Warmth will trigger pollen beetle alert
With some forward oilseed rape crops at the susceptible stage, growers are urged to be on high alert for pollen beetle attack when temperatures rise.
Oilseed rape yields can be decimated when the beetles migrate into the crop to feed and lay eggs at the green- to yellow-bud stage, biting into and killing buds.
After the buds begin to flower they no longer cause economic damage, as pollen is freely available to the adult beetles.
Although the temperatures need to be about 15C for migration to start in earnest, there has been some movement in southern counties according to Bayer’s Pollen Beetle Predictor.
Agrii crop protection technical manager Chris Bean is concerned that if crop development is delayed it may lead to the susceptible period coinciding with peak migration.
| Active ingredients |
|---|
|
“Winter rape can escape the worst of the damage if crops come to flower rapidly, but that is unlikely to happen this season,” he says.
Mr Bean urges growers to monitor crops carefully during the susceptible stage – as soon as buds become visible through to flowering.
The new spray thresholds introduced this season should be used and product choice will be particularly important, with pyrethroid insecticide resistance now widespread.
“Pyrethroids continue to be effective in some areas, but from the eastern seaboard in Kent up to the Scottish Borders repeat pyrethroid sprays are having no effect on pollen beetles,” explains Mr Bean.
With the new thresholds designed to cut the unnecessary spraying that contributed to the resistance problem, Mr Bean advises growers not to spray unless they are exceeded.
However, this year, more than most, it will be vital to protect what is left of the most backward, thin crops from further pest attack.
“A pyrethroid such as Alert at full label rate can remain the first line of defence, unless there are known resistance concerns,” says Mr Bean.
“It’s also important to remember not to spray pyrethroids in overcast conditions, as pollen beetles can virtually disappear if it clouds over and we are relying on the contact activity of the chemistry,” he adds.
Where there are resistant populations, Mr Bean advises that Mavrik, although a pyrethroid, may still be effective because it works on a slightly different target pathway to Alert.
However, Frontier technical support manager Brian Ross would seek an entirely different mode of action where resistance is suspected.
“Steward or Plenum offer good alternatives to overcome the pyrethroid resistance issue and you can also use Biscaya right through the susceptible period,” Mr Ross points out.
However, Mr Bean is cautious about using the neonicotinoid Biscaya so close to flowering following the recent concerns over bee health.
“Steward offers quicker knockdown and is effective for 10 days, which can be valuable during uncertain weather.
“It also has the advantage of being relatively benign to the parasitic wasps that feed on the pollen beetle larvae, which help to reduce future populations,” adds Mr Bean.
Spring rape
Looking ahead, Mr Ross is more concerned about the potential damage to spring oilseed rape crops, with the area set to rise dramatically this season.
“The Bayer Pollen Beetle Predictor has been very useful for indicating migration into winter rape, but won’t help with regards to spring rape,” he explains.
He advises growers to check crops regularly and use yellow sticky traps to monitor local migration from winter to spring rape and treat according to thresholds.
