Abstract

A lobster die-off reduced the 1999 fall landings in western Long Island Sound by more than 99 percent. The die-off corresponded in time with the application of pesticides for the control of mosquitoes that carried West Nile virus, at that time an emerging disease in North America. In order to determine the possible implication of pesticide application as a direct cause or contributing factor in the die-off, we studied the effects of experimental exposure to malathion on the health of lobsters. Lobsters were exposed in 20-gallon tanks, and the direct toxicity as well as sub-lethal effects on the immune system were determined. The 96 hour LC50 for malathion upon single exposure was 33.5 mg/L. Malathion degraded rapidly in our system, with 65-77 percent lost after one day and 83-96 percent lost after three days. Phagocytosis was significantly decreased three days after a single exposure to water concentration as low as 5 ppb, when water concentrations were as low as 0.55 ppb. Phagocytosis was also decreased one, two and three weeks after weekly exposure. Cell counts did not differ significantly upon exposure to malathion. Malathion was not detected in muscle or hepatopancreas of exposed lobsters. Evaluation of phagocytosis is a sensitive indicator of subtle sub-lethal effects of malathion, and relatively small concentrations of malathion (6-7 times lower than the LC50) can affect lobsters defense mechanisms.