Effect of malathion on the demography of Daphnia pulex Leydig and Diaphanosoma birgei Korinek (Cladocera).

Malathion is a common pesticide used to control insects in agricultural, domestic and industrial sectors in different parts of the world. In this work we evaluated the effects of sublethal concentrations of malathion on the survivorship and reproductive variables of two cladoceran species Daphnia pulex and Diaphanosoma birgei using standard life table demography method. Based on preliminary tests, we selected four sublethal concentrations of malathion for each cladoceran species. For D. pulex, the malathion concentrations were 0. 225, 0.45, 0.9 and 1.8ng I(-1) and for D. birgei, these were 0.0281, 0.0562, 0.1125, 0.225ng I(-1). Our results showed that in general, Daphnia pulex was less sensitive than Diaphanosoma birgei to malathion. The average lifespan of Daphnia pulex in controls was about 19 days while under similar conditions, that of D. birgei was about 21 days. For either cladoceran species, increased pesticide concentration resulted in decreased survival; this was more evident in the treatment containing the highest concentration of malathion (0.225 ng l(-1)) for D. birgei. Fecundity of D. pulex and D. birgei also decreased with increase in the concentration of malathion. For a given cladoceran species, compared to controls, the gross reproductive, net reproductive rates and the population growth rate significantly decreased due to malathion. Compared to the survivorship variables (age-specific survival, life expectancy and average lifespan), the reproductive parameters (gross reproductive rate, net reproductive rate and the rate of population increase) of the two cladoceran species were decreased by the pesticide.