Bacillus thuringiensis Berliner is registered for control of several defoliating forest insects, including the spruce budworm (Choristoneura fumiferana Clem.), jackpine budworm (C. pinus Freeman), and gypsy moth (Lymantria dispar L.) (Morris et al. 1986). Product labels and user guidelines of several manufacturers recommend application to dry foliage for best results. Efficacy is presumably reduced when foliage is wet from dew or precipitation. Possible causes are lower impaction efficiency of spray droplets on the wet leaf surface (Spillman 1984) and dilution of the active ingredient. I compared the efficacy of B. thuringiensis applied to wet and dry balsam fir foliage (Abies balsamea L.) in a laboratory bioassay with larvae of the spruce budwom, Choristoneura fumiferana.