Biological Responses and Electrophysiological Effects of Topical Application of Deltamethrin on a Resistant and a Susceptible Strain of Drosophila melanogaster Meig.

Abstract

Two strains of Drosophila melanogaster differing by their sensitivity to deltamethrin, one resistant strain (SR) and one Sensitive Strain (HS1), were selected from a wild-type strain (SS) in the laboratory during the past 10 years. The resistance ratios of the SR strain to the topical application were, respectively, 11 and 27 when compared to the SS and HS1 strain. Relative sensitivity of the two strains to topically applied deltamethrin was compared using coupled behavioral observations and electrophysiological recordings of the induced electrical activity. The recordings were made between a reference extracellutar electrode inserted in the abdomen and an extracellular electrode inserted through the cuticle in the anterior part of the thorax. In the susceptible strains, deltamethrin induced, after a variable delay, typical bursts of activity made of large and usually monophasic excitatory junctional currents (EJCs) and small and faster biphasic action currents. For the three strains, there was a poor correlation between the knockdown (KD) and the lethal (LD) effects, the KD50 being six to seven times lower than the corresponding LD50. The dose which induced activity in 50% of the flies (ED50) was marginally lower than the KD50 for both the resistant and sensitive strains. The latency between the application and the first electrical symptoms was significantly longer for SR flies than for HS1 flies. Duration and frequency of the electrical activity were dose and strain dependent. Flies from the SR strain lacked large deltamethrin-induced EJCs which were present in the two other strains. Interestingly, no distinct electrical activity could be detected when the insects were knocked down. The properties of these two strains are reminiscent of those of kdr or super-kdr strains of Musca domestica. There are indications that the origin of the resistance or the sensitivity of the two strains could correspond to modifications of the main target site of deltamethrin: the voltage-dependent sodium channel.