Science > Research > System to Prevent Douglas-fir Tussock Moth Outbreaks

A Pest Management System to Prevent Douglas-fir Tussock Moth Outbreaks by Early Virus Application

[ Life Cycle | Steps | Results | Contact ]

Outbreak locations - click to enlarge

The Douglas-fir tussock moth is an important native insect that defoliates Douglas-fir trees in the interior dry-belt forests of British Columbia. Tussock moth larvae feed on the needles, causing growth loss, top kill and tree mortality.

Tussock moth outbreaks are cyclic and recur about every 7 to 13 years in British Columbia. Eight outbreaks have been recorded since 1916. A management system was developed to minimize losses caused by this insect.

Outbreak cycles - click to enlarge

Life Cycle of the Douglas-fir Tussock Moth

Adult tussock moths and egg mass

Male moths emerge about one week earlier than females. Soon after they emerge, from mid- to late August, the females emit a pheromone that attracts the males. Female moths are wingless and lay their eggs directly on the cocoon from which they emerge. On average, each female lays about 150 - 200 eggs.

Larvae emerge in late spring and begin to feed on the soft, newly flushed foliage. The newly emerged larvae spin silk which enables them to "balloon" in the wind. Because the females are wingless, ballooning is the main method of dispersal for this species. 

Young larvae hatching from egg mass	Trichogramma minutum parasitizing tussock moth eggs.

A combination of factors including parasites, predators and viral disease can naturally cause the collapse of outbreaks. The viral disease is the most important.

The OpNPV is an effective control agent in nature that usually ends the outbreak late in the cycle. However, by the time the virus spreads through the population naturally and stops the outbreak, extensive damage has already occurred.

Larva killed by OpNPV

As the larvae mature, they begin feeding on the tough, older foliage. At high population densities the larvae can defoliate and kill trees in a single season.

Late instar larvae

Stand severely defoliated by tussock moth infestation, Veasy Lake, BC 1982.

Steps in the Pest Management System for the Douglas-fir Tussock Moth

The management system is applied in the following sequence:

1. Determine the forest stands susceptible to tussock moth outbreaks by overlaying areas previously defoliated over forest type maps and biogeoclimatic zone maps.

Conducting an aerial survey	Inspecting a pheromone trap

2. Establish permanent monitoring stations in susceptible stands to record Douglas-fir tussock moth populations using traps baited with pheromones.

3. Search for egg masses near traps where 25 or more male moths were caught in three consecutive years. Once egg masses are found, conduct intensive egg-mass surveys to determine which areas require protection the following year. Confirm these predictions with larval sampling.

Egg mass

Helicopter application

4. Apply the virus to stands where the surveys predict defoliation will occur.

The Orgyia pseudotsugata nuclear polyhedrosis virus (OpNPV) is the main cause of death of Douglas-fir tussock moth. Applications of OpNPV at the beginning of the outbreak causes infection (epizootic) among the larvae earlier than would occur naturally and stops development of outbreaks in the treated stands. The virus can be applied either from the ground or from the air depending on the size and accessibility of the area.

The virus is specific to the genus Orgyia and is environmentally friendly. For this reason, OpNPV is the principle agent used for the biological control of the Douglas-fir tussock moth.

OpNPV greatly enlarged

Results

Untreated control stand, Veasy Lake, BC, 1983

Depending on the dose, virus application can reduce larval populations by 65 to 95% and reduce the number of trees killed by about 93% in the treated stands.

Treatment and results - click to enlarge

Other publications by Imre S. Otvos

Other publications about Douglas-fir tussock moth

Recovering stand treated with OpNPV the previous year, Veasy Lake, BC, 1983

Contact:

Imre S. Otvos
Natural Resources Canada
Canadian Forest Service
Pacific Forestry Centre
506 West Burnside Road
Victoria, B.C. V8Z 1M5

[ Life Cycle | Steps | Contact]