Effects of insecticide exposure on feeding inhibition in Mayflies and Oligochaetes

The authors evaluated toxicological endpoints and feeding inhibition of Epeorus longimanus mayfly larvae and Lumbriculus variegatus blackworms exposed to pulsed imidacloprid exposures. Imidacloprid concentrations were mixed from the formulated product Admire (imidacloprid). Lethality and immobility tests on the mayflies and blackworms were conducted at 0 – 240 µg/L for 24 and 96 hours. Sublethal tests used 0 – 10 µg/L imidacloprid. The 24h LC50 for mayflies was 2.1 µg/L and the 96h LC50 was 0.65 µg/L. The blackworms were an order of magnitude less sensitive, with a 96h immobility EC50 of 6.2 µg/L. Feeding inhibition was measured by exposing mayflies to 24 hours of imidacloprid and then moving them into an artificial stream to monitor feeding rates. Early instar mayfly feeding was significantly decreased at a concentration of 5 µg/L and no feeding was observed at 10 µg/L because all the mayflies in those replicates died. Later instar mayflies also reduced feeding at the 5 and 10 µg/L concentrations. In the replicates with 0.5 and 1 µg/L, the later instar mayflies increased their feeding rates, suggesting that increased feeding activity could be an adaptive response to low-dose imidacloprid exposure. After the 24h exposure, the mayflies were monitored for four recovery days, and only the 0.1 µg/L treatment group recovered to the control feeding rate. This suggests ongoing sublethal effects for mayflies exposed to imidacloprid that are dependent on the imidacloprid concentration and the time since exposure. Results showed reductions in feeding rate, adult abundance, and body size when mayfly larvae were exposed to low doses of imidacloprid.