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

Abstract

AbstractThe 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.