Gabriella Gibson

Blood-feeding behaviour of the malarial mosquito Anopheles arabiensis: implications for vector control

Abstract Feeding behaviour of the malaria vector Anopheles arabiensis Patton (Diptera: Culicidae) was monitored for 12 months (March 2003−February 2004) in the Konso District of southern Ethiopia (5°15′N, 37°28′E). More than 45 000 An. arabiensis females were collected by host‐baited sampling methods (light‐traps, human landing catches, cattle‐baited traps) and from resting sites (huts and pit shelters). In the village of Fuchucha, where the ratio of cattle : humans was 0.6 : 1, 51% of outdoor‐resting mosquitoes and 66% of those collected indoors had fed on humans, human baits outdoors caught > 2.5 times more mosquitoes than those indoors and the mean catch of mosquitoes from pit shelters was about five times that from huts. Overall, the vast majority of feeding and resting occurred outdoors. In the cattle camps of Konso, where humans slept outdoors close to their cattle, ∼ 46% of resting mosquitoes collected outdoors had fed on humans despite the high cattle : human ratio (17 : 1). In both places, relatively high proportions of bloodmeals were mixed cow + human: 22–25% at Fuchucha and 37% in the cattle camps. Anthropophily was also gauged experimentally by comparing the numbers of mosquitoes caught in odour‐baited entry traps baited with either human or cattle odour. The human‐baited trap caught about five times as many mosquitoes as the cattle‐baited one. Notwithstanding the potential pitfalls of using standard sampling devices to analyse mosquito behaviour, the results suggest that the An. arabiensis population is inherently anthropophagic, but this is counterbalanced by exophagic and postprandial exophilic tendencies. Consequently, the population feeds sufficiently on humans to transmit malaria (sporozoite rates: 0.3% for Plasmodium falciparum and 0.5% for P. vivax, by detection of circumsporozoite antigen) but also takes a high proportion of meals from non‐human hosts, with 59–91% of resting mosquitoes containing blood from cattle. Hence, classical zooprophylaxis is unlikely to have a significant impact on the malaria vectorial capacity of An. arabiensis in Konso, whereas treating cattle with insecticide might do.

Co-adaptation between cassava mosaic geminiviruses and their local vector populations.

Four cassava mosaic geminivirus (CMG) isolates; African cassava mosaic virus from Namulonge, Uganda (ACMV-[Nam]), East African cassava mosaic virus from Mtwara, Tanzania (EACMV-[Mtw]), EACMV-Uganda from Namulonge (EACMV-UG[Nam]) and Indian cassava mosaic virus from Trivandrum, India (ICMV-[Tri]) were compared for their ability to be transmitted by four colonies of cassava whitefly, Bemisia tabaci (Gennadius), collected from Namulonge (NAM), Mtwara (MTW), Kumasi (KUM) and Trivandrum (TRI). With 20 adult whiteflies per test plant, the CMGs from Africa were transmitted by all three of the African B. tabaci populations to 60-79% of the target plants. Indian cassava B. tabaci transmitted ICMV-[Tri] to 89% of the target plants. In contrast, Indian cassava B. tabaci transmitted EACMV-[Mtw] and Tanzanian cassava B. tabaci transmitted ICMV-[Tri] less than a tenth as often, even when using 50 adults per plant and with increased acquisition and inoculation access periods. The complete coat protein genes of the CMGs had sequences typical of their source viruses, the major differences occurring between those originating from India and Africa. Symptom severity of the CMGs was quantified precisely by both visual assessment and image analysis techniques. EACMV-[Mtw] and ACMV-[Nam] were the most and least damaging isolates with 15.4 and 10.0% of the leaf area diseased, respectively. In these tests the transmission frequency was not linked to symptom severity in the source plants. These data support the hypothesis that virus-vector co-adaptation exists in the cassava mosaic disease (CMD) pathosystem and the results are discussed in relation to CMD epidemiology.

Host-plant viral infection effects on arthropod-vector population growth, development and behaviour: management and epidemiological implications

Publisher Summary This chapter reviews the evidence for positive, negative and neutral effects on the population growth of several arthropod vector species when feeding on virus-infected compared with uninfected host plants. The emphasis is on whiteflies and aphids as these have received the greatest attention. Two important examples are the B. tabaci -borne cassava mosaic disease (CMD) pandemic that continues to devastate cassava production in large areas of sub-Saharan Africa (SSA); and the tomato leaf curl disease epidemics in the Indian subcontinent that have had equally serious implications for tomato production in this region. These problems have been considered in detail and experimental data have been included to show, for both pathosystems, that the B. tabaci colonizing virus-infected host plants have significantly higher-population growth rates compared to those colonizing virus-free hosts. This effect has been discussed in the context of the significantly higher densities of B. tabaci present on symptomatic cassava and the behavioral changes associated with this effect. Data has also been presented to show that the concentrations of four amino acids are significantly higher in the phloem sap of CMD-infected cassava plants. These interacting effects are considered in relation to probable mechanisms contributing to the rapid spread of these epidemics.

Flying in Tune: Sexual Recognition in Mosquitoes

Mosquitoes hear with their antennae, which in most species are sexually dimorphic. Johnston, who discovered the mosquito auditory organ at the base of the antenna 150 years ago, speculated that audition was involved with mating behaviour. Indeed, male mosquitoes are attracted to female flight tones. The male auditory organ has been proposed to act as an acoustic filter for female flight tones, but female auditory behavior is unknown. We show, for the first time, interactive auditory behavior between males and females that leads to sexual recognition. Individual males and females both respond to pure tones by altering wing-beat frequency. Behavioral auditory tuning curves, based on minimum threshold sound levels that elicit a change in wing-beat frequency to pure tones, are sharper than the mechanical tuning of the antennae, with males being more sensitive than females. We flew opposite-sex pairs of tethered Toxorhynchites brevipalpis and found that each mosquito alters its wing-beat frequency in response to the flight tone of the other, so that within seconds their flight-tone frequencies are closely matched, if not completely synchronized. The flight tones of same-sex pairs may converge in frequency but eventually diverge dramatically.

Sex Recognition through Midflight Mating Duets in Culex Mosquitoes Is Mediated by Acoustic Distortion

Sexual recognition through wing-beat frequency matching was first demonstrated in Toxorhynchites brevipalpis, where wing-beat frequencies of males and females are similar. Here we show frequency matching in Culex quinquefasciatus, where the wing-beat frequencies of males and females differ considerably. The wing-beat frequencies converge not on the fundamental but on the nearest shared harmonic (usually females third and males second). Frequencies in this range are, however, too high to elicit phasic sensory-neural responses from the Johnstons organ (JO) or to drive the mosquitos motor neurons. Potential cues for frequency matching are difference tones produced by nonlinear mixing of male and female flight tones in the vibrations of the mosquitos antennae. Receptor potentials and neural-motor activity were recorded in response to difference tones produced when a mosquito was stimulated simultaneously by two tones at frequencies outside the phasic response range of the JO but within range of the antennal vibrations. We demonstrate sexual recognition through matching of flight-tone harmonics in Culex mosquitoes and suggest that difference tones are used as an error signal for frequency matching beyond the frequency range of the JOs sensory-neural range. This is the first report of acoustic distortion being exploited as a sensory cue, rather than existing as an epiphenomenon.

Reproductive incompatibility and cytochrome oxidase I gene sequence variability amongst host-adapted and geographically separate Bemisia tabaci populations (Hemiptera: Aleyrodidae)

Abstract.  Reciprocal‐crossing experiments were carried out and mitochondrial cytochrome oxidase I gene (mtCOI) sequences were compared for allopatric and sympatric Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) populations collected from Africa and India, and from the host‐plants cassava, sweet‐potato and a common weed, Euphorbia geniculata. Three incompatible mating groups were discovered, which involved the cassava B. tabaci colonies from Africa and India, the cassava and sweet‐potato B. tabaci populations from Uganda, and the cassava and E. geniculata B. tabaci from India. Successful reciprocal mating occurred between cassava‐specific B. tabaci from Uganda, Tanzania and Ghana, and between two Indian cassava B. tabaci populations. The parsimony and neighbour‐joining analyses of 699 bp mtCOI gene sequences divided the colonies primarily into those originating from Africa and India. Further subgrouping corresponded to host‐plant specialization. Cassava‐specific Ugandan, Tanzanian and Ghanaian colonies formed a single group and the sympatric sweet‐potato colony from Uganda grouped separately from them. The two geographically distant Indian cassava B. tabaci populations were similar and formed a single group, whereas the sympatric E. geniculata colony formed a sister clade. The clades generated by the phylogenetic analyses were maintained, with highly supported bootstrap values, when other published mtCOI gene sequences were included in the tree‐building process and the divisions matched those revealed by the reciprocal‐crossing experiments. These data suggest that biologically discrete populations exist within B. tabaci (sensu Russell, 1957 ).

Could insecticide-treated cattle reduce Afrotropical malaria transmission? Effects of deltamethrin-treated Zebu on Anopheles arabiensis behaviour and survival in Ethiopia.

Abstract.  Anopheles arabiensis Patton (Diptera: Culicidae) is the most widespread vector of malaria in the Afrotropical Region. Because An. arabiensis feeds readily on cattle as well as humans, the insecticide‐treatment of cattle – as employed to control tsetse (Diptera: Glossinidae) and ticks (Acari: Ixodidae) – might simultaneously affect the malaria vectorial capacity of this mosquito. Therefore, we conducted field experiments in southern Ethiopia to establish whether Zebu cattle (Bos indicus L.) treated with a pour‐on pyrethroid formulation of 1% deltamethrin, widely used to control ticks and tsetse, would be effective against An. arabiensis or cause the female mosquitoes to feed more frequently on humans, due to behavioural avoidance of insecticide‐treated cattle. Contact bioassays (3 min exposure) showed that the insecticide remained effective for about 1 month (kill rate > 50%) against mosquitoes feeding on the flanks of treated cattle. A novel behavioural assay demonstrated that An. arabiensis readily fed on insecticide‐treated cattle and were not deflected to human hosts in the presence of treated cattle. DNA‐fingerprinting of bloodmeals revealed that An. arabiensis naturally feeds most frequently on older animals, consistent with the established practice of applying insecticide only to older cattle, while allowing younger untreated animals to gain immunity against infections transmitted by ticks. These encouraging results were tempered by finding that > 90% of An. arabiensis, An. pharoensis and An. tenebrosus females feed on the legs of cattle, farthest from the site of pour‐on application along the animals back and where the treatment may be least residual due to weathering. Observations of mosquitoes feeding naturally on insecticide‐treated cattle showed that the majority of wild female anophelines alighted on the host animal for less than 1 min to feed, with significantly shorter mean duration of feeding bouts on insecticide‐treated animals, and the effective life of the insecticide was only 1 week. Thus the monthly application of deltamethrin to cattle, typically used to control tsetse and ticks, is unlikely to be effective against An. arabiensis populations or their vectorial capacity. Even so, it seems likely that far greater impact on anopheline mosquitoes could be achieved by applying insecticide selectively to the legs of cattle.

Humming in tune: sex and species recognition by mosquitoes on the wing.

Mosquitoes are more sensitive to sound than any other insect due to the remarkable properties of their antennae and Johnston’s organ at the base of each antenna. Male mosquitoes detect and locate female mosquitoes by hearing the female’s flight tone, but until recently we had no idea that females also respond to male flight tones. Our investigation of a novel mechanism of sex recognition in Toxorhynchites brevipalpis revealed that male and female mosquitoes actively respond to the flight tones of other flying mosquitoes by altering their own wing-beat frequencies. Male–female pairs converge on a shared harmonic of their respective fundamental flight tones, whereas same sex pairs diverge. Most frequency matching occurs at frequencies beyond the detection range of the Johnston’s organ but within the range of mechanical responsiveness of the antennae. We have shown that this is possible because the Johnston’s organ is tuned to, and able to detect difference tones in, the harmonics of antennal vibrations which are generated by the combined input of flight tones from both mosquitoes. Acoustic distortion in hearing organs exists usually as an interesting epiphenomenon. Mosquitoes, however, appear to use it as a sensory cue that enables male–female pairs to communicate through a signal that depends on auditory interactions between them. Frequency matching may also provide a means of species recognition. Morphologically identical but reproductively isolated molecular forms of Anopheles gambiae fly in the same mating swarms, but rarely hybridize. Extended frequency matching occurs almost exclusively between males and females of the same molecular form, suggesting that this behavior is associated with observed assortative mating.

Effects of Age and Size on Anopheles gambiae s.s. Male Mosquito Mating Success

ABSTRACT Before the release of genetically-modified or sterile male mosquitoes in an attempt to control local populations of malaria vectors, it is crucial to determine male traits involved in mating success. The effects of male size and age as determinants of male mating success in Anopheles gambiae s.s. were measured in the field and under laboratory conditions in Burkina Faso. First, the body sizes (estimated by wing length) of mating, swarming, and indoor-resting male mosquitoes were compared over a 3-yr period (2006–2009) from July to October in Soumousso and Vallée du Kou, two villages in western Burkina Faso. Second, the age structure of swarming and resting male mosquitoes were characterized based on the number of spermatocysts and the proportion of sperm in the reservoir of wild-caught male testis. Third, male age effects on the insemination rate of female An. gambiae were investigated in the laboratory. The mean size of males collected in copula was significantly larger than the mean for swarming males and indoor-resting males. The optimum male age for successful insemination of females was 4–8 d. These results suggest that male size is an important trait in determining male mating competitiveness in the field. Although age was not found to be a significant factor in mating competitiveness, it was significantly correlated with swarming behaviors in the field and insemination success in the laboratory. The implications of these results in terms of sexual selection in An. gambiae and vector control programs are further discussed.

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.