Last spring there were more pollen beetles than in the previous year. In fact, the number caught in sticky traps has been creeping up for a few years and this spring could see even more following the cold winter, warns one expert.

Cold winters send adult pollen beetles into deep hibernation, allowing them to conserve energy for their migration into oilseed rape crops in March, explains Sam Cook, a senior research scientist at Rothamsted Research. Once they find the buds, they feed on the pollen inside, causing the buds to abort.

However, that doesn’t necessarily mean you’ll need to control the pest. Spraying is unnecessary unless you can find, on average, 15 beetles per plant (five in backward crops) across the field between green and yellow bud stage, she stresses. Once flowering has begun, the beetles focus on the pollen in open petals.

“We know from recent observations that many crops are treated even though the economic threshold is not reached. Last season, 16 out of the 53 sites monitored received one or more sprays when only three justified an insecticide.”

Dr Cook recognises the difficulties in monitoring and risk assessment. Walking through crops to monitor beetle numbers can give a skewed picture as there are likely to be more at field edges and in areas in which the crop turns yellow earlier.

New research has shown the adults are by choice attracted to bright yellow petals – where the pollen is readily accessible – rather than buds. A variable crop can be harbouring different beetle concentrations across the field.

With this problem in mind, Dr Cook and a team of scientists have been developing a yellow sticky trap with a natural brassica attractant to lure the beetles, as part of a DEFRA Link project.

“We are hopeful the field traps we have designed will give a true representation of numbers and be cost-effective to use. We are also close to validating a German risk assessment program called ProPlant, which is helping other European growers decide when it is necessary to monitor their crops and whether or not to use an insecticide.”

ProPlant should allow growers to minimise variable costs and environmental impact, as it predicts the start and the end of beetle migration into the crop, as well as the peak. It promises to go a step beyond the Crop Monitor decision-support system, which can only alert growers to the risk based on temperature and growth stage of the crop.

While resistance is less severe in the UK than in other parts of Europe, it is becoming more widespread, according to work by the Rothamsted insecticide resistance group. Scientists don’t yet understand how the pyrethroid-resistant populations are spreading. They were limited to the eastern counties in 2009, but that is no longer the case, warns Dr Cook.

“Routinely applying pyrethroid insecticides can only worsen the situation,” she adds. “Until we have traps and ProPlant to assist us, carefully monitor crops for beetles and spray only if you can find an average of 15 per plant at the green-to-yellow bud stage. Stop once the crop is flowering because plants can, thereafter, compensate well for feeding damage.”

Alternative overcomes growing pyrethroid resistance problem

The cold winter has not only increased the probability of greater numbers of pollen beetles this spring, but it has slowed the development of oilseed rape crops, which could make them more susceptible to feeding damage, warns Syngenta’s oilseed rape manager, Gary Jobling.

“In a more normal season, the beetles start to come in during the green-to-yellow bud stage. However, most invade when the crop is in flower and can generally withstand attack. This season, with the predicted later flowering, crops will be at greater risk.”

Controlling the pest may also be more difficult with the recent spread of pyrethroid resistance, he stresses. More than 60% of populations tested in 2010 showed some degree of resistance.

This season, growers have a new alternative to the neonicotinoid, thiacloprid (Biscaya), explains Mr Jobling. “Plenum (pymetrozine) offers control of all pollen beetle populations, including those fully resistant to pyrethroids. It will prove hugely advantageous at green-to-yellow bud because growers can reserve pyrethroids to target pests such as pod weevil and pod midge later in the season.”

It works by contact action in OSR, so application techniques must achieve good coverage of beetles within the flower buds between green and yellow bud. The recommended rate is 0.15kg/ha.

Take part

If you are growing oilseed rape in the North East, the West Country or Kent, and would be willing to place pollen beetle traps in your crop to further the research, please contact Sam Cook as soon as possible sam.cook@bbsrc.ac.uk