March 21, 2013
Pollen Beetle Alert
A large area of backward oilseed rape and the continued spread of pyrethroid resistance mean pollen beetle could prove a particular challenge for many this season.
“Crops can be more susceptible to damage if the weather is dry and warm at the green/yellow bud stage” explains Agrii crop protection technical manager, Chris Bean.
“Winter OSR can escape the worst of the damage if crops come to flower rapidly but where development is delayed – as it is in many crops this season – the period of peak susceptibility is more likely to coincide with beetle migration.
“We’re also seeing more spring rape this year as growers fill land left bare by the wet winter, so this is likely to add to the summer risk. A warm spell coinciding with bud development could see a lot of crops with relatively high beetle numbers, even by new threshold guideline standards.
“Pyrethroids continue to be effective in some areas,” he continues. “However, there’s a big swath of the eastern seaboard from Kent right up to the Scottish Borders where repeated sprays are having no effect on pollen beetles. We can find resistance in most places if we look for it these days. It’s nowhere near as bad as the continent. But resistant populations have been found as far inland as Shropshire and they’re becoming a real problem for many.”
Under these circumstances, Chris Bean advises careful monitoring once flower buds become visible through to the beginning of flowering. After this the beetles pose no risk as they have free access to pollen so no longer have to bite through the buds to feed.
He strongly recommends spraying once threshold levels have been reached and urges particular care in both chemical choice and application practice.
“We don’t want to spray unless we need to,” he insists. “Equally this season, more than most, we don’t want to sacrifice any more of our winter OSR potential to pests. And the last thing we want to do is go through the crop two or three times and have no effect.”
As it’s so difficult to be sure if resistance has developed, Chris Bean suggests pyrethroids like Alert (alpha-cypermethrin) at full label rate can remain the first line of defence unless there are particular concerns. Where resistance is suspected, he sees Mavrik
(tau-fluvalinate) as a valuable alternative. Although a pyrethroid, its different activity pathway means it has continued to retain good efficacy where others have not, while delivering quick, cost-effective knockdown.
“Because beetles all over the place on a sunny day can virtually disappear if it clouds over, it’s important not spray a pyrethroid when conditions are overcast,” he warns. “After all, we’re relying on good contact activity.”
Cautious about using neonicotinoids so close to flowering, Chris Bean prefers Steward (indoxacarb) or Plenum (pymetrozine) where resistance is a problem. Both are highly effective against resistant beetles, with Steward offering quicker knockdown and a residuality of up to 10 days – which can be particularly valuable under uncertain weather conditions.
“Like Mavrik, Steward has the extra advantage of being relatively benign to the parasitic wasps that feed on pollen beetle larvae, helping to restrict future populations,” he adds.