Researcher William Deasy found that sulphur compounds released from broccoli roots attracted D. radicum larvae. As the roots were eaten, more compounds were released, attracting more larvae. He investigated the potential of "switching on" inducible plant resistance in Parthenon using plant-defence elicitors.
Methyl jasmonate and D-fructose were selected on the basis of previous research showing they could enhance pest resistance. But in glasshouse and field trials they caused increased egg laying by cabbage root fly. D-fructose did not affect larval behaviour while methyl jasmonate reduced crop damage. Unfortunately, there was a trend across several experiments and rates of methyl jasmonate for phytotoxicity, reducing yield.
Dazitol, a caliente mustard derivative, was also applied to modules. Experiments showed it to reduce larval numbers but it was severely phytotoxic at the rates applied. A third category of product, plant-derived insecticide dimethyl disulphur, was safe to the crop but showed no significant effect on cabbage root fly damage.
If chlorpyriphos is withdrawn the only effective cabbage root fly controls will be Tracer treatment and mesh covers. It was considered worthwhile to explore alternatives but those that showed effects on the pest would need research on formulation and rates before they could be safely applied on brassicas.
Horticultural Development Company
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