Douglas-fir Tussock Moth Management
The course of action selected in an area should be consistent with the long-term stand-management objectives. Different treatments meet different objectives and each stand has to be considered individually. Long-term management strategies are stand-manipulation tactics, including:
- Conversion to alternative species
- Promotion of species mixes
- Stand-structure manipulation (harvesting, thinning)
Successful management of the Douglas-fir tussock moth depends on carefully monitoring populations within high-hazard stands during the non-outbreak and building phases. Once an outbreak begins, viable treatment options decrease significantly.
Where management objectives warrant, treatments should be applied prior to noticeable defoliation, to reduce the tussock moth population, rather than after defoliation has appeared with its associated growth loss, dieback or mortality.
The preferred treatment is the application of the biological insecticide Bacillus thuringiensis var Kurstaki (Btk). Btk is applied aerially and kills young tussock moth caterpillars after they’ve eaten treated foliage.
Total removal of an infested stand, including all logging debris, could control localized outbreaks. However, unless regeneration was possible or conversion to other land uses was the objective, this is not a recommended treatment. Shelterwood cutting within an infestation would not produce any protection for the remaining trees and, if tops and branches of logged trees containing egg masses are left on site, newly hatched larvae may concentrate on the remaining trees.
Thus, harvesting (clearcut or shelterwood) of an infested stand may not be feasible unless the regeneration is already established and can be protected by a virus or insecticide treatment.
Virus (NPV) application
Resource issues, stage of outbreak, and condition of infested trees should be considered when deciding which insecticide to use. A five- to eight-week incubation period ensues after virus application before the larvae cease feeding, and unless the trees can withstand additional defoliation, a chemical insecticide treatment may be necessary. Therefore, the virus is best used when populations are very low, as seen in an outbreak's early building stage.
The advantages of virus treatment are species specificity and the need for only one application. This virus is highly contagious to tussock moth larvae, so only small amounts are needed to bring an outbreak under control. NPV is ideal for reducing the population a year before significant defoliation is expected. One year into an outbreak, biological (btk) insecticide treatment is often a better choice.
Harvesting susceptible trees before damage occurs may be a viable management strategy. If harvesting is preferred, then all logging, processing and slash burning should be completed before egg hatch to prevent the spread.
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, but the salvage value obtained from dead trees may be considerably lower.