Effect of a Feed-Through Insecticide (Imidacloprid) on the Flea Index of a Norway Rat (Rattus norvegicus) Focus in Los Angeles, California

Norway rats and their fleas are associated with transmitting diseases, such as murine typhus and plague, to humans. The use of rodenticide baits may increase human exposure to rodent fleas due to loss of their preferred host. Therefore, flea control is an important component of risk reduction. However, the use of insecticide powder or spray can be difficult in certain situations. In this field study we used a feed-through insecticide, imidacloprid, in a grain-based pellet formulation without a rodenticide to determine the effect on the flea index of a well defined, problematic Norway rat focus in a dense urban area of Los Angeles. The flea index was determined to be 12.9 (116 fleas/9 rats) 5 days prior to applying the product. Rats were allowed to feed on the pellets ad libitum for 48 hours, in the presence of previously existing competing food sources. Flea counts were then taken, and again 7 days later, resulting in flea indices of 2.3 (25/11) and 1.5 (23/15), respectively. Since this field trial was conducted in a very dynamic urban area, attempts to survey a concurrent control group were not successful. However, Norway rat surveys conducted in the previous 4 years from the same urban area for the same months, July and August, produced flea indices of 7.4 and 10.1, respectively. In this field trial, we found that a rodent bait formulation containing imidacloprid reduced the flea index of a Norway rat focus from 12.9 to 2.3 (82%) in 48 hours, and to 1.5 (88%) 7 days later