Structure-activity relationships in glucosinolates as oviposition stimulants of the cabbage root fly, Delia radicum (L.)
A statistically sound and quantifiable bioassay procedure was developed for determining the response of adult Delia radicum to oviposition stimulants. The method was based on the use of surrogate leaves coated with test compounds. All of the test compounds were of purity 99% or greater. Twelve of the tested compounds were synthesised the remaining twenty four were either donated or available commercially. Attempts to follow literature methods to synthesise glucosinolates were unsatisfactory and an investigation of the procedures led to a number of synthetic modifications. Despite various attempts, the failure to sulphate a thiohydroximate to produce a glucosinolate was not resolved. Attempts to correlate oviposition stimulus with chemical structure produced a very significant finding. The results showed that a wide variety of chemically dissimilar compounds were effective stimulants providing they contained an S=O group. Thus sulphoxides, sulphones, sulphinic, sulphonic acids and their derivatives were all effective. Thiols and thioethers were non-stimulant, as were naturally occurring glucosinolates which had been chemically modified by the removal of the oxime sulphate group. The relative effectiveness of the oviposition stimulants was examined by determining the number of eggs laid on surrogate leaves relative to a prop-2-enylglucosinolate (sinigrin) standard over a range or concentrations. Statistical modelling of the data collected produced a maximum relative number of eggs laid (YMAX) at an optimum concentration (C) for each compound. It was not found possible to produce a single parameter combining YMAX and C, neither did any other structure-activity feature emerge from the study.