Dosage-mortality studies with commercial Bacillus thuringiensis sprayed in a modified Potter's tower against some forest insects

Dosage-mortality tests were carried out with commercial Bacillus thuringiensis (B.T.) (Dipel)^® against various instars of the spruce budworm, Choristoneura fumiferana, the white-marked and Douglas fir tussock moths, Hemerocampa leucostigmata and Orgyia pseudotsugata, and the gypsy moth, Porthetria dispar. Dipel was applied as a dilute (10^?2) molasses suspension onto artificial diet surface in a spray tower designed to simulate aerial application. Probit analysis of the results showed that LD₅₀s expressed both in terms of gallons deposited per acre and as spores and crystals deposited per cm² increased with larval age for all species. The spruce budworm was the most sensitive to the bacteria, followed in decreasing order of sensitivity by the white-marked tussock moth, Douglas fir tussock moth, and the gypsy moth. The mean slopes for all instars of the four species were 1.6, 3.1, 2.6, and 2.2, respectively, indicating that precision-wise, the assay of B.T. on artificial medium was good. The relatively low slope for spruce budworm is explained by its peculiar feeding habit. Among all species tested, bacteria-treated larvae gained weight at a considerably reduced rate compared with untreated ones. Reduction in weight resulting from lowered feeding activity intensified as dosage rates increased. The implication of this in terms of mortality assessments in microbial control operations is discussed.It is suggested that 0.02 gallon (4 × 10⁶ International Units) of Dipel Molasses deposited per acre may achieve economic control of fourth- to sixth-instar budworm and first-to second-instar gypsy moths. A deposit rate for second- to fifth-instar white-marked or Douglas fir tussock moths appears to be in the vicinity of 0.01 gallon (2 × 10⁶ IU) per acre.