Abstract Susceptibility to insecticides in larvae of Costelytra zealandica (White) has been studied in the laboratory by the topical application of insecticides in solution. Since the onset of mortality, especially that due to DDT, is slow, the loss or retention of the capacity for burrowing in loose soil was adopted as the criterion of effect. Response to DDT occurred over a wide range of dosage when acetone was employed as the solvent; with non-volatile solvents, olive oil or dioctyl phthalate, the range was less and log-dosage probit response lines were steeper. Conditions in the post-treatment period affected the slope of the regression line, which was steeper when the larvae were kept in a damp atmosphere than when they were covered with earth. Four main variants in technique are described, and their advantages discussed in relation to the detection and measurement of susceptibility or tolerance to DDT in larval populations, and possible analysis of these into genetically differing fractions.