Comparison of the activity of selamectin, fipronil, and imidacloprid against flea larvae (Ctenocephalides felis felis) in vitro

The activity of selamectin, fipronil and imidacloprid against larval cat fleas (Ctenocephalides felis felis) was evaluated in an in vitro potency assay system. One hundred microliters of each compound at various concentrations in acetone were added to glass vials (1.5 by 3 cm) to which had been previously added 20 mg of sand and 10 mg of flea feces. Vials were then ball milled to allow the acetone to evaporate. Selamectin and fipronil were tested at 0.001, 0.003, 0.005, 0.01, 0.03, 0.05, 0.11, 0.3, and 0.5 mug of active compound per tube. Imidacloprid was tested at 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 1.0, 3.0, and 5.0 mug of active compound per tube. Thirty first instar C. felis larvae were added to each vial. The number of larvae remaining alive in each vial was determined once daily for 72 h. With selamectin, reductions of greater than or equal to93.5% were achieved at 24 h after exposure at doses of greater than or equal to0.3 mug. In contrast, at 24 h neither fipronil nor imidacloprid reached 90% reduction, even at the highest doses tested (0.5 mug for fipronil and 5.0 mug for imidacloprid). Selamectin was significantly (P less than or equal to 0.05) more potent than imidacloprid and fipronil at levels greater than or equal to0.03 mug. A similar pattern of activity was observed at both 48 and 72 h, but higher percentages of larvae were killed for each of the compounds as the incubation time increased. At 72 h selamectin was significantly (P less than or equal to 0.05) more potent than imidacloprid at levels of 0.01-0.1 mug and significantly (P less than or equal to 0.05) more potent than fipronil at levels of 0.003-0.01 mug. Therefore, selamectin was more potent than either fipronil or imidacloprid in killing flea larvae in this in vitro assay system. (C) 2003 Published by Elsevier B.V.