A previous study showed for Anopheles gambiae s.s. a gradation of feeding preference on common plant species growing in a malaria holoendemic area in western Kenya. The present follow-up study determines whether there is a relationship between the mosquito's preferences and its survival and fecundity.

The close association of Anopheles gambiae Giles with humans and its females’ ability to live on human blood alone suggest that females may ignore sources of sugar in favour of human blood as a source of energy. They have limited energy reserves at emergence, and at 27°C both sexes generally die if they do not feed during night 1, 24–36 h after emergence. Food preferences during this critical period were tested by measuring responses to volatiles from honey and soiled socks, which served as surrogates for nectar-related and human-related volatiles in a wind-tunnel olfactometer. Both sexes responded more strongly to honey than to human volatiles, and given a choice, preferred honey over human volatiles. After 5 days of sugar access and maturation, males continued to prefer honey volatiles, whereas females changed behaviour, responding almost exclusively to human volatiles. Night 1 experiments also demonstrated that: (i) females previously having had sugar during the night of emergence responded more strongly to human volatiles; (ii) large-bodied mosquitoes of both sexes responded more strongly to honey than small-bodied ones; and (iii) females were equally responsive to honey in both early and late scotophase but were slightly more responsive to human volatiles in late scotophase. These results indicate that for a female’s first meal, sugar is a viable option and is preferred when nectar-related stimuli are strong. This supports field evidence that sugar-feeding is a significant component of A. gambiae female behaviour.

Vector control has been the most effective preventive measure against malaria and other vector-borne diseases. However, due to concerns such as insecticide resistance and budget shortfalls, an integrated control approach will be required to ensure sustainable, long-term effectiveness. An integrated management strategy should entail some aspects of environmental management, relying on coordination between various scientific disciplines. Here, we review one such environmental control tactic: invasive alien plant management. This covers salient plant-mosquito interactions for both terrestrial and aquatic invasive plants and how these affect a vector’s ability to transmit malaria. Invasive plants tend to have longer flowering durations, more vigorous growth, and their spread can result in an increase in biomass, particularly in areas where previously little vegetation existed. Some invasive alien plants provide shelter or resting sites for adult mosquitoes and are also attractive nectar-producing hosts, enhancing their vectorial capacity. We conclude that these plants may increase malaria transmission rates in certain environments, though many questions still need to be answered, to determine how often this conclusion holds. However, in the case of aquatic invasive plants, available evidence suggests that the management of these plants would contribute to malaria control. We also examine and review the opportunities for large-scale invasive alien plant management, including options for biological control. Finally, we highlight the research priorities that must be addressed in order to ensure that integrated vector and invasive alien plant management operate in a synergistic fashion.

Background: Anopheles gambiae plant-sugar feeding was thought to be rare and physiologically optional. Unlike adult females, males have no alternative source of energy and soon die with only water, yet they might be competent to inseminate all females within their brief lifespan. This study was designed to detect sugar's effect, if any, on male performance. Methods: Males with and without 20% sucrose were evaluated at two body sizes and two temperatures, 23° and 27°C. Survival was recorded twice daily, and sexual behaviour was recorded each night after adult emergence. Insemination at a 2:1 male:female ratio was examined in three cage sizes, including walk-in mesocosms. Results: Without sugar, males of both sizes lived longer at 23° than 27°C, and large males lived longer at each temperature. Survival of large males at low temperature averaged 3.7 days, small males at high temperature, 1.9 days. With sugar, males in all four treatments suffered minimal mortality. With sugar, in small cages, large males at 27°C matured most rapidly. A few erected fibrillae and inseminated females on night 1. On night 2, maximal proportions erected fibrillae and swarmed, and over one-third of females became inseminated. Small sugar-fed males at 23°C matured most slowly but had achieved nearly maximal levels of swarming by night 3. By night 5, small males had inseminated more than half the females, and large males had inseminated nearly all of them. Without sugar, large males progressed similarly during the first two nights. On night 3, however, the proportion erecting fibrillae and swarming declined precipitously at 27°C, and to a lesser degree at 23°C. Cumulative insemination never reached high levels. Small males never achieved high levels of fibrillar erection or swarming and inseminated few females, even at 23°C. In larger cages and under more semi-natural conditions, regardless of body size, without sugar male insemination capacity was virtually nonexistent. Conclusion: Under some conditions, a limited number of sugar-deprived males can survive long enough to inseminate females. However, in nature males that cannot obtain sugar at frequent intervals will not be competitive with those that can, suggesting that male performance is closely tied to plant communities.

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.

The purpose of this study was to determine whether the sugar-or-blood meal choice of Anopheles gambiae females one day after emergence is influenced by blood-host presence and accessibility, nectariferous plant abundance, and female size. This tested the hypothesis that the initial meal of female An. gambiae is sugar, even when a blood host is available throughout the night, and, if not, whether the use of a bed net diverts mosquitoes to sugar sources.