Oilseed growers warned against insecticide misuse

Oilseed rape growers wanting to control aphids migrating into crops this autumn are being warned against misuse of the neonicotinoid insecticide Biscaya (thiacloprid).

Having become aware that some advisers are considering this tactic, Rothamsted Research insecticide resistance specialist Dr Steve Foster points out that Biscaya is approved for control of pollen beetle at the green bud stage but not aphids in the preceding autumn.

"Most winter oilseed rape crops are now grown from seed protected by one of the second generation neonicotinoid seed treatments – clothianidin in Modesto or thiamethoxam in Cruiser OSR – so for resistance management a product with a different mode of action must be used for the first subsequent foliar spray.

"Applying another neonicotinoid at this point would favour selection pressure and that’s why it’s ruled out," Dr Foster explains. Furthermore, Broom’s Barn’s Dr Mark Stevens adds that the one Biscaya application permitted per crop should be saved for pollen beetle control, particularly in the South and East where there is a high risk of this pest developing pyrethroid resistance.

On the need for aphicide sprays to oilseed rape crops this autumn he advises growers to think about drilling date and peach–potato aphid numbers. "Typically the second generation neonicotinoid seed treatments persist for up to two months from drilling so protection of crops drilled in the second half of August will be running out of steam around now," he says.

Based on the low number of aphids currently being found in oilseed rape crops by Broom’s Barn’s nationwide surveying, Dr Stevens doubts much follow-up aphicide spraying will be warranted this autumn but advises growers to continue monitoring crops into November and make decisions based on their individual findings.

"If a treatment is needed, pyrethroids are currently the only real option," he says. "In Rothamsted’s monitoring of aphid resistance kdr has only been found in about 20% of this year’s peach–potato aphid samples so pyrethroids should be reasonably effective and also control any cabbage stem flea beetle present. With MACE resistance now close to 100% pirimicarb will be ineffective for aphid control and should not be used."