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Douglas-fir tussock moth management

Last updated on January 30, 2025

Outbreaks occur every 10 to 12 years, causing significant damage and mortality to Douglas-fir stands in B.C.'s interior. These outbreaks tend to last up to four years before natural controls such as predators, parasites, pathogens and starvation lead to population collapse.

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Detection 

Building phase

The building phase of a tussock moth outbreak takes one to two years. Detection of increasing insect populations during the building phase is critical, and unless detected at this stage, significant damage could occur. High population levels persist for one to four years, then collapse due to natural control agents which include parasites, predators (mainly birds and ants), pathogens, and starvation due to the forced consumption of older, less nutritious foliage.

Nucleopolyhedrosis virus (NVP)

Another factor in the collapse of the population is caused by a species-specific NPV, which is always present in the population at low levels. The virus is spread through insect-to-insect contact, causing populations to decline rapidly. Six to eight years elapse before populations again reach damaging levels.

If dead larvae are commonly found, or if egg masses are small, distorted and incompletely covered with hairs, the population is infected with virus and no significant additional defoliation will occur.

Strategies 

Successful management depends on carefully monitoring populations within high-hazard stands during the non-outbreak and building phases. Once the outbreak begins, viable treatment options decrease significantly.

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Long-term strategies

Long-term management strategies are focused on host trees including:

  • Conversion to alternative species
  • Promotion of species mixes
  • Stand-structure manipulation (harvesting, thinning)

Harvesting and thinning

Harvesting and thinning susceptible trees before damage occurs may be a viable management strategy.

When harvesting or thinning susceptible trees should be removed before any eggs hatch in order to reduce spread. 

Replanting

Replanting with a non-susceptible species such as ponderosa pine or changing to some other land use such as grazing may also be considered.

Alternatively, the outbreak may be allowed to run its course, and dead trees may be salvaged. 

Short-term strategies

Treatments should be applied prior to noticeable defoliation.

Treatments

Bacillus thuringiensis var Kurstaki (Btk)

The preferred treatment is the application of the biological insecticide Btk. Btk is applied aerially and kills young tussock moth caterpillars after they’ve eaten treated foliage.  

Virus (NPV) application

Because a five- to eight-week incubation period is needed after NPV virus application before larval mortality occurs, trees must be health enough to withstand defoliation while this treatment takes effect. Therefore, the virus is best used when populations are low during an outbreak's building stage.

NPV virus only needs one appliction and is specific to Douglas-fir tussock moth. In additon, this virus is highly contagious to tussock moth larvae, so small amounts are needed for outbreak control. NPV is ideal for reducing the population before significant defoliation is expected. If one year of defoliation has already occurred, biological (btk) insecticide treatment is a better choice.

Management programs

Under B.C.'s Integrated Pest Management Act, defoliator spraying in B.C.'s forests must be authorized by the B.C. Government through an approved pest management plan.

Information for landowners

Information for landowners on spray programs, protecting trees and preventing spread.

Contact information

Contact us if you have further questions about Forest Health in B.C.

 

Email
Forests.ForestPracticesBranchOffice@gov.bc.ca