The first cases of resistance to insecticides have been found in flea beetles in the UK, adding to growers’ troubles following the neonicotinoid ban. Nick Fone sought expert advice from Germany, where resistance has been an issue for six years
Last week Rothamsted researchers confirmed resistance to pyrethroid insecticides in populations of cabbage stem flea beetles in the UK.
The news adds to concerns for oilseed rape growers who, for the first time this autumn, have no neonicotinoid insecticide seed treatments to protect their crops from pest attack.
It is estimated that 70% of the UK crop is affected by flea beetle, and foliar-applied pyrethroid insecticides are now the only method of combating damage caused by the pest – and if pyrethroid-resistant flea beetles are widespread, control will be challenging.
UK spray thresholds
The HGCA suggests considering spraying crops from the cotyledon stage to two true leaves when 25% of the green-leaf area has been eaten. At the three- to four-leaf stage, the UK advice is to consider spraying when 50% of the green-leaf area has been eaten.
Resistance was confirmed in Germany in 2008 and has spread across continental Europe, with increasing use of pyrethroids helping to propagate the problem.
German lessons learned
Dr Udo Heimbach is a pest control specialist at Germany’s Julius Kühn Institute for plant protection research in Brunswick, east of Hanover.
Following six years of experience with resistance, he outlines his views on the most practical approach to flea beetle control in the absence of neonicotinoids:
- Do not sow too early. Beetles prefer developed crops to lay their eggs in but of course small plants may be more at risk from adult beetles feeding on them.
- Some growers in Germany sow a small area early in a corner of the field to attract the beetles and encourage them to lay their eggs there, hoping to reduce the problem in the main crop.
- There is evidence that direct drilling can reduce infestation levels. Research with aphids has seen them settle less on direct-sown fields with mulch on the soil surface, compared with ploughed ground. Lower stubble disturbance also provides useful habitats for beneficial insects and spiders.
- Use slightly higher seed rates to compensate for plants being lost to beetles. In Germany, the typical rate is about 40 seeds/sq m. An increase of about 10% should cover the losses, but this is obviously dependent on the numbers of the pest found on the ground.
- Check carefully from emergence onwards if shot holing is visible and use yellow water traps. If numbers exceed 50 a trap within three weeks, apply pyrethroids to avoid damage caused by larvae (any spray decisions should be made at least three weeks after the two true leaf stage).
- Do not apply too early to cover beetles coming into the crop late. If you do decide to use pyrethroids when the population reaches threshold levels, leave spraying as late as possible to hit both beetles and larvae.
- If the number of beetles in traps exceeds threshold after just two-and-half weeks, do not rush in with the sprayer because you will probably get another 50 in the next 10 days. They can all be dealt with in one hit and their offspring will have had a chance to hatch, too.
Free yellow water traps
Agrichemical company BASF is to supply free yellow water traps to OSR growers to help determine the threat from flea beetles following the discovery of pyrethroid resistance. Visit the BASF website for your local contact.
- Sticking to a single pyrethroid application is critical in stopping resistance developing further and a second should be avoided at all costs.
- The level of resistance is rarely 100% and is often much lower, so pyrethroids can still work even where partial resistance is present. The main thing to remember is that reducing the use of pyrethroids will help to limit the intensification of resistance while alternative controls are being developed.
- There are new seed treatment products in the pipeline, but they are not approved yet.
- We have limited knowledge of the efficacy of registered actives used in the past (tefluthrin in Scandinavia and methiocarb in France). Methiocarb is reported to have poor performance. Tefluthrin was not used on a practical field-scale, so again not much is known.
- An equal threat to rape crops in northern Germany is cabbage root fly. Without neonicotinoid seed treatments, we have no idea of how to control it at the moment. We know other actives show some promise, but nothing concrete at present.