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Thresholds for cabbage stem flea beetle

An on-going HGCA study is investigating the thresholds for cabbage stem flea beetle in winter oilseed rape to ensure that early decisions on control can be made where necessary.

"Reliable detection of cabbage stem flea beetle in time for an application of a pyrethoid spray with a herbicide or fungicide will enable cost-effective control of an economically-damaging pest and can reduce the need for unnecessary insecticide applications," said David Green, ADAS, who is leading the project.

The study is looking at the use of water traps to catch adult cabbage stem flea beetles and to relate this to larval infestations levels so that a decision on the need to treat can be taken in the autumn. This is the final year of the study and trapping for adult beetles will be completed in early November.

Cabbage stem flea beetle is the main autumn pest of winter oilseed rape. A revised threshold for control, an average of around two larvae per plant, has recently been calculated. An infestation of this magnitude would be expected to result in a yield loss averaging 0.16 t/ha.

 "This year's results show a higher incidence of beetle activity than last year, particularly in the Midlands and in northern England. The mean number of cabbage stem flea beetle per site was 1.9 times as high in autumn 2006 compared with autumn 2005," said David Green.

Calibration of the traps using combined data for harvest years 2005 and 2006 indicated that the threshold of two larvae per plant was likely to be attained if an average of around 30 beetles were caught per water trap in September and October.

"Variation between seasons is enabling us to test the robustness of the relationships between the number of adult beetles and the likely incidence of larval damage," said David Green.

"Much of the insecticide usage targeted on cabbage stem flea beetle is disproportionate  to the likely risk of attack due to difficulties in assessing risk in time to take action," said David Green. "This HGCA-funded study will enable growers to decide on the need to spray based on risk. The potential environmental impact of the study is consequently high."

Release date: 2 November 2006

For further information contact

David Green, ADAS, David.B.Green@adas.co.uk

Madeleine Archer, HGCA press officer, 020 7520 3926 madeleine.archer@hgca.com