Plant-sugar feeding and vectorial capacity

Abstract

Sugar feeding is a common behaviour of male and female mosquitoes, sand flies, and other Dipteran vectors. In some species it is essential to one or both sexes; in others it is facultative. Even among females of anthropophilic species that are predisposed to a diet of frequent blood meals sugar is often taken, depending on internal state and opportunity. This opportunism is expressed as an increased likelihood of feeding on nectar when access to blood and oviposition sites is limited. Newly emerged Anopheles gambiae females sometimes show a preference for sugar before mating even when blood hosts are available, likely depending both on the strength of plant and animal kairomones and on the attractive qualities of each. Incorporation of sugar in the diet by mosquitoes affects certain components of their vectorial capacity. Environmental conditions, such as bed net coverage and abundance of nectar sources, will affect the extent to which mosquitoes feed on sugar. If the effect of sugar on vectorial capacity is significant, these conditions will impact transmission rates of vector-borne diseases and should be included in epidemiological models. Vectorial capacity is pulled in opposing directions by sugar feeding, through its effect on the two most important components, survival and biting rate. Survival of females feeding on sugar and blood is greater than that of females restricted to a blood-only diet, according to the vast majority of studies, whereas biting rates usually are depressed when sugar is available, but field evidence is scarce. Vector density results from survival and fecundity. Most studies on vectors suggest that although fecundity per gonotrophic cycle is enhanced by sugar feeding, long-term reproductive fitness in anthropophilic species is slightly depressed. Vector competence appears to be negatively affected by sugar feeding. In certain cases plant nectar contains factors that inhibit development of the parasite in the vector. More common may be positive effects on the vector’s immune response, but this appears to depend heavily on the host-parasite system, condition of the vector, and possibly genotype-by-environment interactions. Estimating the combined effect of these factors at different levels of sugar intake remains difficult at this point, but an overall impression is that vectorial capacity is somewhat decreased in environments where sugar is readily accessed. Sugar feeding behaviour can be exploited for control, the most promising methods employing sugar solutions combined with attractants and oral insecticides for direct control and attractive phytochemicals for surveillance. Main questions facing both approaches are their suitability in verdant areas where attractants will compete with a diverse flora. For females of anthropophilic species in settings with abundant blood hosts, the question may be whether populations can be effectively suppressed by targeting male mosquitoes.