Natural Variation: A Complicating Factor in Bioassays with Chemical and Microbial Pesticides

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Our investigation of responses of Colorado potato beetle, Leptinotarsa decemlineata (Say), to Bacillus thuringiensis subsp. tenebrionis over 83 wk; diamondback moth, Plutella xylostello (L.), to B. thuringiensis subsp. kurstaki (cloned into Pseudomonas florescens) over 37 wk; and western spruce budworm, Choristoneura occidentalis Freeman, to pyrethrins and DDT over 91 generations indicated that ratios at LC50, LC90, and LC99 varied among pesticides tested on the same species and among insect species tested with the same pesticide. Frequencies with which LCs were significantly different (based on bracketing of the value 1.0 by 95% CI of each ratio) compared with the standard (the lowest LC in the data set) were extremely high (>95%) in all tests except those with pyrethrins on western spruce budworm. For example, toxicity ratios for Colorado potato beetle larvae fed B. thuringiensis subsp. tenebrionis were as high as 12.8 at LC50, 37.0 at LC90, and 155 at LC99 over 83 wk; the maximum upper 95% CL for a ratio were 18.6, 168, and 1,000, respectively, at these LCs. We conclude that the extent of natural variation must be investigated before biologically important changes can be identified with any certainty. The conventional practice of using ratios of one LC to another in studies to detect resistance and other biological changes may lead to erroneous conclusions if natural variation in cohorts of a population and subsequent generations of the same genetic strain are unknown.

Document Type: Research Article

Publication date: February 1, 1995

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