Field and laboratory toxicity tests were carried out, to evaluate the effects of atrazine on population abundance, distribution pattern, fecundity and instar duration of Entomobrya musatica (Collembola). In field experiments atrazine treatments at doses of 1.6 kg a.i. ha(-1), 4.0 kg a.i. ha(-1) and 6.3 kg a.i. ha(-1) were adopted and Collembola densities were estimated four times during 1 year after treatment. A significant effect on the abundance of Collembola in the 4.0 kg a.i. ha(-1) treated plot was observed. However, at a dose of 1.6 kg a.i. ha(-1), intensive vertical migration was recorded. In the plot treated with 6.3 kg a.i. ha(-1), population abundance was severely decreased. In an atrazine-free plot, E. musatica showed a high tendency of less frequent spatial aggregation. The application of atrazine had a marked influence on the aggregation indices. Laboratory toxicity tests using different atrazine concentrations (0.58, 1.46 and 2.33 mg a.i. g(-1) fresh weight of food) were conducted and observations on moulting, growth and egg production were made for a period of 2 months. A slight effect, at the dose of 0.58 mg a.i. g(-1) fresh weight, on fecundity was observed but no significant effect of that dose on instar duration period could be detected. Fecundity and instar duration period were clearly affected as the atrazine dose was increased to 1.46 mg a.i. g(-1). Moreover, at the dose 2.33 mg a.i. g(-1), a marked negative effect on egg production and the duration length of instars was shown. Extrapolation of data between field populations and laboratory conditions are discussed. It is concluded that the atrazine treatment of 4.0 kg a.i. ha(-1) has an adverse effect on the abundance and the development of the test species.
