Studies on the toxicity of insecticidal drift to the first instar larvae of the Large White butterfly Pieris brassicae (Lepidoptera: Pieridae)

Abstract

SummaryThe toxicity (LD,) of eight insecticides to 2–day‐old first instar larvae of Pieris brassicue was determined by topical application. On this basis, the insecticides could be ranked in ascending order viz. dimethoate > pirimicarb > phosalone > endosulfan > fenitrothion > pirimiphos‐methyl > fenvalerate > diflubenzuron. Relative to endosulfan, dimethoate and pirimicarb were 30–25 times less toxic while at the other extreme, fenvalerate (x 12) and diflubenzuron (x 26) were much more toxic to the larvae of P. brassicae.A methodology was developed for examining the effects of spray drift through bioassay using these 2–day‐old larvae. Six field trials with commercial formulations were carried out in which variables other than the insecticide and wind speed were held constant as far as possible. Phosalone (700 g active ingredient ha) produced no effect beyond 1 m downwind at low wind speed (2 m s); at high wind speed (4 m s‐1). larval mortality (5.3%) was recorded up to 2m. Fenitrothion (1050 g a.i. ha‐1) at moderate wind speed (3 m s‐l) caused mortality up to 4 m while fenvalerate (30 g a.i. ha‐l) at the same wind speed affected larvae up to a distance of 8 m. Diflubenzuron at a very low dosage (6.5 g a.i. ha‐1) also produced effects up to 8 m. At the maximum approved dosage (100 g a.i. ha‐1), and at high wind speed (4.25 m s‐l), diflubenzuron killed 8.4% of the larvae at 16 m distance.At wind speeds of 2 ‐ 3.5 m s‐1, spray deposits on the upper surfaces of water sensitive papers were high at 0 m and declined exponentially with distance. At higher wind speeds (4 ‐ 4.25 m s‐I) turbulence produced irregular deposits. Deposition on lower surfaces was much lower than on upper surfaces but the trends with distance and wind speed were the same.The exponential relationships between mortality and distance, and drift deposition and distance, were examined. The results are discussed with respect to laboratory toxicities, application rates, wind speeds and ecological factors.