Validation of a process-oriented model of Bacillus thuringiensis variety kurstaki efficacy against spruce budworm (Lepidoptera: Tortricidae)

A model simulating the efficacy of Bacillus thuringiensis variety kurstaki Berliner against populations of the spruce budworm, Choristoneura fumiferana (Clemens), on balsam fir, Abies balsamea L., was validated with extensive field data. The model, a detailed description of the interactions between the insect and its pathogen, requires as input daily minimum and maximum air temperature, spray deposit measurements (product potency, droplet density, diameter spectrum), initial budworm density and stage-specific survival rates of untreated populations. The model simulated the efficacy of treatments in 24 plots that received single or double applications of Foray 48B and 76B at rates of 30 and 50 BIU/ha. Observations also were made in 5 control (untreated) plots. The model accurately predicted both foliage protection and population reduction, and constitutes a valid tool for decision making concerning optimal application rates, atomization, and timing with respect to control objectives. Comparisons between simulated and observed patterns of development, defoliation and population reduction indicated several areas where additional knowledge on the interactions between B. thuringiensis and spruce budworm would be beneficial. In particular, a better understanding of the influence of plant quality on feeding, development and behavior of spruce budworm is needed for hosts other than healthy, mature, nonflowering balsam fir. Additional information is needed concerning variation in the degree of exposure and susceptibility to B. thuringiensis among the early larval stages of spruce budworm. Future models should consider the interactions between B. thuringiensis and natural enemies, especially parasitoids, delayed effects of exposure to B. thuringiensis on subsequent stages and on the progeny of surviving spruce budworm. Models such as this can be used to establish treatment recommendations based on efficacy targets and cost-benefit analyses.