Western spruce budworm (WSB) and Douglas-fir tussock moth (DFTM) are primary defoliators of interior Douglas-fir. Larval feeding during outbreaks can cause tree growth loss, topkill, stem deformities and mortality. Other important negative effects in an urban setting include increased fire hazard, reduced aesthetic values and in the case of DFTM, significant toxic and allergic reactions by people and animals.
Douglas-fir stands cover about 7,000 ha of the Logan Lake Community Forest. All of these stands are susceptible to WSB and more than half are susceptible to DFTM. Both insects are native to BC and have reached outbreak proportions in the past. Douglas-fir tussock moth outbreaks are fairly cyclical and occur approximately every decade for two to five years, developing very quickly and subsiding abruptly. Compared to WSB outbreaks, DFTM outbreaks tend to be shorter and cover less area, but are more damaging. A DFTM outbreak is currently underway in the Kamloops Forest District. Western spruce budworm damage in the district rose substantially in 2006 through 2008 and though recorded infestations dropped in 2009, 70,000 ha were still defoliated within the district. No DFTM damage is currently visible within the community forest, but light western spruce budworm damage can be observed throughout the Douglas-fir stands.
The first fundamental step in a strategy to reduce the impact of these insects is to conduct annual monitoring. The best initial method for WSB is egg mass sampling in the fall and for DFTM pheromone trapping of male moths in the summer. Further surveys are conducted if these monitoring techniques indicate that treatment is advisable. Egg mass sampling was conducted in the fall of 2009 at twelve sites within susceptible stands in the community forest, and results indicated that defoliation should remain light in 2010 with no treatment required. It was too late in the year to conduct pheromone trapping for DFTM, so walkthroughs were employed at fourteen sites to look for evidence of populations reaching outbreak levels and they appeared to be low.
To reduce populations during an outbreak situation, short term control strategies primarily consist of aerial application of a product harmful to the insects. One of the best options for both WSB and DFTM for the Logan Lake Community Forest, if the need arises, is the application of Bacillus thuringiensis var. kurstaki (Btk). This organically registered biological control is specific to lepidopteron larvae and it has been used in the Kamloops Forest District very successfully for many years to control WSB. It was registered last year for DFTM control as well.
Recommended long term strategies employ stand manipulation tactics to reduce site hazard, such as thinning and encouraging a mixed tree species composition.