Deltamethrin multifactorial activity toward carp larva mobility related to calcium, humic acids, and pH

The toxicity of pyrethroid insecticide deltamethrin was assessed by the "escaping capability" of carp larvae out of a trap. Mobility was tested after 1, 4, and 12 hr exposure to eight deltamethrin concentrations in standard water and after 24 hr exposure to seven deltamethrin concentrations in 18 media derived from the combinations of pH levels of 6.9, 7.8, 9.0, calcium concentrations of 2 . 10(-4) and 2 . 10(-2) M, and humic acid concentrations on 0, 5 and 100 mg/liter. In standard water, a 1-hr exposure at 4 microg/liter deltamethrin increased the mobility, while a 4-hr 32 microg/liter exposure decreased it. After 24 hr without deltamethrin, mobility was reduced at pH 6.9 and 7.8 and 2 . 10(-4) M calcium. It was also reduced in 100 mg/liter humic acids, especially when the former pH and calcium conditions were used. Humic acid effects could partly result from a calcium concentration reduction in water, and darkness due to humic acid coloration could play a minor role. Increasing humic acid concentration, calcium concentration, and pH reduced deltamethrin activity. In 0 or 5 mg/liter humic acids the No Observed Effect Concentration was 1 microg/liter, and in 100 mg/liter humic acid it was 2 microg/liter. Compared to previous results on deltamethrin-induced lethality, the escape test appeared less reproducible, but was 2 or 4 times more sensitive.