Deltamethrin lethal multifactorial activity toward carp larva related to pH, calcium, and humic acid concentrations

Acute 24-hr embryotoxicity of the pyrethroid insecticide delta-methrin toward carp larvae was tested in multifactorial combinations of three pHs (6.9, 7.8, 9.0), two calcium concentrations ([Ca] = 2 x 10(-4) and 2 x 10(-2) M), and three humic acids concentrations ([HA] = 0, 5, and 100 mg/liter). Lethal effects were determined and compared to the distribution and hexane extractibility of radiolabeled deltamethrin in the solution and on the vessel walls, either in the presence or in the absence of the larvae. As a function of the three combined factors tested, the no observed effect concentrations (NOECs) differed to a maximum by a factor of 8, while they differed by only a factor of 2 with pH alone, and by a factor of 4 with calcium or humic acids alone. No contact toxicity was observed with adsorbed deltamethrin on the vessel walls, on which adsorption and hexane extractibility was increased in the presence of larvae. The most plausible interpretation for the toxicological influence of the physicochemical factors tested on deltamethrin toxicity deals with the deltamethrin distribution in the medium for humic acids, changes in biological targets for calcium influence, and, for pH influence, degradation speed or one of the two other explanations.