Researchershave confirmed that pyrethroid resistant cabbage stem flea beetles are presentin the UK. This information is vital for oilseed rape growers who are alreadysowing crops following this years early harvest.
Workcarried out by Rothamsted Research, as part of HGCA Project 2140019,shows that knock down resistance (KDR) is found in UK populations of cabbagestem flea beetle. KDR usually confers moderate resistance to all pyrethroidsapplied at recommended field rates. Following the restriction on neonicotinoidseed treatments, controlling cabbage stem flea beetle will be reliant onpyrethroid insecticides, so control may be more challenging this season.
Samplesfor testing were sent in mainly from the South East so we dont know howwidespread the resistant populations are. It is therefore possible that the KDRmutation is localised making resistance management a priority this autumn.
Closemonitoring will be crucial this season and the presence of KDR is an added complication.It is therefore essential to only apply a pyrethroid spray where it isabsolutely necessary in order to prevent the spread of resistance. explainsHGCAs Caroline Nicholls.
Researchfrom ADAS (HGCAProject 2140009) has shown that oilseed rape can cope with severe shotholing even at the cotyledon stage. This demonstrates that the crop is mostvulnerable at emergence where any damage to the growing tip can be lethal. Itis this type of damage, at emergence, that can wipe out an entire crop.
Itis prohibited to use pyrethroids on bare soil, so the spray window for treatingat emergence is quite narrow. There are no set spray thresholds at emergencebut monitoring local pest pressure will give an indication of whether it isnecessary. There are three ways to monitor pest pressure:
Checkingthe number of cabbage stem flea beetles in the previous crops harvested seed
Usingwater traps to check beetle numbers
Assessingdamage to volunteer oilseed rape plants
Oncethe crop reaches the cotyledon stage and beyond, the following spray thresholdsare recommended:
- Cotyledon 2 true leaves: consider spraying when 25% of the green leaf area of thewhole crop has been eaten
- 34leaf stage: consider spraying when 50% of the green leaf area of the wholecrop has been eaten
Withno foliar alternatives to pyrethroids, these thresholds are crucial forresistance management. Any unnecessary sprays are likely to speed up the rateat which KDR becomes widespread.
Theappearance of KDR underlines that good establishment and a fast getaway for thecrop is more important than ever given the reduced number and efficacy ofpesticides. It is important to pay attention to a wide range of factors such assoil moisture, temperature, seed rate, variety and fertiliser choice to givethe crop the best possible start.