The efficacy of Bacillus thuringiensis was evaluated against the large aspen tortrix on trembling aspen in Alaska. A concentration of 4 × 109 IU per 378.5 L of water was applied to infested trees using a hydraulic sprayer. Treatments with Dipel 4L and Thuricide 32LV resulted in population reductions of 69 and 76% respectively. Both products provided significant foliage protection. Timing of treatment is critical to successful population suppression and foliage protection.