Oilseed rape crops should be closely monitored for cabbage stem flea beetles as they reach their most vulnerable stage and treated when spray thresholds are reached.

Reports in eastern and southern regions of UK have seen high levels of infestations cause damage to young oilseed rape plants, with some growers in Suffolk and Essex having to consider redrilling their crops.

Independent entomologist Alan Dewar says milder weather has rapidly escalated the problem, and he warns growers to be careful not to be caught out and risk losing their crop.

HGCA cabbage stem flea beetle spray thresholds

Consider applying if:

  • Adults have eaten more than 25% of leaf area at the cotyledon – two true leaf growth stage
  • Adults have eaten more than 50% of the leaf area at the three to four true leaf stage
  • The crop is growing more slowly than it is being destroyed.

“I think we are staring disaster in the face. In some areas there are fields that are being massacred before growers have even had the chance to spray.”

Concerns about the threat from the pest were heightened following the EU-wide ban on neonicotinoid seed treatments for this autumn.

See also: Big changes for disease control in oilseed rape

These had originally given crops protection for the first six to eight weeks, but now growers only have pyrethroid sprays left in their armoury,

Despite confirmed reports of resistance in the UK earlier this summer, Dr Dewar points out it may still be worth spraying to get some degree of control.

Issues further north

Problems have not just been isolated to eastern and southern regions, according to Stuart Hill of Frontier, who reports growers in Yorkshire are also seeing infestations in some fields.

Mr Hill says while most oilseed rape crops have got away well, it will now be a race between the plant and the pest.

He also stresses that even where crops have managed to grow away from the threat, growers will still need to closely monitor the crops for larvae, with a pyrethroid spray justified if there are more than two larvae/plant.

The high levels of cabbage stem flea beetle being found in some of the typically higher-risk regions of the country are not, however, being seen everywhere.

Steve Ellis from Adas says the company has yet to find significant numbers at any of its trial sites in Herefordshire, Cambridgeshire and Yorkshire.

“We will undoubtedly get some damage and some hotspots, but I am not anticipating there will be significant levels and most of the people we’ve spoke to will get away without spraying.”

Meanwhile, Dr Ellis advises those growers who are planning to spray not to go on too early and to stick to the HGCA guidelines on thresholds.

He warns against a knee-jerk reaction, suggesting that oilseed rape crops will be able to cope with some shot-holing and recover later in the season with little to no effect on biomass and yield.

Syngenta technical manager Simon Roberts advises growers to keep using water traps, but also keep a close eye on the germinating crops in the field, checking for signs of damage above and below the surface.

He suggests using a quality pyrethroid and says growers should keep the rates up while also monitoring control from the pesticides by checking flea beetle populations the next day.

“If you are not getting control, you may have a resistance issue and more applications will only increase this pressure.”