TEMPORAL VARIABILITY IN REPEATED BIOASSAYS OF FIELD POPULATIONS OF EUROPEAN RED MITE (ACARI, TETRANYCHIDAE) - IMPLICATIONS FOR RESISTANCE MONITORING

Discriminating bioassays are increasingly being used to detect resistance and estimate the frequency (F) of resistant (R) phenotypes in field populations. The proportion of survivors in discriminating bioassays is thought to measure the frequency of R phenotypes in the population. External factors that cause physiological stress in a field population may alter response to a discriminating concentration and affect estimates of F. Because susceptible (S) phenotypes already die at the discriminating concentration, only the response of R phenotypes changes. Thus, estimates of F will be biased and populations under stress will appear to be more susceptible than they really are. Repeated discriminating bioassays of the European red mite, Panonychus ulmi (Koch), showed variability in estimates of F. Repeated bioassays of P. ulmi from commercial apple orchards were used to construct a cumulative distribution function that describes the probability that R phenotypes die in the discriminating bioassay. This distribution function was then used in simulations to explore the effect of day-to-day variability in tolerance of R phenotypes on resistance monitoring programs.