BULK DEPOSITION OF PESTICIDES IN A CANADIAN CITY: PART 2. IMPACT OF MALATHION USE WITHIN CITY-LIMITS
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Malathion is an organophosphate insecticide registered for use in cities throughout North America to control adult mosquitoes. The objective of this study was to determine the impact of urban malathion applications on the levels of malathion detected in bulk deposition. In 2010, malathion was applied by the City of Winnipeg’s Insect Control Branch for a total amount of 6,632 kg in the city, as well as by the general public in relatively small amounts. In 2011, no malathion was applied by the city. Malathion was detected in 41% of the samples in 2010 with deposition rates ranging from 0.5 to 107.7 μg/m2/week. Only 9% of the samples contained malathion in 2011 with deposition rates always being < 0.4 μg/m2/week. Between 6 to 25% of the samples in 2010 exceeded the toxicological threshold levels of malathion to a range of freshwater amphipods, water fleas and stoneflies, including Daphnia magna which is a bioindicator of good environmental health. The weekly maximum malathion concentration detected in this study (5.2 μg/L for a week in June 2010) was at least 26 times greater than the maximum concentration of malathion reported in other atmospheric deposition studies. For the two insect management areas (7.4 and 37.6 km2) where the bulk deposition samplers had been placed, calculations suggested that between 1.2 to 5.1% of the malathion applied by the city became bulk deposition. Percutaneous absorption by humans of malathion in rainfall is unknown.