Discriminative feeding behaviour of Anopheles gambiae s.s. on endemic plants in western Kenya

Abstract

Anopheles gambiae Giles s.s. (Diptera: Culicidae) is known to feed on plant sugars, but this is the first experimental study to consider whether it discriminates between plant species. Thirteen perennial plant species were selected on the basis of their local availability within the vicinity of human dwellings and larval habitats of An. gambiae s.s. in western Kenya. Groups of 100 or 200 mosquitoes were released into cages either with a cutting of one plant type at a time (single-plant assay) or with cuttings of all 13 plants simultaneously (choice assay), respectively, and left overnight. In the choice assay, direct observations of the percentages of mosquitoes perching or feeding on each plant were recorded over four 1-h periods each night. For both types of assay, mosquitoes were recaptured and the percentage that had fed on plants was assessed by testing them individually for the presence of fructose. To identify which plants the choice-assay mosquitoes had fed on, gas chromatography (GC) profiles of samples of mosquito homogenates were compared with GC profiles of extracts from relevant parts of each plant. Four of the plants that were observed to have been fed on most frequently in the choice assay (Parthenium hysterophorus L., Tecoma stans L., Ricinus communis L., and Senna didymobotrya Fresen) were also shown to have been ingested most often by mosquitoes in both types of assay, suggesting that An. gambiae is differentially responsive to this range of plants, regardless of whether the plants were presented singly or mixed together. Significantly more females than males fed on plants, with the exception of P. hysterophorus L., one of the plants most frequently fed on. For most plant species (ten of 13), GC profiles indicated that An. gambiae obtained sugars primarily from flowers. The exceptions were P. hysterophorus L., Lantana camara L. and R. communis L., on which An. gambiae fed more often from leaves and stems than from flowers.

Fig. 1

Percentages (mean ± SE) of Anopheles gambiae males and females that were (A) observed to be perching and (B) observed to be feeding in the choice assay (C) found to be positive for fructose after the single-plant assay and (D) found to have ingested sugars in the choice assay that matched the sugar profiles of particular plants. Columns with differ ent letters indicate significant differences and same letters indicate columns that are not significantly different (Tukey–Kramer multirange comparison, α = 0.05). Number of replicates, 39 (A, B), 3 (C) and 80 D (40 males and 40 females). GC, gas chromatography.

Fig. 2

Percentages (mean ± SE) per night of mosquitoes seen perching and feeding on plants at 20.00 h, 21.00 h, 22.00 h and 23.00 h (39 replicates). Different letters in superscripts indicate significant differences (Tukey–Kramer multirange comparison, α = 0.05).

Fig. 3

Chromatogram of sugar derivatives from extract of mixed sugar standards (multiple peaks of the same sugar, anomeric forms). a, Fructose; b, sucrose; c, galactose; d, mannose; e, gulose; f, raffinose; g, glucose; h, allose; i, altrose; j, lactose; FID, flame ionization detector.

Fig. 4

Representative chromatograms showing match between the sugar derivatives from extracts of (A) Hamelia patens flower and (B) one sugar-fed female Anopheles gambiae. (C) Representative chromatogram showing sugar profile of one newly-emerged An. gambiae female. Peaks with the same number or letter indicate same sugar type. Matching sugar peaks in (B) and (C) presumed to be metabolites derived from larval feeding. Extra peaks in (B), and enhanced ones also found in the unfed females (C), indicate sugars acquired from the plant (A). Letters on top of peaks represent identified types of sugars. Peaks without a letter are sugars not identified. b, Sucrose; d, mannose; f, raffinose; g, glucose; FID, flame ionization detector.