NFU adviser Don Pendergrast:
"Obviously the investigation into pyrethroid resistance is ongoing and growers will wait to see how widespread resistance in CSFB is. However these findings emphasise the concern that growers have that the crop protection toolbox is nearly empty.
"The NFU has expressed concern that restriction on important chemistry like neonicotinoids could have unintended consequence for crop production if populations of economically important pests are not controllable with pyrethroids due to resistance."
Work carried out by Rothamsted Research, as part of an HGCA project, has found ‘knock down resistance’ in UK populations of the beetle, providing the beetles with a moderate resilience to pyrethroids applied at recommended field rates.
This is particularly worrying for growers following restrictions on neonicotinoid seed treatments, which will make controlling cabbage stem flea beetle reliant on pyrethroid insecticides.
“Samples for testing were sent in mainly from the South East so we don’t know how widespread the resistant populations are. It is therefore possible that the KDR mutation is localised making resistance management a priority this autumn,” said HGCA’s Caroline Nicholls.
“Close monitoring will be crucial this season and the presence of knock down resistance is an added complication. It is therefore essential to only apply a pyrethroid spray where it is absolutely necessary in order to prevent the spread of resistance.”
“Research from ADAS has shown that oilseed rape can cope with severe shot holing even at the cotyledon stage.
This demonstrates that the crop is most vulnerable at emergence where any damage to the growing tip can be lethal. It is this type of damage, at emergence, that can wipe out an entire crop.”
Using pyrethroids – HGCA advice
It is prohibited to use pyrethroids on bare soil, so the spray window for treating at emergence is quite narrow. There are no set spray thresholds at emergence but monitoring local pest pressure will give an indication of whether it is necessary.
Once the crop reaches the cotyledon stage and beyond, the following spray thresholds are recommended:
- Cotyledon – 2 true leaves: consider spraying when 25% of the green leaf area of the whole crop has been eaten
- 3–4 leaf stage: consider spraying when 50% of the green leaf area of the whole crop has been eaten.
With no foliar alternatives to pyrethroids, these thresholds are crucial for resistance management. Any unnecessary sprays are likely to speed up the rate at which KDR becomes widespread.