Extension)

Over the next several weeks you’ll see an increase in moth activity around outdoor lighting. One of these species will be our native Fall Cankerworm. Another visitor to your porch (and landscape) will very possibly be the invasive and devastating Winter Moth. Beyond answering your entomological curiosity, this alert will outline the limited control measures that you might consider taking as this serious pest moves into Carlisle. Since UMass and other New England extension services have several factsheets readily found on the web, I’ll spare the reader boring identification and treatment details.



Winter Moth first became established from Europe to Nova Scotia in the 1950s. Since then it has become established in the Pacific Northwest but has only recently been found in the eastern U.S. in Plymouth County, Massachusetts in 2003. Since then both the North and South Shores have been hammered with wide areas of trees severely defoliated each spring. It has been slowly moving into the western suburbs within Route 495. After repeated seasons of defoliation many trees decline and die.The Fall Cankerworm and other native caterpillars may experience boom years periodically but natural control agents eventually bring populations to a tolerable level, usually within a season or two. The Winter Moth does not appear to have any truly effective native predators.

Life cycle

The life cycle of the Winter Moth is similar to the Fall Cankerworm, and they are often found together. Both moths avoid bat predation by delaying pupation until most of our bats have either migrated or hibernated. They emerge from their pupa cases just below the soil surface in late November/December after a blast of cold weather followed by some warming.

The females crawl up trunks of host plants which include oak, maple, elm, apple, cherry and blueberry. The males fly around looking for them and partying at street lights (actually there are several theories about the disorienting effects of artificial lights on moths). The eggs are laid on host plant twigs and again the winter weather favors the survival of the eggs because general predatory insect activity has ceased. These eggs hatch around March when the temperatures average about 55 degrees.

The scientific reader might know that there is a more accurate method of predicting life-cycle events of various garden and landscape pests based on counting the accumulated number of warm days above a certain temperature threshold. This is known as Growing Degree Days or GDD, which can be used to time control measures for maximum efficacy.

In very early spring, at about 35 GDDs, the newly hatched, tiny larva will crawl out to twigs. Some will spin short lengths of silk and “balloon” up into the forest canopy. At the twigs they will burrow between the scales of recently swelled leaf and flowers buds and commence eating. The damage is worst when returning cold weather delays the opening of the buds and one larva can consume multiple buds. Expanding leaves emerge from their buds with large holes already in them. Flower buds can be completely wiped out with a huge impact on apple and blueberry yields. When the leaves have fully opened, the caterpillars “free feed,” and this is when you might see the generally green, white striped “inchworm.” Once these inchworms reach full size of about one inch long in early to mid-June, they drop to the ground and pupate within an earthen case.

Are they here in Carlisle?

Yes, but as far as I can tell, the populations are still small. The species does not migrate very quickly as the females are wingless and only move from tree to tree by ballooning on their silken threads just after hatching. Landscape activities such as bringing in large plants from nurseries in infested areas can accelerate pest movements so it’s very likely that we have several spotty populations at a minimum. Fortunately, from what I understand, Winter Moth in the western suburbs was set back considerably last year with the combined effects of the early snow and well-timed spring frosts. The adults are easier to identify than the caterpillar stages. Look for moths that are light brown with a wingspan of about one inch, and then compare with the UMass factsheet. If in doubt, please mail a couple of suspects to me along with your email address, and I’ll report back.

Control measures

There might be more publicity on this pest if there were an eradication program such as the $30 million tree removal and chipping process prescribed for the Asian Longhorned Beetle in Worcester (see Mosquito, October 31, 2008). Unfortunately we are limited to only partial control within the planted landscape as containment is just not feasible. The first choice for homeowners might be the sticky bands sold at garden stores for trunk-climbing insects. These are not generally recommended by the experts, however, because the young larvae will still balloon to banded landscape trees from the surrounding woodland.

In addition, as with the Gypsy Moth, when population levels are high, the sticky bands saturate quickly with dead insects and a new wave of invaders simply crawls across the bodies of their compatriots. Horticultural oil is effective on any eggs that are directly contacted, and since it is relatively benign and works for other pests, it might be a good defense strategy for particularly vulnerable crops such as apple and blueberry. A third option is well-timed applications of bacteria (B.t.K) or bacterium-derived sprays such as Spinosad, applied with care to protect beneficial insects.

These sprays are tricky because they work only when the insect eats them or is directly contacted in the early larval stages. During this time the Winter Moth is generally protected within the host plant buds. Although permitted by the EPA, there is a general consensus among arborists that the more toxic, systemic insecticides are not sufficiently “eco-rational” for use on this pest. One of the best things a homeowner can do with any defoliated plant is irrigation as needed and light organic fertilization.

Natural controls

Birds do eat the larger caterpillars but they just don’t eat enough. Several decades ago Canada introduced a European Tachinid Fly that targets solely the Winter Moth. This strategy appears to be successful, although it takes many years for this predator to become established in sufficient numbers for control. One thousand parasitic flies were released in Wellesley and other eastern Massachusetts towns last spring, and more releases are planned.

Another natural control is a variety of ground beetles that are believed to prey on pupae within the soil. Over time these might also effect some control, but judging from the damage to the landscape in Natick and Wellesley, it takes a long time for these predators to catch up. ∆

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