Science into practice: Ways to control thrips on onion and leek crops

Onion thrips are the biggest pest problem for leek growers and can also attack other Allium crops such as salad onions.

Large populations of thrips can develop, causing leaf blemishes that reduce quality and may make the crop unmarketable. Insecticide resistance to pyrethroids (which had been used for successful control) was confirmed three years ago. Growers need alternative strategies for season-long control.

A one-year HDC project, FV 339, has highlighted some experimental seed-and-spray treatments that reduce thrips damage on leek and onion. Observations on crops in 2008 demonstrated that thrips damage got worse over time, with most occurring from early July onwards. With leeks, sticky trap counts reached their highest levels in August.

Previous Defra-funded work has shown control is required until thrips numbers decrease in October. This suggests that there is little benefit in applying sprays early to attempt to suppress infestations. Instead, growers should apply spray treatments when thrips numbers are increasing (as indicated by sticky traps or plant samples). Of the approved insecticides, Tracer and Dursban were the most effective, but Methiocarb 500 SC (awaiting approval) was even more effective than Tracer. Two novel compounds also showed promise.

Experiments with seed treatments confirmed they give a certain amount of thrips control in onion and leek crops but this diminishes by midsummer. On leeks, imidachloprid and particularly fipronil reduced damage. Three novel seed treatments reduced onion seedling losses, which was assumed to be because they reduced damage caused by bean seed fly.

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