19 April 2001
Pollen beetle threat to oilseed crops

By Andrew Swallow

GROWERS should check oilseed rape crops for pollen beetle, warns plant breeder CPB Twyford.

Beetles flew into many crops during warm weather before Easter and, where crops have not yet flowered, will cause severe damage to buds, says head of oilseeds Liz Williams.

If beetle numbers reach five per plant on conventional varieties, then an insecticide is justified, she says.

But on varietal mix hybrids such as Gemini or Synergy, a threshold of just one beetle per plant should be applied, she says.

“Varietal associations such as Gemini have just 20% of plants that bear pollen and, as a result, the beetles will seek out these plants in the crop,” she explains.

However, entomologist Jon Oakley from consultants ADAS believes Ms Williamss thresholds to be over-cautious.

He recommends a spray at five beetles per plant for varietal associations and backward conventional crops, or 20 beetles per plant for normally developed conventional varieties.

If an insecticide is used it should be “bee-safe” and tank-mixes with fungicides should be checked to make sure they do not affect that safety, he adds.

Once flowering has started beetle damage is insignificant, so insecticides are no longer worthwhile.