A laboratory study of the effects of some soil-applied organophosphorus pesticides on Carabidae (Coleoptera)

Abstract

Time/mortality studies with Carabidae exposed to thionazin, phorate, disulfoton and menazon in an acid sandy loam soil were done in controlled laboratory conditions. Carabids were killed by soil treated with thionazin within dosage rates normally required for satisfactory control of nematode or insect pests of crops, i.e., 2·24–8·96 kg/ha, but menazon was virtually non-toxic. Soil moisture increased the speed of kill of Carabidae in thionazin-treated soil presumably by affecting processes of adsorption and desorption of the pesticide to soil colloids and by influencing burrowing behaviour. The speed of kill increased with a rise in temperature, mortalities occurring 2·2 × faster at 10°C, 8·1 × faster at 15°C and 11·8 × faster at 22°C than at 7°C, but was decreased by illumination which decreased the activity of the nocturnal Carabids. The addition of calcium carbonate to increase soil pH did not alter immediate mortality by thionazin but persistence was decreased. Speed of kill was negatively correlated with adult size of Carabid species that behaved similarly, small species such as Bembidion lampros (Hbst.) (mean weight 2·1–2·2 mg) dying 12–13 × faster than female adults of Pterostichus vulgaris (L.) (mean weight 195 mg). Species which burrowed in soil were more susceptible than those which did not, as were newly moulted or starved adults. At 15°C and in wet conditions the “ half-life ” of thionazin based on speed of kill in a soil of pH 6·1–6·7 was 1–2 weeks but when determined by gas-liquid chromatography was 3–4 weeks at 22°C. Leaching could account for the loss of up to 12% of thionazin from the top 9 cm of soil.