The phytotoxicity of insecticidal seed coatings and granular and spray soil treatments to grain yields in field experiments on the control of larvae of wheat bulb fly (Delia coarctata Fall.)

A simple statistical procedure has been derived to measure the direct effect of insecticides, particularly any phytotoxic effect, on cereal grain yields. Data from several years' field experiments on the use of insecticides to control larvae of the wheat bulb fly (Delia coarctata Fall.) (Diptera: Anthomyiidae) have been used to determine the ratio of (grain yield ÷ percentrage of undamaged shoots) for plots treated with an insecticide to (grain yield ÷ percentage of undamaged shoots) for untreated plots. A ratio < 1 indicated that the insecticide was phytotoxic; a ratio > 1 suggested that the compound was phytostimulant. The ratios of all the insecticidal seed coatings and soil treatments of granules and sprays which were tested in the field experiments have been measured. The ratios for all insecticides (apart from fonofos microencapsulated seed coating at one site in 1974–1975) were <1. The phytotoxicity of the insecticides was found to vary between years. Insecticidal soil treatments (granules and sprays) seemed to be as phytotoxic as seed coatings. Synthetic pyrethroid seed coatings were slightly less phytotoxic than organophosphorus seed coatings. In field experiments on the control of wheat bulb fly larvae by insecticides, phytotoxicity has usually been estimated by a subjective system of scoring plots visually before larval invasion of plants. This method was compared with the objective method of determining pesticide/untreated (or P/U) ratios. Visual scoring appears to be the more efficient method of determining the most phytotoxic insecticide, whereas the least phytotoxic insecticide is determined more accurately by the method of P/U ratios.