COMPARISON OF LEAF DISC AND PETRI DISH ASSAYS FOR THE ASSESSMENT OF DICOFOL RESISTANCE IN POPULATIONS OF EUROPEAN RED MITE FROM SOUTHERN ONTARIO

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Abstract
Two bioassay methods using leaf discs and Petri dishes as the treated medium were compared. Resistance levels were approximately 11-fold with the leaf disc method and 7-fold using the Petri dish assay. Comparisons of the two methods on six wild populations indicated that the Petri dish assay was more sensitive in identifying low frequencies of resistance. Advantages of the Petri dish assay over the previously used leaf disc method for rapid, field-type surveys are discussed.Surveys of populations from the Simcoe and Niagara areas, using the Petri dish assay, indicated resistance to dicofol in 11 of 22 samples. Resistance was generally correlated with sites where dicofol had been used at least once a year for 2–3 consecutive years. The data indicate a resurgence of dicofol resistance since 1984 when resistance was detected at 4% of the surveyed sites.