Comparative toxicity of two herbicides, molinate and thiobencarb, to Brachionus calyciflorus

Acute and chronic toxicity tests with the rice herbicides molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) and thiobencarb (S-4-chlorobenzyl diethyl thiocarbamate) were conducted on the rotifer Brachionus calyciflorus. The 24-hr static LC50's were 11.37 and 47.82 mg.L-1 for molinate and thiobencarb, respectively. The effects of sublethal levels of both herbicides on the rotifer B. calyciflorus were evaluated using some demographic parameters: intrinsic rate of natural increase (r), generation time (T) and net reproductive rate (Ro). All the parameters studied decreased with increasing toxicant concentrations. The maximum acceptable toxicant concentration (MATC) was calculated for both herbicides using the intrisic rate of natural increase (r) as parameter of evaluation and the chronic data was used to calculate aplication factors (AF) of 0.062 and 0.066 mg.L-1 for molinate and thiobencarb, respectively. Finally, the EC50 values were derived for the selected life history parameters (Ro, T, r) of B. calyciflorus after exposure to both herbicides.