Large-scale monitoring of imidacloprid susceptibility in the cat flea, Ctenocephalides felis

Abstract

Abstract. Although on-animal topical treatment with compounds such as imidacloprid has revolutionized the control of the cat flea, Ctenocephalides felis (Bouché) (Siphonaptera: Pulicidae), the development of insecticide resistance is a continuing threat. As part of a highly co-ordinated and unprecedented resistance monitoring programme for C. felis, 1437 flea isolates were collected by veterinary clinics in Australia, Germany, France, the U.K. and 29 states in the U.S.A. from 2002 to 2009. About 65% of the collections were made from June to October each year and 71% of the collections were from cats. Collections of flea eggs were sent to one of five different laboratories, where they were tested with a diagnostic dose of imidacloprid (3 p.p.m.) applied to larval flea-rearing medium. Of the 1437 collections received, 1064 contained adequate numbers of eggs for testing. Of these isolates, untreated eggs failed to hatch in 22.7% and were not considered valid bioassays. Survival rates >5% and development of adult fleas (a threshold for further testing) occurred in only 22 isolates. They were re-tested with the same diagnostic dose and none produced >5% adult emergence. Complete dose-response bioassays were performed on three of the isolates that had triggered a second test and produced slopes, intercepts and LC 50 values similar to those for existing susceptible laboratory strains. Results confirmed sustained susceptibility of C. felis to imidacloprid, despite its widespread use for over a decade.

Susceptibility of Adult Cat Fleas (Siphonaptera: Pulicidae) to Insecticides and Status of Insecticide Resistance Mutations at the Rdl and Knockdown Resistance Loci

“…The susceptibility of field-collected isolates of C. felis from Europe, Australia and the USA to imidacloprid has been investigated since 2002 by exposing larvae to a diagnostic concentration of insecticide in a diet-impregnation bioassay (Rust et al 2002(Rust et al , 2005(Rust et al , 2011Schroeder et al, 2003). To date, testing of over 100,000 individuals from over 1500 isolates has disclosed no evidence for a directional shift in response or a decline is susceptibility (Rust et al, 2011;Kopp et al 2013).…”

“…To date, testing of over 100,000 individuals from over 1500 isolates has disclosed no evidence for a directional shift in response or a decline is susceptibility (Rust et al, 2011;Kopp et al 2013). However, it could be argued that over-reliance on a larval bioassay, despite its operational advantages for resistance monitoring, could disguise the emergence of a resistance phenotype expressed only in adults.…”

“…Monitoring for changes in response requires access to sensitive bioassays appropriate to the chemicals and species under consideration. For example, to provide scientifically sound data on responses of the cat flea (Ctenocephalides felis) to the insecticide imidacloprid, a larval bioassay was developed and evaluated, and field-collected isolates from three continents have been tested since 2002 (Blagburn et al 2006;Rust et al 2002Rust et al , 2005Rust et al , 2011.…”

Monitoring Field Susceptibility to Imidacloprid in the Cat Flea: A World-First Initiative Twelve Years on

“…Crucially, none of these isolates reached or exceeded the 5 % survival threshold on repeat testing, hence they could not be characterised as possessing field resistance to imidacloprid. As commented an earlier review of this initiative (Rust et al 2011), it is likely that initial survivorship in the DD bioassay that cannot be replicated represents a chance finding, and not the systematic development of resistance in that population. It is worth reflecting upon data pertaining to the flea isolate submission process itself, since this can provide insight into seasonal prevalence of C. felis.…”

“…These isolates were further propagated on cats, and eggs were collected for use in full-dose response bioassays, conduct of which has been previously described (Rust et al 2011). Briefly, larval rearing medium was treated to achieve a gradient of nine imidacloprid concentrations (0.005, 0.01, 0.05, 0.1, 0.3, 0.5, 1, 1.5 and 3 ppm).…”