Abstract
BackgroundIncreased understanding of the dry-season survival mechanisms of Anopheles gambiae in semi-arid regions could benefit vector control efforts by identifying weak links in the transmission cycle of malaria. In this study, we examined the effect of photoperiod and relative humidity on morphologic and chemical traits known to control water loss in mosquitoes.MethodsAnopheles gambiae body size (indexed by wing length), mesothoracic spiracle size, and cuticular hydrocarbon composition (both standardized by body size) were examined in mosquitoes raised from eggs exposed to short photoperiod and low relative humidity, simulating the dry season, or long photoperiod and high relative humidity, simulating the wet-season.ResultsMosquitoes exposed to short photoperiod exhibited larger body size and larger mesothoracic spiracle length than mosquitoes exposed to long photoperiod. Mosquitoes exposed to short photoperiod and low relative humidity exhibited greater total cuticular hydrocarbon amount than mosquitoes exposed to long photoperiod and high relative humidity. In addition, total cuticular hydrocarbon amount increased with age and was higher in mated females. Mean n-alkane retention time (a measure of cuticular hydrocarbon chain length) was lower in mosquitoes exposed to short photoperiod and low relative humidity, and increased with age. Individual cuticular hydrocarbon peaks were examined, and several cuticular hydrocarbons were identified as potential biomarkers of dry- and wet-season conditions, age, and insemination status.ConclusionsResults from this study indicate that morphological and chemical changes underlie aestivation of Anopheles gambiae and may serve as biomarkers of aestivation.
Highlights
Increased understanding of the dry-season survival mechanisms of Anopheles gambiae in semi-arid regions could benefit vector control efforts by identifying weak links in the transmission cycle of malaria
Development time was significantly associated with wing length (Table 1), with mean wing length of early emerging mosquitoes estimated to be 2.9% greater than that of mosquitoes emerging on day 2 (Figure 3)
Females reared under dry-season conditions (SPLRH), had 28% greater total cuticular hydrocarbon quantity (tCHC) than females reared under wet-season conditions (LP-HRH)
Summary
Increased understanding of the dry-season survival mechanisms of Anopheles gambiae in semi-arid regions could benefit vector control efforts by identifying weak links in the transmission cycle of malaria. Since An. gambiae eggs, larvae and pupae are non-resistant to seasonal desiccation [8,9,10] and since embryonic and larval development of Anopheles require at least nine days for adult emergence [11], this rapid population build-up cannot be explained by local population growth Instead, these early wet-season adult mosquitoes are either migrating from surrounding areas with permanent water sources capable of harboring a continuously active population [5], or represent a local adult population that has undergone aestivation during the dry season and that emerge from their shelters following the first rains [6,7,12,13]. Wetseason An. gambiae (M-form) density was reported to be 30% lower in sites that received pyrethrum treatments [6], suggesting that adults were present during the dryseason, and that adult aestivation plays a significant role in An. gambiae dry-season survival and subsequent population buildup
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