Résultat de la recherche
Quinine and artesunate inhibit feeding in the African malaria mosquito Anopheles gambiae: the role of gustatory organs within the mouthparts
A membrane feeding assay in which the effects of the antimalarial drugs quinine and artesunate are tested on Anopheles gambiae Giles sensu stricto is described. In the present study, 87% of female A. gambiae are shown to feed on whole defibrinated bovine blood alone, whereas only 47% and 43.5% feed on saline and on saline + bovine serum albumin (BSA) solutions, respectively, suggesting that additional components in the blood stimulate mosquito feeding. The addition of 1 mm quinine or artesunate to the BSA solution results in a significant reduction in percentage engorgement to 16.2% and 14.1%, respectively. However, the feeding rate is higher when 1 mm artesunate and quinine are mixed in the blood because 67.8% and 78.4% of females engorge on these solutions respectively. Artesunate (10 mm) in the blood reduces percentage engorgement to 20%. Because circulating doses of quinine and artesunate affecting Plasmodium in humans are much lower than those affecting feeding by A. gambiae in the in vitro assay, these two antimalarial drugs should have no effect, or only a minor effect, on the infection rate of mosquitoes feeding on treated patients. Because only the stylets penetrate the membrane and not the labellar lobes, the results of the present study suggest that both blood phagostimulants and feeding deterrents are detected by internal gustatory organs in A. gambiae, namely sensory cells in the apical and subapical labral pegs, in sensilla on the inner face of the labellar lobes, or by cibarial receptor cells. The neuroanatomy of gustatory sensilla on the apical and subapical labral pegs and on the inner face of the labellar lobes of female A. gambiae is described in the present study.
Responses of Anopheles gambiae, Anopheles stephensi, Aedes aegypti, and Culex pipiens mosquitoes (Diptera: Culicidae) to cool and humid refugium conditions
Like all terrestrial arthropods, mosquitoes must cope with the threat of desiccation. To gain insight into their survival strategies, we recorded the behavioral responses of Anopheles gambiae, Anopheles stephensi, Aedes aegypti, and Culex pipiens offered zones of different microclimatic conditions in laboratory cages. The cooled refugium was at 25.6° C, 86% RH and the control was at 28.5° C, 75% RH, i.e., a difference in saturation deficit of 3.9 mm Hg between the two zones. We show that newly-emerged adults, with no access to water or sugar, prefer the cooler and more humid refugium with a saturation deficit half that in the control and where the mosquitoes could reduce their metabolic rate. This response is delayed in Ae. aegypti, perhaps because the energy reserves accumulated as larvae are higher in this species. This study shows that mosquitoes under stress can use their thermohygroreceptor cells to guide them to locations that facilitate survival.
The sugar meal of the African malaria mosquito Anopheles gambiae and how deterrent compounds interfere with it: a behavioural and neurophysiological study
In this study, we show that female African malaria mosquitoes Anopheles gambiae starved for 3–5 h start to engorge on sucrose at concentrations between 50 and 75 mmol l−1. Half of the feeding response (ED50) is reached at 111 mmol l−1 and the maximum response (0.4 mg) occurs at 250 mmol l−1. Two receptor cells in a trichoid sensillum of the labellum, called the ‘sucrose’ and ‘water’ neurones, are activated by sucrose and water, respectively. The electrophysiological response of the sucrose receptor cell starts well below the level of sugar necessary to induce engorgement. The sugar receptor cell is most sensitive to small increments in sucrose concentration up to 10 mmol l−1 with a response plateau from 25 mmol l−1. Fructose has a mild phagostimulatory effect on A. gambiae, whereas no significant differences in meal sizes between water and glucose were found. However, when 146 mmol l−1 fructose plus glucose are mixed, the same engorgement as on 146 mmol l−1 sucrose is observed. Likewise, even though the sucrose receptor cell is not activated by either fructose or glucose alone, equimolar solutions of fructose plus glucose activate the neurone. We conclude that there is a behavioural and neurophysiological synergism between fructose and glucose, the two hexose sugars of sucrose. We show that some bitter-tasting products for humans have a deterrent effect on feeding in A. gambiae. When 1 mmol l−1 quinidine, quinine or denatonium benzoate is added to 146 mmol l−1 sucrose, feeding is almost totally inhibited. The effect of berberine is lower and no significant inhibition on engorgement occurs for caffeine. The deterrent effect depends on the concentration for both quinine and quinidine. Capillary feeding experiments show that contact chemosensilla on the mouthparts are sufficient for the detection of sucrose and bitter products. The feeding assay findings with deterrents correlate with the neurophysiological responses of the sucrose and water labellar neurones, which are both inhibited by the bitter compounds denatonium benzoate, quinine and berberine between 0.01 and 1 mmol l−1, but not by the same concentrations of caffeine. In conclusion, sucrose stimulates feeding and activates the labellar sucrose neurone, whereas feeding deterrents inhibit both the sucrose and water neurones. This study provides an initial understanding of the physiological mechanisms involved in sugar feeding in A. gambiae and shows how some bitter products interfere with it.
Host plant preferences of Hyalesthes obsoletus, the vector of the grapevine yellows disease ‘bois noir’, in Switzerland
Bois noir is an important grapevine yellows disease in Europe that can cause serious economic losses in grapevine production. It is caused by stolbur phytoplasma strains of the taxonomic group 16Sr-XII-A. Hyalesthes obsoletus Signoret (Hemiptera: Cixiidae) is the most important vector of bois noir in Europe. This polyphagous planthopper is assumed to mainly use stinging nettle [Urtica dioica L. (Urticaceae)] and field bindweed [Convolvulus arvensis L. (Convolvulaceae)] as its host plants. For a better understanding of the epidemiology of bois noir in Switzerland, host plant preferences of H. obsoletus were studied in the field and in the laboratory. In vineyards of Western Switzerland, adults of H. obsoletus were primarily captured on U. dioica, but a few specimens were also caught on C. arvensis, hedge bindweed [Calystegia sepium (L.) R. Brown (Convolvulaceae)], and five other dicotyledons [i.e., Clematis vitalba L. (Ranunculaceae), Lepidium draba L. (Brassicaceae), Plantago lanceolata L. (Plantaginaceae), Polygonum aviculare L. (Polygonaceae), and Taraxacum officinale Weber (Asteraceae)]. The preference of the vector for U. dioica compared to C. arvensis was confirmed by a second, more targeted field study and by the positioning of emergence traps above the two plant species. Two-choice experiments in the laboratory showed that H. obsoletus adults originating from U. dioica preferred to feed and to oviposit on U. dioica compared to C. arvensis. However, H. obsoletus nymphs showed no host plant preference, even though they developed much better on U. dioica than on C. arvensis. Similarly, adults survived significantly longer on U. dioica than on C. arvensis or any other plant species tested [i.e., L. draba and Lavandula angustifolia Mill. (Lamiaceae)]. In conclusion, although nymphs of H. obsoletus had no inherent host plant preference, adults tested preferred to feed and oviposit on U. dioica, which is in agreement with the observed superior performance of both nymphal and adult stages on this plant species. Urtica dioica appears to be the principal host plant of H. obsoletus in Switzerland and plays therefore an important role in the epidemiology of the bois noir disease in Swiss vineyards.