Pyrethroid-resistant pollen beetles could become a major threat to UK oilseed rape productivity unless growers adopt sensible anti-resistance measures this season.
Resistance was first detected in the UK in 2006, but last year it had spread to most coastal areas of East Anglia and the south and it could extend much further this year, he warned. “Pollen beetles are active fliers, so once you get a problem, it will spread very quickly. We are at the start of a potentially significant problem, which could have knock-on effects for other brassicas and ornamentals.”
In many of the cases where pyrethroid-resistant beetles were found in 2007, there were relatively low numbers of “highly-resistant” individuals. “This means you’d probably still see good control from pyrethroid sprays, but you would be selecting for, and leaving behind, these highly resistant beetles, which can survive something like five-times the normal field dose of pyrethroid.”
All pyrethroids are affected by resistance, so growers should not apply one unless it is absolutely necessary, Mr Richardson stressed. “There is a tendency because of the [low] price of pyrethroids to put one in the tank when you go in with a fungicide. We’d advise not to do that.”
In many cases not spraying would not pose a problem, he added. “Even in the highest pollen beetle year, 2005, most crops only had about five pollen beetles per plant, which is below the existing threshold for winter rape anyway.”
Using a pyrethroid could also kill beneficial parasitoid insects, which attack the pollen beetle larvae after the normal pyrethroid spray timing. “You can get up to 80% control from parasitisim,” he said.
Mr Richardson hoped that getting the message to growers early, would help avoid a repeat of the situation in Germany, where over-reliance on pyrethroids and a lack of clear, unified advice meant the problem spread from a few isolated cases to most of the country in just a few years. Some 200,000ha (494,200 acres) of oilseed rape are now severely affected, with about 30,000ha (74,130 acres) destroyed each year, costing E22-25m.
At present the neo-nicotinoid Biscaya (thiacloprid) is the only non-pyrethroid form of control approved in the UK, although an OP product was in the pipeline, he noted.
Pollen beetle treatment thresholds
- Winter osr: 15 beetles per plant (five per plant backward crops)
- Spring osr: Three per plant
- Restored hybrids: Potentially more susceptible (five per plant threshold suggested), but no evidence to substantiate yet
- Pollen beetle only a threat during green bud stage
- Beetles tend to overwinter in leaf litter fairly frost-resilient
- Inspect crops at headland and midfield
- Only spray if absolutely necessary (see thresholds)
- Apply no more than one pyrethroid
- If surviving pollen beetles are found after treating with a pyrethroid, consider using thiacloprid