Classifying Resistance Severity in Field Populations: Sampling Inspection Plans for an Insecticide Resistance Monitoring Program

Abstract

Detecting insecticide resistance before it increases above a critical level has been a driving force in developing tests to estimate resistance severity in field populations. Here, estimates of resistance severity (r) are classified into one of three categories. The classificationis based on comparing r with two preselected levels of resistance severity (θ) to obtain a three decision sampling plan. Using the example of fen valerate resistance in beet armyworm, Spodoptera exigua (Hübner), each θ was selected by comparing resistance test responses of populations that differed in resistance and exposure to insecticides. Test response wascorrected for survival of moths after exposure to a 1,000 μg/g concentration of fen valerate. This concentration was selected by comparing probit lines of susceptible and fen valerate-selected strains. The determination that r was greater or less than each θ was made using sampling inspection plans based on the binomial distribution. A sequential sampling plan using the sequential probability ratio test (SPRT) required less sampling effort than single sampling plans of equal strength and was suited to our operational needs. Using the same susceptibility test, sequential analysis of survival proportions from the 1,000 μg/gconcentration was also compared with probit analysis of survival proportions from multiple concentrations. Sampling effort required for the SPRT was substantially lower than that required for probit analysis. Despite differences in probit analysis and SPRT assumptions, probit estimates of resistance severity at 1,000 μg/gagreed with sequential classificationof r using a 1,000 μg/gconcentration.