Dosage reduction to improve the selectivity of deltamethrin between aphids and coccinellids in cereals.

Abstract

The toxicity of the pyrethroid insecticide deltamethrin to a cereal aphid and a coccinellid beetle predator was assessed. Deltamethrin gave effective aphid control in winter wheat at dose-rates of 6.25, 3.13 and 1.56 g a.i./ha. The direct exposure of adult Coccinella septempunctata L. (Coleoptera: Coccinellidae) to spray drops was estimated at a range of positions in a cereal crop canopy from volumetric analysis of fluorescent tracer deposits. These measurements were used to calculate exposure to deltamethrin at the three experimental dose-rates. Observations of coccinellid beetle distribution through a cereal crop canopy permitted a realistic range of direct contact doses to be calculated and the toxic effects of these levels of exposure to be predicted from laboratory dose-response data. Estimated beetle mortalities from direct exposure were 19, 8 and 3% at the three experimental dose-rates. In situ bioassays with adult C. septempunctata which exposed beetles continuously to deltamethrin residues on flag leaves, resulted in 100, 94 and 39% mortality respectively at these dose-rates during the 10 days after spray application. Additional in situ bioassays exposed beetles to deltamethrin residues on flag leaves for 24 h and then transferred surviving beetles to the soil under the cereal crop canopy for a further 9 days. This resulted in 89, 69 and 29% beetle mortality respectively at the three dose-rates. Mortality predictions combining both direct contact and residual exposure were made for the three dose-rates to determine the maximum impact of summer sprays of deltamethrin on adult coccinellid populations in cereals. These worst case predictions suggested that a reduction in dose-rate by as much as three quarters of the recommended application rate in UK cereals may be necessary to preserve approximately 60% of adult C. septempunctata in the crop over the 10 days after a deltamethrin spray application. The methodology described may be appropriate for estimating selective dose-rates for key enemies in a range of crops.