GROWERS IN the Tweed Valley and Scottish borders region should be aware that high numbers of cabbage stem flea beetle larvae have been detected in crops of oilseed rape.

Field analysis of a range of varieties in this area has found very high larvae numbers in untreated and Chinook (beta-cyfluthrin + imidacloprid) – seed treated crops, said CPB Twyford’s Lee Bennett.

Because the larvae are within the plant’s stem, there is nothing growers can do to kill them, he said.

“The first effect farmers may see is lodging in the crop as the larvae eat through the stems. Also if the season gets drier, the larvae will reduce nutrient uptake, producing lower than expected yields.”

He believes this highlights the importance of considering a second pyrethroid, once activity of any seed treatment has worn off.

Adult beetle activity went on longer last season, which meant crops which did not receive a later pyrethroid could be exposed to larvae infestation, he said.

Growers need to look closely at crops to identify whether they have a problem – this can only be done by bisecting stems, he noted. It is also important to distinguish the stem flea beetle from the similar rape winter stem weevil.

“The larvae look very similar, but the flea beetle has definite mouth parts, whereas the weevil doesn’t.”

While nothing can be done to control larvae within stems this season, growers must start to plan for next and make sure they are forewarned and forearmed to any threat, he concluded.