Studies on the persistence and effects on soil fauna of some soil-applied systemic insecticides

Granular formulations of menazon, phorate and thionazin were applied to a sandy loam soil (pH = 6·1)in April and May as in-row treatments at commercial rates. They were also broadcast and mixed into the top 4 in of soil at concentrations (10 and 250 ppm of dry soil) which simulated the local in-row concentrations in areas large enough for sampling. Bioassays showed that 50 % of phorate, thionazin and menazon equivalents had disappeared in about 68, 57 and 23 days respectively from soil treated with 10 ppm of the insecticides. Small residues of the 250 ppm treatments still remained 2 years later. The initial rate of loss of activity of thionazin applied at 250 ppm was much slower than from a 10 ppm application whereas the activities of 250 and 10 ppm of menazon disappeared at similar rates. Phorate at 250 ppm killed almost all earthworms, Collembola, Acarina, free-living saprophytic and parasitic nematodes and Protozoa; 10 ppm phorate and 250 ppm of menazon also killed almost all Collembola and Acarina but the menazon was relatively harmless to earthworms. Collembola and mite populations began to increase when residues of phorate and menazon equivalents had decreased to about 2 and 20 ppm respectively, and after 15 months they were similar to those in untreated plots. The broadcast treatments initially decreased the rate of breakdown of leaf discs put in the soil but, after about 4 weeks, breakdown increased, sometimes above that in untreated plots. This was associated with a large increase in numbers of Enchytraeidae which were apparently unaffected by the insecticides. Four months after application, 1 lb/acre of phorate and 2 lb of menazon applied in seed drills 2 ft apart were not affecting Collembola in soil between the rows but were still decreasing the numbers within them. In -row phorate distributed along a 1·5 in diameter band of soil, 3 in deep, killed Collembola 3 in on either side but not 6 in away. It did not spread upwards in toxic quantities, but after rainfall, sufficient to kill Collembola leached at least 3 in downwards. We conclude that commercial in-row applications of chemicals like phorate are most unlikely to harm soil fertility, especially as the leaf-litter-destroying function of Collembola and other animals killed by phorate may be taken over by Enchytraeidae.