Tussock moth invasion

Colorado may be at the beginning of a multi-year infestation that’s destroying Douglas-fir trees — but there are ways to fight back

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Courtesy of the U.S. Forest Service

If the tops of your Douglas-fir trees are looking a little bare and brown, there’s a possibility the Douglas-fir tussock moth has moved into your neighborhood.

The caterpillars can come in and destroy parts of a forest in what residents claim to feel like just a day or so.

“One day, we were just driving along, we looked up and the trees that are normally green were brown, and it was just shocking,” says Boulder resident Diana Meyer.

The Colorado State Forest Service has reported that Douglas-fir trees growing on roughly 2,500 acres in the foothills are currently experiencing defoliation caused by the Douglas-fir tussock moth and caterpillar. The insect feeds off the needles of fir trees and experts say damage by the pest can result in a high degree of mortality in severely affected trees.

Due to natural variables, outbreaks occur years and even decades apart. During the beginning of an outbreak, the tussock moths lay egg masses on trees and even on the sides of nearby houses, which are easily visible. The moths spend the winter as an egg within the mass. Eggs hatch in late spring, often in May when the tree buds begin to break. The small and hairy caterpillars begin to migrate and move upwards within the tree. They are extremely mobile and can actually stand straight up, allowing them to move from branch to branch easily.

The caterpillars consume needles off the top edges of fir trees in early spring, first feeding off of newer foliage. The half-eaten needles will then weaken and turn brown and possibly fall off the tree. “They start off eating the newest growth and then they work their way to the oldest growth,” says Ben Pfohl, assistant district forester with the Colorado State Forest Service Boulder District.

In doing so, the tussock moths eliminate the fir trees’ ability to photosynthesize, the process by which trees covert light energy from the sun into chemical energy that can later be released to fuel their activities.

By August, the larvae reach their full size and can consume the entire upper crown of the tree, leaving them bare and completely needless.Then they migrate away from the infested tree and look for their next host plant to occupy until the next season begins in April or May.

Many trees may survive partial defoliation — even after an entire branch no longer has needles. Small needles that look dead can produce fresh buds and even the barest of trees can make it through the season and start to grow again next season. A tree can usually put out new growth and survive the following year, but with repeated defoliation, a tree will die.

Additionally, damage from tussock moths can make trees more susceptible to bark beetle attacks as the trees are already in a weakened state.

The moth is not only a danger to fir trees but to humans as well.

The tussock moth gets its name from the tufts of hair that can be found on their bodies.

“Hairs found on the caterpillars and cocoons can cause severe allergic reactions in some people who come into direct contact with them, so those who live, work and recreate in affected areas should take caution,” Pfohl warns.

The most common symptom of this condition, formally known as “tussockosis,” is a rash, which occurs when the skin is exposed to the larval hairs. In addition, people exposed to more heavy populations of the moth can develop respiratory tract symptoms.

According to Pfohl, the tussock moth caterpillars are affecting 2,500 acres in Colorado and he believes that this is the start of an outbreak.

While outbreaks are relatively short — two to four years — they can be very damaging when they occur. Once egg masses hatch, the cycle starts again; so, the best way to stop an infestation is to destroy the eggs early before they have a chance to hatch and circulate around the area.

When tussock moth outbreaks become severe and large numbers of trees are being destroyed, it may be necessary to intervene with an insecticide. Only during the most extreme outbreaks are helicopters used to release pesticides over a small area of the most infected trees. The conditions have to be just right to use a helicopter drop as the pesticides could be blown away easily if there is a strong breeze and other parts of the environment could be inadvertently destroyed. If residents are considering going the route of spraying a pesticide, treehelp.com recommends standard chemical insecticides such as acephate (Orthene), cyfluthrin (Tempo), bifenthrin (Talstar), fluvalinate (Mavrik) or carbaryl (Sevin), applied when the new foliage first appears, and providing good control.

The pesticides used to control these outbreaks are harmful to the environment and therefore are only recommended for the most dangerous of outbreaks — always check first to see which pesticides are legal in your area. Consulting with a local, licensed tree-care company before taking any action is the best option.

Without these interventions, the majority of tussock moths die because they fail to find a host plant. The larvae/ caterpillar need a host plant, off the ground away from predators, to spin their cocoons or they will not be able to survive. Female tussock moths do not have wings, and therefore can’t fly, making it critical for them to find a host plant high off the ground where predators can’t reach them. Natural predators of the tussock moths include birds, large ground beetles and, most notably, a virus known as “wilt disease,” which generally reduce populations within a few years.

The infestation doesn’t occur every summer but is hard to treat once the cycle has begun, so it’s important to control the larvae just after the eggs have hatched.

During the outbreak this year in Colorado, the caterpillars have been feeding since May and are migrating from affected trees to areas where they will spin their cocoon and turn into adult moths. Spraying is not recommended after the larvae season, and most of the defoliation has already occurred. Colorado residents that are currently experiencing defoliation should not panic now unless many of their trees are completely bare. At this point, residents generally should wait until spring to try to get ahead of the tussock moths and destroy next season’s eggs before they hatch.

It looks as though the summer of 2015 may be the beginning of a multi-year infestation, so make sure you’re ready for another possibly insect ridden season when spring rolls around.

Respond: info@boulderganic.com