The effect of artificial cold selection on the behaviour and immune response of the major malaria vector Anopheles arabiensis (Diptera: Culicidae)

The physiological response of mosquitoes to cold is poorly understood, particularly for Anopheles mosquitoes, where overwintering is rarer than in Aedes and Culex. The effect of cold selection was examined in two laboratory strains of the major malaria vector An. arabiensis that differ in insecticide resistant phenotypes. This is due to previous studies that demonstrated an increased tolerance for high temperatures in insecticide resistant An. arabiensis. The insecticide unselected SENN strain and the insecticide-resistant SENN-DDT strain were reared at decreasing fluctuating temperatures for three generations until adults emerged at 18°C. Adults from this generation was exposed to 18°C and 25°C and their longevity compared to unselected counterparts. Furthermore, their landing and biting behaviour at 20 and 25°C were assessed. Finally, the expression of the antimicrobial peptides Attacin, Cecropin, Defensin and Gambicin was assessed. The cold selection had variable effects on the two strains. Cold selection significantly increased adult female longevity at 18°C in SENN-DDT but not SENN. By contrast, cold selection increased landing at 20°C in SENN but not SENN-DDT. However, it decreased biting success at 20°C in both strains. Cold treatment increased Attacin and cecropin expression in both strains but had no effect on Defensin and Gambicin expression. This study suggests that winter survival in An. arabiensis is associated with increased longevity and reduced feeding success. Increased antimicrobial peptide activity was generally observed under cold conditions. These factors combined may contribute to reduced transmission of Plasmodium under winter conditions, over and above the low population density.