Some responses of cabbage root fly (Delia brassicae) to ally I isothiocyanate and other volatile constituents of crucifers

Vapour components emanating from disrupted cauliflower, turnip, radish, wallflower and brown mustard tissue were assessed for their effects on the cabbage root fly (Delia brassicae). Of about 20 vapour components detected and separated by gas chromatography, six elicited sufficiently large electroantennal responses to warrant further testing, but of these only allyl isothiocyanate and hexyl acetate markedly affected the behaviour of gravid flies in an olfactometer. In wind-tunnel tests at a wind speed of 1–2-1-7 m/s, the numbers of females caught was increased when allyl isothiocyanate vapour was released at 32–130 mg/h but decreased at higher evaporation rates. The only effect of hexyl acetate vapour released at 40–140 mg/h was a reduction in the numbers caught at the highest concentration. In a cabbage crop, yellow water traps fitted with allyl isothiocyanate sources, each evaporating 2–3 g/day, caught 11 times as many female and seven times as many male flies as unmodified traps during the early period of the third generation but the improvement in trap efficiency later diminished. Trap efficiency was slightly reduced when the rate of evaporation of allyl isothiocyanate from a trap was decreased from 2–3 to 0–51 g/day. On fallow ground, allyl isothiocyanate improved trap performance in catching female flies by about seven-fold, but along a hedgerow adjacent to Brussels sprouts the improvement was barely two-fold. Hexyl acetate did not improve the performance of traps in a cabbage crop.