Stability of dicofol resistance in populations of European red mite (Acari: Tetranychidae) on apples in southern Ontario

Fluctuations in levels of resistance to dicofol in populations of European red mite, Panonychus ulmi (Koch), from several orchards with different frequencies of resistant individuals were compared with responses of laboratory colonies established from the same sites. Considerable variability in responses occurred in observations spaced over 2 yr, but the frequency of dicofol-resistant individuals in these populations did not decline in the absence of selection pressure. Laboratory colonies reared through 32 generations displayed relatively constant resistance frequencies, suggesting that dicofol resistance is stable for a time equivalent to at least 3 yr in the field. Relative fecundities of mites resistant and susceptible to dicofol were examined by comparing the number of eggs laid by females from intra- and interstrain crosses, as well as the percentage hatchability, percentage survivorship, and sex ratio of the resulting progeny. Females from a 12-yr-old susceptible laboratory colony laid more eggs than females from a more recently established susceptible colony and than females from two colonies resistant to dicofol. Resistant and susceptible mites did not differ for any of the other parameters examined. It is concluded that the absence of a reproductive detriment is a major factor in the apparent stability of resistance to dicofol in these mites. Implications of these findings for management of dicofol resistance in P. ulmi are discussed