Energy-state dependent responses of Anopheles gambiae (Diptera: Culicidae) to simulated bednet-protected hosts

In nature, Anopheles gambiae mosquitoes are found at various energy levels and such females must choose between seeking somatic energy from sugar sources and obtaining both somatic and gametic energy from blood hosts. We used a straight-tube olfactometer containing a simulated unobtainable blood host (human foot smell protected by a net) as well as a sugar source (honey odor). We assessed female probing rate and residence time at the net as a function of energy state (0, 24, 48, 72-h starved). In our trials, 0-h starved females showed low response to human odor, low probing rate, and residence time at the human odor site. By contrast, both 48 and 72-h individuals showed high response to foot odor, longer residence time, and higher probing rates. Seventy-two-h females also flew towards the honey source less often than other groups. Our findings suggest that managing sugar sources might be a viable strategy for influencing mosquito biting behavior.     

Variation in energy reserves and role of body size in the mating system of Anopheles gambiae

Anopheles gambiae mates in flight. Males gather at stationary places at sunset and compete for incoming females. Factors that account for male mating success are not known but are critical for the future of any genetic control strategy. The current study explored variations in nutritional reserves (sugars, glycogen, lipids, and proteins) in wild‐caught swarming and resting males and evaluated the effect of body size and wing symmetry on male mating success. Our results showed that glycogen and sugar reserves are mobilized for flight. Males consume proportionally 5.9‐fold as much energy derived from sugars in swarming activities than when they are at rest. Mated males were on average bigger than unmated ones (P<0.0001). A strong correlation between the left and right wings in both mated and unmated males was found and additional analysis on fluctuating asymmetry did not show any indication of mated males being more symmetrical than unmated ones. The distribution of wing size of mated males was focused around a central value, suggesting that intermediate size of males is advantageous in the An. gambiae mating system. The results are discussed in the context of sexual selection.     

Effects of larval density and feeding rates on larval life history traits in Anopheles gambiae s.s. (Diptera: Culicidae)

Mosquito larval habitat determines the fitness, survivorship, fecundity, and vector capacity of emerging adults. We manipulated larval density and food provisioning in the laboratory to determine their effects on a number of life history parameters of Anopheles gambiae. Larval mortality was positively correlated with larval density. In addition, increased density skewed sex ratios that favored females.     

Behavioural and electrophysiological responses of the malaria mosquito Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to human skin emanations

Behavioural and electrophysiological responses of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to human skin emanations collected on glass beads were studied using a dual-port olfactometer and electroantannography. Glass beads to which skin emanations from human hands had been transferred elicited a level of attraction similar to a human hand. The attractiveness of these handled glass beads faded away 4 h after transfer onto the beads. Storage at -20 degrees C for up to 8 weeks showed a decreased but still attractive effect of the beads. In a choice test between one individual and four others, the emanations from the reference individual were significantly more attractive in three out of four cases. The headspace of handled glass beads elicited a dose-dependent EAG response. The substances causing EAG activity could be removed partially by dichloromethane, ethanol and pentane-ether. Glass beads provide a suitable neutral substrate for the transfer of human odour to enable chemical analysis of the human skin emanations for identification of kairomones of anthropophilic mosquitoes.     

Flight tone of field‐collected populations of Anopheles gambiae and An. arabiensis (Diptera: Culicidae)

Abstract.Laboratory colonies of the human malaria vectors Anopheles gambiae Giles and An. arabiensis Patton have distinct flight tones. If flight tone similarly distinguishes natural populations of these sympatric sibling species, it may play a role in reproductive isolation of swarms that are otherwise behaviourally identical. To assess the fidelity of flight tone differences in natural populations, flight tone was measured in the F1 progeny of mosquitoes of both species captured in western Kenya. Flight tone distributions of wild An . gambiae and An. arabiensis were similar to their laboratory conspecifics. However, interspecies comparisons of flight tone of wild mosquitoes revealed significantly different but overlapping distributions for both sexes. Furthermore, when the effect of body size on flight tone was determined, there was a positive correlation between wing length and flight tone for both sexes of An. gambiae and An. arabiensis , suggesting that mosquito size is a significant variable affecting flight tone. Although these findings diminish any practical benefit of flight tone as a diagnostic tool in species identification, its potential role in pre‐mating species recognition needs further investigation.     

A low‐cost mesocosm for the study of behaviour and reproductive potential in Afrotropical mosquito (Diptera: Culicidae) vectors of malaria

A large‐scale mesocosm was constructed and tested for its effectiveness for use in experiments on behaviour, reproduction and adult survivorship in the Afrotropical malaria vector Anopheles gambiae s.s. Giles (Diptera: Culicidae) in temperate climates. The large space (82.69 m³) allowed for semi‐natural experiments that increased demand on a mosquito's energetic reserves in an environment of widely distributed resources. A one‐piece prefabricated enclosure, made with white netting and vinyl, prevented the ingress of predators and the egress of mosquitoes. Daylight and white materials prompted the mosquitoes to seclude themselves in restricted daytime resting sites and allowed the easy collection of dead bodies so that daily mortality could be assessed accurately using a method that accounts for the loss of a proportion of bodies. Here, daily, age‐dependent mortality rates of males and females were estimated using Bayesian Markov chain Monte Carlo simulation. In overnight experiments, mosquitoes successfully located plants and took sugar meals. A 3‐week survival trial with a single cohort demonstrated successful mating, blood feeding, oviposition and long life. The relatively low cost of the mesocosm and the performance of the mosquitoes in it make it a viable option for any behavioural or ecological study of tropical mosquitoes in which space and seasonal cold are constraining factors.     

Olfactory basis of floral preference of the malaria vector Anopheles gambiae (Diptera: Culicidae) among common African plants

Mosquitoes of both sexes feed on plants to obtain sugar. Nocturnal species probably locate the plants primarily by their volatile semiochemicals that also form the basis for the mosquitoes’ innate plant‐species preferences. To evaluate these olfactory preferences quantitatively, we used a two‐choice wind‐tunnel olfactometer to measure the upwind orientation of Anopheles gambiae Giles, an important vector of malaria in equatorial Africa, toward odor plumes produced by nine plant species common where this mosquito occurs. These plants are reported to induce feeding behaviors in An. gambiae and to produce floral or extrafloral nectar. Results presented here demonstrated that the volatiles of S. didymobotrya, P. hysterophorus, S. occidentalis, and L. camara, in descending order of numbers of mosquitoes responding, were all attractive, compared to a control plant species, whereas D. stramonium, R. communis, S. bicapsularis, T. stans, and T. diversifolia were not. As expected, chromatographic analysis of the headspace of attractive plants whose volatiles were captured by stir‐bar sorptive extraction revealed a wide range of compounds, primarily terpenoids. Once their bioactivity and attractiveness for An. gambiae, alone and in blends, has been firmly established, some of these semiochemicals may have applications in population sampling and control.     

尼日利亚南部阿贝奥库塔地区冈比亚按蚊复合体的形态特征研究（双翅目：蚊科）

对采自尼日利亚南部城市阿贝奥库塔冈比亚按蚊复合体Anopheles gambiae complex的形态特征进行了研究。依据2005年8月至2006年7月灯诱捕获的364个冈比亚按蚊复合体标本,分别对它们的触角、翅、喙、前足、中足和后足6个部位的长度进行了测量,对月平均值进行回归分析,同时利用差异系数（co-efficient of difference,CD）进行近缘分析。分析显示,各特征的长度平均值雨季大于旱季,但是回归分析表明长度变化与季节不显著相关（P>0.05）。差异系数分析结果表明,仅触角长度和翅长显示此复合体为两个不同的种群(CD>1.28),而其他特征值表明为同一种群。因此,该研究结果提示触角长度及翅长对冈比亚按蚊复合体近缘种的区分有重要参考价值。     

Diel timing and frequency of sugar feeding in the mosquito Anopheles gambiae, depending on sex, gonotrophic state and resource availability

Little is known about the sugar-feeding behaviour of equatorial Africa's principal vector of malaria, Anopheles gambiae Giles (Diptera: Culicidae). It is suspected to feed on plant sugar infrequently, but possibly the timing depends on environmental circumstances, and males may differ markedly from females. These points of uncertainty were clarified in the laboratory, by monitoring both diel and longterm sugar-feeding activity in both sexes. Males fed on sugar in a nocturnal diel rhythm closely approximating non-specific flight activity. Female diel sugar-feeding patterns resembled published rhythms and cycles of host seeking. Males sugar fed nightly at an average frequency of about twice per night, sustained over 17 days. This was substantially higher than the sugar-feeding frequency of females that were allowed both blood and oviposition sites every night: they averaged about one sugar feed in every 4 nights. These females fed on sugar between gonotrophic cycles, after eggs were mature but before the next bloodmeal. They did not sugar feed during the 2 days after blood feeding, while blood was being digested and the eggs developed. A slight delay in the availability of either the oviposition site or blood led to an increase in female sugar-feeding frequency: they averaged more than once per night until the delayed resource was made available. These observations support the conclusion that sugar feeding is a normal part of the biology of both sexes of An. gambiae.     

Efficacy of Bacillus sphaericus 2362 against larvae of Anopheles gambiae under laboratory and field conditions in West Africa

A flowable concentrate of Bacillus sphaericus (Neide) strain 2362 was applied against Anopheles gambiae Giles s.l. mosquito larve in small plot field trials in Bobo-Dioulasso area. Burkina-Faso. Third and fourth instar larvae were controlled for 10-15 days with a dosage of 10 g/m2, 3-10 days with 1 or 0.1 mg/m2, and 2 days with 0.01 g/m2. Complete elimination of larval populations required 1 x 10(2) to 2 x 10(3) viable spores/ml in the larval feeding zone. After treatment, the total numbers of viable spores decreased in the ponds, due to ingestion of spores by non-target as well as target organisms and/or loss of viability of some spores by sunlight. This formulation was less effective against An. gambiae than against Culex quinquefasciatus Say larvae, both in laboratory bioassays and under field conditions.     

Biting activity and host preference of the malaria vectors Anopheles maculatus and Anopheles sawadwongporni (Diptera: Culicidae) in Thailand

Adult mosquitoes in the Anopheles maculatus group were surveyed from different regions of Thailand and five different species were morphologically identified, including Anopheles maculatus, Anopheles sawadwongporni, Anopheles notanandai, Anopheles dravidicus, and Anopheles willmori. Blood‐feeding activity and host preference of two species, Anopheles maculatus and Anopheles sawadwongporni, were observed during a one‐year period at Pu Teuy Village, Sai Yok District, Kanchanaburi Province, west‐central Thailand. Both species were more prevalent during the wetter period of the year and each had a greater predilection to feed on cattle than humans. Primary feeding activity occurred between 20:00‐23:00 and a smaller peak at 01:00‐03:00. Findings are discussed relative to the importance of these two vectors for malaria transmission in Pu Teuy.     

Assortative mating in mixed swarms of the mosquito Anopheles gambiae s.s. M and S molecular forms,in Burkina Faso,West Africa

The molecular form composition of Anopheles gambiae Giles s.s. (Diptera: Culicidae) mating swarms and the associated mating pairs (copulae) were investigated during two rainy seasons (July to October, 2005 and July to November, 2006) in the villages of Soumousso and Vallée du Kou (VK7). Although the habitats of these villages differ markedly, sympatric populations of M and S molecular forms of An. gambiae s.s. occur in both places periodically. The main aim was to assess the degree to which these molecular forms mate assortatively. In Soumousso, a wooded savannah habitat, the majority of swarm samples consisted of only S‐form males (21/28), although a few M‐form males were found in mixed M‐ and S‐form swarms. In VK7, a rice growing area, the majority of swarm samples consisted of only M‐form males (38/62), until October and November 2006, when there were nearly as many mixed‐form as single‐form swarms. Overall, ～60% of M‐ and S‐form swarms were temporally or spatially segregated; the two forms were effectively prevented from encountering each other. Of the remaining 40% of swarms, however, only about half were single‐form and the rest were mixed‐form. Of the 33 copulae collected from mixed‐form swarms, only four were mixed‐form pairs, significantly fewer than expected by random pairing between forms (χ² = 10.34, d.f. = 2, P < 0.01). Finally, all specimens of inseminated females were of the same form as the sperm contained within their spermatheca (n = 91), even for the four mixed‐form copulae. These findings indicate that assortative mating occurs within mixed‐form swarms, mediated most probably by close‐range mate recognition cues.     

土荆芥的石油醚提取物及精油对冈比亚按蚊的杀虫活性

在室内条件下测试了土荆芥Chenopodium ambrosioides 叶片的石油醚提取物及精油对冈比亚按蚊Anopheles gambiae 0～2 d龄成虫和1～4龄幼虫的毒性。每一龄期各取30头幼虫分别接触50,100,250,500,750和1 000 mg/L提取物和精油,于24 h和48 h后记录死亡幼虫的数目。30头成虫在密封的长方形玻璃笼子中分别接触0.8,1.6,2.4 μL/L土荆芥精油蒸气, 24 h后记录死亡率。结果表明,测试的提取物和精油对冈比亚按蚊的各龄幼虫和成虫均具有毒性。48 h致死中浓度（LC50）的测试结果表明,石油醚提取物对1龄幼虫的毒性最强（14.89 mg/L）,其次是对4龄幼虫（18.90 mg/L）,对3龄幼虫的毒性最低（183.77 mg/L）; 精油对4龄幼虫毒性最强（36.62 mg/L）,其次是对1龄幼虫（90.75 mg/L）。推算的土荆芥精油对冈比亚按蚊的LC50为1.01 μL/L。本研究揭示了土荆芥对冈比亚按蚊的防治潜力。     

The effect of sugar meals and body size on fecundity and longevity of female Aedes communis(Diptera: Culicidae)

Abstract. Starved females of Aedes communis (DeGeer) had significantly shorter mean lifespans than females with access to one blood meal and/or sugar meals. For the starving females, lifespan was significantly correlated with wing length. This correlation was not found in sugar- and/or blood-fed females. Fructose and sucrose had similar effects on mosquito lifespan. The 25% and 50% (w/w) solutions of each, increased mean lifespans to 37.8 ± 7.2 and 34.9 ± 9.1 days, respectively. A 10% solution, of either sugar, was significantly inferior, mean lifespan 23.2 ± 10.2 days. The initial sugar meal was very important for female longevity. Subsequent meals aided in ovarian development. One sugar meal, one blood meal, and several sugar meals during oogenesis was found to be the optimal diet for the female mosquitoes in this study.     

Feeding response of Aedes aegypti and Anopheles dirus (Diptera: Culicidae) using out‐of‐date human blood in a membrane feeding apparatus

The colonization of Aedes aegypti and Anopheles dirus was performed using out‐of‐date human blood from a blood bank as a nutritional supply dispensed from a common artificial feeder. Preserved human blood was collected and used for feeding on days 5, 15, and 25 after date of expiration and dispensed from a common artificial feeder to rear the mosquitoes. Ae. aegypti had a feeding rate of 78.7, 62, and 18% at the respective intervals while An. dirus had a rate of 80, 56.8, and 7.3% on the same respective days. Direct feeding on live hamsters resulted in a rate of 96 and 90% for Ae. aegypti and An. dirus, respectively. Although egg production rates decreased from the day 5 feeding to the day 25 feeding, all of the developmental stages resulting from An. dirus fed at day 5 and 15 showed insignificant differences when compared with direct feeding on the blood of a hamster.     

Anopheles arabiensis and An. gambiae chromosomal inversion polymorphism, feeding and resting behaviour in relation to insecticide house-spraying in Tanzania

Abstract. Differential responses of the mosquitoes Anopheles arabiensis and An. gambiae sensu stricto to house-spraying with DDT or lambda-cyhalothrin were evaluated in relation to chromosomal inversion polymorphism, feeding and resting behaviour of these malaria vectors in Tanzania. Blood-fed mosquitoes from pit traps outdoors, exit traps on windows and indoor-resting catches were identified cytogenetically and the chromosomal inversion frequencies compared between samples and species. Their outdoor-resting behaviour was assessed by a mark–release–recapture experiment and by determining the proportion of freshly blood-fed individuals in exit traps. The source of bloodmeals was analysed by an ELISA method. Endophagic females of An. arabiensis were more likely than those of An. gambiae to exit from a house on the night of blood-feeding. Only in one out of three villages was there evidence that chromosomally distinct individuals within a species had different preferences for resting sites. There were indications, but not conclusive evidence, that mosquitoes caught indoors or outdoors had a tendency to return to the same type of resting site. In villages sprayed with either insecticide, the mean age of the vector populations was greatly reduced, compared with those in the unsprayed villages. An. arabiensis females exited from DDT sprayed houses after blood-feeding, whereas with lambda-cyhalothrin those exiting were mostly unfed and there was a decline in the human blood index. The excitorepellency of DDT was perceived as a disadvantage, whereas lambda-cyhalothrin apparently had more impact on malaria transmission by An. arabiensis.     

南美斑潜蝇寄主选择性与植物次生化合物及叶毛的关系

采用非自由选择法,以刺伤孔（包括取食孔和产卵孔）数为指标,在室内测定了南美斑潜蝇对6科、16种、22个蔬菜品种的寄主选择性。结果表明,南美斑潜蝇对供试蔬菜品种的选择性存在显著差异,选择性大小依次为豆科>菊科>葫芦科>茄科>伞形科>十字花科; 菜豆和茼蒿是南美斑潜蝇最喜欢产卵和取食的寄主,而甘蓝和番茄是其选择性最差的寄主。化学分析和解剖镜观察表明,叶片单宁酸、黄酮含量及叶毛密度在供试品种间存在显著差异： 苦瓜的单宁酸含量最高,其次为豌豆、番茄和青椒,含量最低的为金丝搅瓜、茼蒿、菜豆和美国西芹; 黄酮含量最高的为甘蓝和番茄,其次为苦瓜,最低为茼蒿、菜豆和美国西芹; 南瓜和小白菜的叶毛密度最高,其次为西葫芦、金丝搅瓜和甘蓝,最低为青椒、莴苣和苦瓜。相关分析表明,南美斑潜蝇寄主选择性与叶片单宁酸和黄酮含量呈显著的负相关关系,相关系数分别为-0.4425（ P=0.0392）和 -0.5728（ P=0.0053）,而与叶毛密度相关关系不显著（ R＝-0.1807,P=0.4211）。说明黄酮和单宁酸对南美斑潜蝇产卵和取食具有明显的抑制作用。     

Spatio‐temporal variations of Anopheles coluzzii and An. gambiae and their Plasmodium infectivity rates in Lobito,Angola

From 2003 to 2007, entomological surveys were conducted in Lobito town (Benguela Province, Angola) to determine which Anopheles species were present and to identify the vectors responsible for malaria transmission in areas where workers of the Sonamet Company live. Two types of surveys were conducted: (1) time and space surveys in the low and upper parts of Lobito during the rainy and dry periods; (2) a two‐year longitudinal study in Sonamet workers' houses provided with long‐lasting insecticide‐treated nets (LLIN), “PermaNet,” along with the neighboring community. Both species, An. coluzzii (M molecular form) and An. gambiae (S molecular form), were collected. Anopheles coluzzii was predominant during the dry season in the low part of Lobito where larvae develop in natural ponds and temporary pools. However, during the rainy season, An. gambiae was found in higher proportions in the upper part of the town where larvae were collected in domestic water tanks built near houses. Anopheles melas and An. listeri were captured in higher numbers during the dry season and in the low part of Lobito where larvae develop in stagnant brackish water pools. The infectivity rates of An. gambiae s.l. varied from 0.90% to 3.41%.     

Effect of larval rearing temperature on adult female morphology of Anopheles (Nyssorhynchus) albimanus (Diptera: Culicidae)

Summary

Time to pupation, percent survival to pupation, and percent adult emergence of Anopheles albimanus Wiedemann decreased at higher larval rearing temperature. Mosquitoes reared at 30°C experienced higher mortality during the pupal stage than did mosquitoes reared at 22°C. Analysis of variance revealed that wing length and costal wing spot patterns of adult female A. albimanus were affected by larval rearing temperature. Female A. albimanus reared at 22°C had longer wings, and larger basal pale + prehumeral pale, prehumeral dark, and humeral pale costal wing spots than did female siblings reared at 30°C. Female A. albimanus reared at 30°C had larger subcostal pale spots than did female siblings reared at 22°C. Analyses of 2x2 contingency tables indicated that sex ratio was independent of larval rearing temperature, whereas survival to the adult stage and coalescence of wing spots were not independent of rearing temperature. The need to examine stability of morphological characters under differing environmental conditions is discussed.     

Studies on the resting behaviour and host choice of Anopheles gambiae and An. arabiensis from Muleba,Tanzania

The relative efficacy of a mechanical (Prokopack) collection method vs. manual aspiration in the collection of resting mosquitoes was evaluated in northern Tanzania before and after an intervention using indoor residual spraying and longlasting insecticide‐treated nets. In smoke‐free houses mosquitoes were collected from the roof and walls, but in smoky houses mosquitoes were found predominantly on the walls. Anopheles gambiae (Diptera: Culicidae) constituted 97.7% of the 312 An. gambiae complex specimens identified before but only 19.3% of the 183 identified after the intervention. A single sampling with the Prokopack collected a third of the available insects. Anopheles gambiae completed its gonotrophic development indoors, whereas Anopheles arabiensis did so outdoors. In both species gonotrophic development took 2 days. Most unfed resting An. arabiensis collected outdoors were virgins, whereas the majority of engorged insects were parous (with well‐contracted sacs). Daily survival was estimated to be 80.0%. Only 9.4% of the engorged An. arabiensis collected outdoors and 47.1% of those collected indoors had fed on humans. Using the Prokopack sampler is more efficient than manual methods for the collection of resting mosquitoes. Malaria transmission may have been affected by a change in vector composition resulting from a change in feeding, rather than reduced survival. Monitoring the proportions of members of the An. gambiae complex may provide signals of an impending breakdown in control.