The Invasion
A little caterpillar with a big appetite is chomping down on some of our finest furs, across the Front Range. A Colorado native, the black-tusked or Douglas fir tussock moth is an apt defoliator of spruce, Douglas-fir, true fir and other conifers in the Rocky Mountain region.
In forests, tussock moth outbreaks usually develop quickly and then abruptly subside after about three years. In Colorado’s residential areas, however, populations seem to remain relatively stable at higher than normal levels for long periods of time.
Young tussock moth caterpillars are blackish with long body hairs, producing brightly colored tufts of hair as they grow larger. A mature larva is 1.2-1.4 inches long, with a gray to brown body and shiny black head. Two long tufts of black hairs project forward from the head, and a similar tuft projects backward from the rear of the body. Dense, light brown patches of hairs and red spots occur on the first four and the last abdominal segment and there is an orange stripe on each side.
The cocoon surrounding the pupal stage is brownish gray and covered with hairs from the body of the larva. They are usually attached to foliage but may be found on tree trunks, rock, or other objects in the vicinity of a previously infested tree. The egg mass, laid on the pupal cocoon of the female, contains about 300 white spherical eggs laid in several layers. The entire mass is covered with a frothy substance that is intermixed with body hairs of the mother.
The adult males are moths with rusty-colored forewings and gray-brown hind wings, with a wing-span of about one inch. Females are thick-bodied and wingless, found in close association with the spot where they earlier pupated.
The Life Cycle
Tussock moth spends the winter as an egg within the egg mass. Eggs hatch in the spring, often in late May, typically following bud break. The small, hairy caterpillars migrate, moving to the new growth but also often traveling upwards in the trees. This habit allows some of the caterpillars to be dispersed by strong winds. Since the adult female moths do not fly, wind-blown movement of young larvae is an important means of initiating new infestations.
By mid-July or August, the larvae become full grown and many may migrate away from the infested tree. The males are winged and are strong fliers, but as the females have only minute, non-functional wings, mating occurs in the immediate vicinity of the female pupal case. They then lay their characteristic mass of eggs covered with grayish hairs. There is one generation produced per year.
The Damage
Larvae first feed on the new needles, killing them but not consuming the entire needle. The dead needles remain on the tree, giving it a reddish cast for a short time. Later, the reddish appearance is lost as the damaged needles drop. As the larvae mature, they begin to feed on older foliage, leaving a heavily infested tree completely defoliated.
Infected trees normally experience growth losses and some mortality, although death is restricted to trees losing more than 90 percent of their needles. More commonly, the top of the tree is killed. Severe defoliation for more than one year does cause mortality. Any infestation may cause serious aesthetic damage in residential situations.
The Defense
The tussock moth has a number of natural enemies, including parasites, predators and pathogens. These enemies, along with environmental factors, keep populations at low levels under most circumstances.
However, tussock moth populations in Colorado urban areas seem to somehow escape these natural control factors. Perhaps the artificial nature of these urban “forests,” and the associated stresses on the trees, allows the tussock moth to survive at higher levels than is possible in a natural forest.
Control measures include biological insecticides such as Bacillus thuringiensis (Bt), but to date these have shown only fair success. Standard chemical insecticides such as acephate (Orthene), cyfluthrin (Tempo), bifenthrin (Talstar), fluvalinate (Mavrik) or carbaryl (Sevin), applied when the new foliage first appears, give good control. Check to see which ones are legal in your area. Certain Front Range forests with tussock moths also contain the endangered Pawnee montane skipper butterfly. Spraying is carefully controlled in such situations.
One important aspect of control is the proper assessment of damage. When is a tree top, or entire tree, really dead? The experience of Colorado urban foresters suggests that a “dead” tree or tree top be given at least one year to recover before removing or reshaping the tree. Trees as much as 50 percent defoliated have been known to recover. To help them recover, such trees should be properly watered, especially in winter, and fertilized.
Contact a licensed tree care or landscape company. Response time is critical when it comes to saving your landscape from the mouths of these eating machines. If you see signs of the tussock moth, contact Plant Escape immediately, so that our trained horticulturalists can begin evaluation and treatment of your plants, shrubs and tress, ASAP.
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