Relying solely on neonicotinoid insecticidal seed treatments, such as Gaucho, in sugar beet will almost inevitably result in resistance developing to the chemical class in aphids, particularly as they become more widely available for use in other crops.

The warning came from Broom’s Barn entomologist Alan Dewar at the Association of Applied Biologists conference in Cambridge.

“It is only a matter of time,” he said.

Growers were relying heavily on Gaucho because pesticides in other chemical groups had been withdrawn or had already developed resistance and with good results, he said.

“Control levels have been very high, especially with foliar pests, with recorded yield improvements of up to 28%.

It is no wonder they’re so popular.”

But, while there had been no indications of any control failures in sugar beet so far, registration of neonicotinoid products in
other crops would only increase selection pressure and the likelihood of resistance developing, he suggested.

“Resistance to imidacloprid has already developed in other insects, such as whiteflies, Colorado beetle and brown plant hoppers.

There is no reason why it couldn’t happen in aphids.”

It meant growers could not afford to be complacent.

“There is a possible solution.

An earlier forecasting system for virus yellows in sugar beet has proved to be accurate and robust, which means in a low risk year, there would be no need to use a seed treatment.”

That would both save growers money and reduce the risk of resistance developing, he said.

Unfortunately, the current system of growers ordering their seed in the summer before use made its use impossible, practically.

“Both the forecasting system and the current practices of ordering and treating seed needs to be modified.

“The seed industry needs to change.

It should look at the possibility of supplying seed treatments just before the seed is sown, in response to the risk,” he said.

Other neonicotinoid seed treatments, such as Bayer CropScience’s Poncho Beta launched last year and Syngenta’s thiamethoxam-based treatment due for introduction later this year, were coming onto the market, which would help challenge Gaucho’s worldwide dominance, he noted.

“74% of sugar beet crops were treated with Gaucho last year.”

There is not much difference between the three, he said.

“All of them give reliable control of most soil pests, although none have activity against leatherjackets or beet cyst nematode.

“And they all give good control of foliar pests such as leaf miners and aphids, but are not effective against late season pests such as spider mites and caterpillars.”

Other similarities included little or no effect on non-target organisms and compatibility with carbamate granules.

“Where they differ is in dose rates and their interaction with lenacil; both imidacloprid and thiamethoxam have adverse interactions when lenacil is applied pre-emergence.

“But, pre-emergence spraying of lenacil very rarely occurs in the UK, so it is largely irrelevant.”

fwarable@rbi.co.uk