Effect of emamectin benzoate on the molt cycle of ovigerous American lobsters Homarus americanus is influenced by the dosing regimen

Emamectin benzoate, the active ingredient in SLICE", targets the nervous system of arthropods. Ingestion of high doses causes ovigerous American lobsters Homarus americanus to molt before their eggs hatch. To determine the effect of repeated exposure to doses at or below the no-observed-effect level (NOEL) of a single exposure (NOEL: 0.12 mu g g-1), ovigerous lobsters were given either a single dose of 0.5 mu g g-1 or a succession of lower doses at 14 d intervals that provided similar cumulative exposures (0.06 mu g g-1 t 8, 0.125 mu g g-1 t 4, or 0.25 mu g g-1 t 2). The study was conducted between July 2003 and September 2004. Groups repeatedly administered doses of 0.06 and 0.125 µg g-1 had higher rates of premolt induction (87 and 89%, respectively) than groups given 1 or 2 higher doses (35%). Lobsters in the 8- and 4-dose groups had difficulty molting, and a significant proportion died at molt stages D3 to B (35 and 30%, respectively, versus 0 and 5% in the 1- and 2-dose groups). The results demonstrate that repeated exposure to emamectin benzoate (1) produces effects not seen when an equivalent quantity of the drug is administered in 1 or 2 doses, and (2) causes a dose less than the single-dose NOEL to become a very potent dose. However, the potential risk of emamectin benzoate to lobsters foraging near salmon farms cannot be determined from these results because the cumulative dose is greater than lobsters are likely to acquire. Further work is needed using lower, more realistic doses.