Behaviour and fitness of gamma HCH/dieldrin resistant and susceptible female Anopheles gambiae and An.stephensi mosquitoes in the absence of insecticide.

The effects of gamma HCH/dieldrin resistance genes on various fitness components of mosquito larvae and adult females in the absence of insecticide were investigated in backcrossed strains of Anopheles gambiae Giles and An.stephensi Liston. Among larvae, heterozygotes (RS) developed slightly but significantly faster than homozygotes for resistance (RR) or susceptibility (SS). The lifetime fecundity of RR females in population cages was only half to two-thirds that of SS and RS females despite similar longevities; several reasons were identified: RR gravid females were less responsive to oviposition-site stimuli, their spontaneous activity--as measured in an acoustic actograph--was only half that of SS or RS females, and RR females produced fewer eggs per unit bloodmeal. When inseminated females were recorded in LD 12:12, RR were again less active than SS or RS. When the lighting was switched to a regime simulating full-moonlight, the activity pattern of SS and RS changed and they flew for longer periods. In contrast, the activity of RR females was the same in LD 12:12 as in 'moonlight'. In a test simulation of potential predation, RR mosquitoes took to flight least readily. All component tests on adult females therefore point to RR as being the least fit of the three genotypes. The behavioural tests suggest that resistance has raised the response threshold of RR females to diverse stimuli. A possible physiological mechanism underlying RR behaviour is that a change in the cyclodiene receptor on the chloride channels has increased their permeability to chloride ions, causing hyper-inhibition of the nervous system.