Biological traits, plant sugar feeding patterns, and chemical ecology of Aedes aegypti

Abstract

Increased frequency of Aedes aegypti-borne diseases demands for improved vector monitoring and control. This requires a good understanding of the behaviour, biology, and ecology of this vector. This study began (Chapter 2) with an assessment of biological traits for their possible contribution to local scale differences in vector potential of Ae. aegypti populations from two areas of contrasting dengue endemicity in Kenya. Specifically, wing lengths (proxy for body size) and total carbohydrates and lipids (energy reserves) were compared among Ae. aegypti populations from Kilifi (endemic) and Isiolo (non-endemic) during the wet and dry seasons. Body size varied by site with the population from Isiolo being generally smaller. Energy reserves varied by sex and season with significantly higher amounts during the dry season and three times more in males than females. As both biological traits are linked to plant nutrition, Chapter 3, examined the plant feeding patterns of the two Ae. aegypti populations by evaluating: 1) seasonality in the extent of plant feeding by measuring fructose positivity rates using the cold anthrone test; 2) the composition of plant associated sugars in wild Ae. aegypti using coupled gas chromatography-mass spectrometry (GC/MS), and 3) the putative plant sources of the sugar meals using polymerase chain reaction (PCR) employing the chloroplast ribulose-1,5 bisphosphate carboxylase/oxygenase large chain (rbcL) gene and analysis of sequenced amplicons. Plant sugar feeding rates varied by sex and season with a three-fold increase in males during the dry season. A broad range of plant associated sugars and host plant species were detected in the profiles of the wild-caught Ae. aegypti with a greater diversity in females than males. Generally, Fabaceae and Malvaceae were the most dominant plant families associated with both sexes. Plant-derived kairomones have previously been exploited to improve trapping of Ae. aegypti as part of surveillance and control. Chapter 4 builds on a previous study that reported the attractive potential of a blend of three compounds ((E)-linalool oxide, ocimene and (E)-2-hexen-1-ol) isolated from a host plant of Ae. aegypti. The blend was further evaluated to assess seasonal performance and selectivity in trapping male and female Ae. aegypti with and without carbon dioxide (CO2) in Kilifi only. Without CO2, the blend was more effective in trapping female than male Ae. aegypti, but was a weak attractant. Although CO2 increased trap catches, no additive effects were observed, necessitating that the lure be further improved and field tested. I conclude that, variations in biological traits may contribute to differences in vectorial capacity of dengue vectors in the two areas. Of the two traits, body size appears to be the more sensitive indicator of population level differences. Seasonal and sex-specific differences in the plant feeding patterns of the wild-caught Ae. aegypti populations potentially reflects their habits in nature and may be leveraged for targeted vector monitoring and control. Although the blend was effective in selectively trapping female Ae. aegypti only in the absence of CO2, the findings demonstrate the utility of plant-derived olfactory cues as attractants for field monitoring of this vector species.