Laboratory investigation into the development of resistance of Daphnia magna to the herbicide molinate.

Abstract Daphnia magna (F 0 generation) was exposed to different sublethal molinate concentrations (0, 3.77, 4.71, 6.28, 9.42, and 18.85 mg/L) during 21 days. Chronic toxicity tests, using the same herbicide concentrations, were also carried out during 21 days using neonates of F 1 first brood (F 1 -1st) and F 1 third brood (F 1 -3rd) offspring generations from the parentals (F 0 ) preexposed to the herbicide. Finally, offspring (from F 1 -1st and F 1 -3rd broods) were transferred to herbicide-free medium during a 21-day recovery period. The alga Nannochloris oculata (5×10 5  cells/mL) was used as food in all the experiments. The effect of molinate on survival, reproduction, and growth was monitored for the selected daphnid generations. The parameters used to evaluate herbicide effect on reproduction were mean total young per female, mean brood size, time to first reproduction, mean number broods per female, and intrinsic rate of natural increase ( r ). Survival and growth (body length) were also determined after 21 days of exposure to molinate. Reproduction was significantly reduced when molinate concentration was increased in the medium. This effect was higher in the parental (F 0 ) daphnids than in the F 1 -1st and F 1 -3rd offspring. In the recovery study, reproduction was still reduced in F 1 generation daphnids (1st and 3rd), but only in those animals from parentals exposed to the highest molinate concentrations. The decreased with increasing concentrations of molinate in daphnids from the parental generation (F 0 ). Significant differences were also found in daphnids from the F 1 generation (exposure). The growth of the exposed organisms (F 0 and F 1 ) decreased, although the greatest decrease was found in the parental animals (F 0 ) (25%) exposed to 9.42 mg/L molinate. F 1 daphnids (1st and 3rd broods) from the recovery period did not show any significant difference in their growth after 21 days of study. Finally, survival was not affected after exposure to the selected molinate concentrations except in those daphnids from the F 0 generation in which survival decreased 51% and 78% at the highest herbicide concentrations tested (9.42 and 18.85 mg/L). Our results suggest that the offspring daphnids seem to be adaptated to the herbicide molinate, showing more longevity and reproduction than their parentals.