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() Characteristics of malathion toxicity to amphibians.
Pauli, B.1, Gallant, N. 2, Charbonneau, M.3, Trudeau, V.2, Berrill, M.3, 1 Canadian Wildlife Service, Ottawa, Ontario, Canada2 University of Ottawa, Ottawa, Ontario, Canada3 Trent University, Peterborough, Ontario, Canada
ABSTRACT- Malathion is an organophosphorus insecticide with numerous registrations including agriculture and mosquito control. Malathion can be applied by truck-mounted or aircraft-mounted sprayers as an ultra-low volume (ULV) spray. Because of the potential for aquatic contamination from this use pattern, it is important to determine the toxicity of malathion formulations to larval amphibians, particularly given the concern about the influence of contaminants in the global decline of amphibian populations. Using a standard protocol and laboratory exposures, we determined the acute toxicity of two malathion formulations and major formulation ingredients to native amphibian species as well as Xenopus spp. Static renewal exposures were conducted over 96 h with one renewal, or over 8 d with renewal every 24 h. Mortality was recorded daily, and LC50 concentrations were calculated for each species and formulation. In experiments with Fyfanon®, a formulation which is almost pure active ingredient, the insecticide was less toxic to Northern Leopard Frog (Rana pipiens) tadpoles than was Malathion 50, a formulation consisting of 50% active ingredient plus adjuvants. In fact, in 96 h experiments, the 50% formulation proved to be very toxic to R. pipiens tadpoles, causing 100% mortality at a concentration that was 100 000 times lower then the dilution suggested by the manufacturer. The LC50 was approximately 2.14 
g/L for the 50% formulation. In addition, developmental abnormalities in tails of the tadpoles, and impaired swimming ability were observed. Because these effects were observed at an environmentally relevant concentration, a careful determination of the toxicity of different malathion formulations to amphibians, and the influence of specific formulation ingredients, should be completed. This is especially the case given the possibly increased use of this compound for mosquito control. Supported by Canadian Network of Toxicology Centres, Environment Canada, and NSERC.
Key words: amphibians, malathion, formulations, Xenopus