Effects of permethrin at different temperatures on pyrethroid-resistant and susceptible strains of Anopheles

The influence of temperature (16, 22, 28, 37 degrees C) on effects of perme thrin was investigated for susceptible and pyrethroid-resistant strains of the mosquitoes Anopheles gambiae and An. stephensi (Diptera: Culicidae). Yo ung unfed female adult mosquitoes were exposed to 0.25% permethrin test pap ers or to polyester netting treated with permethrin 500mg a.i./m(2). The ti me to 50% knock-down (KT50) declined as temperature increased, i.e. there w as a positive temperature coefficient of this effect of the pyrethroid. Res istance ratios (comparing KT50 values) between resistant and susceptible An . stephensi ranged between 2.5 and 4.4 at the different temperatures. Compa rative tests of pyrethroid tolerance of different strains would be valid ov er the 22-28 degrees C range but, when using a discriminating dose to detec t resistance, more precise temperature control is desirable. Mortality 24h after exposure to 0.25% permethrin of both susceptible and re sistant strains of Art. stephensi showed a negative correlation with temper ature between 16 and 22 degrees C and a positive correlation at higher temp eratures. In An. gambiae, however, the correlation was positive over the wh ole range. Irritancy of permethrin-treated netting to Anopheles females (measured as t ime lapse until first flight take-off, and the number of take-offs during 7 .5min exposure) was positively correlated with temperature in all four stra ins and was much greater for the susceptible than the resistant strains.