Arop L. Deng

Mediation of oviposition site selection in the African malaria mosquito Anopheles gambiae (Diptera: Culicidae) by semiochemicals of microbial origin

Laboratory studies were carried out to investigate the role of larval habitat-derived microorganisms in the production of semiochemicals for oviposition site selection by Anopheles gambiae Giles sensu stricto mosquitoes. Dual-choice bioassays with gravid females were conducted in standard mosquito cages. Field-collected or laboratory-reared mosquitoes, individually or in groups, were offered a choice between unmodified (water or soil from a natural breeding site) or modified substrates (filtered water, autoclaved soil or sterile media to which bacterial suspensions had been added). Egg counts were used to assess oviposition preferences. Mosquitoes preferred to oviposit on unmodified substrates from natural larval habitats containing live microorganisms rather than on sterilized ones. Variable responses were observed when sterile substrates were inoculated with bacteria isolated from water and soil from natural habitats. We conclude that microbial populations in breeding sites produce volatiles that serve as semiochemicals for gravid An. gambiae. These signals, in conjunction with other (non-olfactory) chemical and physical cues, may be used by the female to assess the suitability of potential larval habitats in order to maximize the fitness of her offspring.RésuméDes études de laboratoire ont été conduites afin de déterminer le rôle de substances chimiques attractives de microorganismes présents dans les sites larvaires dans la préférence de ponte des femelles gravides d’Anopheles gambiae s.s. pour ces sites. Un test biologique à double choix a été mis en place dans des cages à moustiques standards contenant des femelles gravides. Un choix entre substrat naturel (eau et sol non modifiés provenant des sites larvaires naturels) et substrat modifié (eau filtrée, sol stérilisé, ajout de suspensions bactériennes) est offert individuellement ou en groupe aux femelles collectées sur le terrain ou élevées en laboratoire. Le nombre d’oeufs pondus dans chaque substrat a été utilisé comme critère pour évaluer la préférence des femelles gravides pour un substrat particulier. Les femelles ont préféré pondre dans le substrat naturel contenant des microorganismes vivants plutôt que dans le substrat stérile. Des réponses variables ont été observées lorsque des bactéries provenant des sites larvaires naturels ont été inoculées dans le substrat stérile. Nous concluons que la population microbienne, présente dans les site larvaires, produit des substances chimiques odorantes attirant les femelles gravides d’Anopheles gambiae s.s. Ces signaux, associés à d’autres substances non odorantes et à des facteurs physiques, peuvent être utilisés par les femelles pour évaluer le potentiel d’un site larvaire, en vue du développement optimal de leur progéniture.

Daily oviposition patterns of the African malaria mosquito Anopheles gambiae Giles (Diptera: Culicidae) on different types of aqueous substrates

Background Anopheles gambiae Giles is the most important vector of human malaria in sub-Saharan Africa. Knowledge of the factors that influence its daily oviposition pattern is crucial if field interventions targeting gravid females are to be successful. This laboratory study investigated the effect of oviposition substrate and time of blood feeding on daily oviposition patterns of An. gambiae mosquitoes. Methods Greenhouse-reared gravid and hypergravid (delayed oviposition onset) An. gambiae sensu stricto and wild-caught An. gambiae sensu lato were exposed to three types of substrates in choice and no-choice cage bioassays: water from a predominantly anopheline colonised ground pool (anopheline habitat water), swamp water mainly colonised by culicine larvae (culicine habitat water) and distilled water. The daily oviposition pattern and the number of eggs oviposited on each substrate during the entire egg-laying period were determined. The results were subjected to analysis of variance using the General Linear Model (GLM) procedure. Results The main oviposition time for greenhouse-reared An. gambiae s.s. was between 19:00 and 20:00 hrs, approximately one hour after sunset. Wild-caught gravid An. gambiae s.l. displayed two distinct peak oviposition times between 19:00 and 20:00 hrs and between 22:00 and 23:00 hrs, respectively. During these times, both greenhouse-reared and wild-caught mosquitoes significantly (P < 0.05) preferred anopheline habitat water to the culicine one. Peak oviposition activity was not delayed when the mosquitoes were exposed to the less preferred oviposition substrate (culicine habitat water). However, culicine water influenced negatively (P < 0.05) not only the number of eggs oviposited by the mosquitoes during peak oviposition time but also the overall number of gravid mosquitoes that laid their eggs on it. The differences in mosquito feeding times did not affect the daily oviposition patterns displayed. Conclusion This study shows that the peak oviposition time of An. gambiae s.l. may be regulated by the light-dark cycle rather than oviposition habitat characteristics or feeding times. However, the number of eggs laid by the female mosquito during the peak oviposition time is affected by the suitability of the habitat.

Regulation of oviposition in Anopheles gambiae s.s.: role of inter- and intra-specific signals.

Females of Anopheles gambiae Giles normally oviposit in a large number of fresh, small, sunlit, and spatially spread temporary pools. Such pools are associated with lower levels of predation compared to large, longer-lasting habitats. We compared oviposition levels on preferred (water collected from natural anopheline larval habitats) and non-preferred (distilled water) aqueous substrates by gravid females that contained different densities of conspecific eggs or early and late instar larvae. The presence of conspecific larvae, but not eggs, had a positive or negative effect on the ovipositional responses of gravid An. gambiae females, depending on the quality (preferred or non-preferred by the mosquito) of the oviposition water and the density of larvae. Presence of larvae, at all densities, in distilled water deterred oviposition. However, in natural anopheline pool water, a low density of larvae increased oviposition, whereas a higher density inhibited oviposition. Our results suggest that two signals produced by this mosquito may be involved in regulating oviposition: a volatile pheromone emitted by conspecific larvae, which augments the effect of a volatile signal emitted by preferred habitats, and a non-olfactory cue associated with high densities of larvae that deters oviposition.

Comparison of Toxic and Repellent Effects of Lantana Camara L. with Tephrosia Vogelii Hook and a Synthetic Pesticide Against Sitophilus Zeamais Motschulsky (Coleoptera: Curculionidae) in Stored Maize Grain

The insecticidal and repellent properties of Lantana camara and Tephrosia vogelii were evaluated against Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) in stored maize grain. Five treatment rates (1.0, 2.5,5.0,7.5 and 10.0% w/w) of each powdered plant material, an untreated control and a synthetic insecticide (Actellic Super™ 2% dust) were used to investigate treatment efficacy on mortality of the adult insect (five to eight days old), F1 progeny emergence and repellency against S. zeamais adults. After 21 days, L. camara and T. vogelii caused 82.7–90.0% and 85.0–93.7% insect mortality, respectively. The mean lethal exposure times (LT50) to achieve 50% mortality varied from five to six days (7.5–10.0% w/w) to seven to eight days (2.5–5.0% w/w) for both plants. Probit regression analysis showed a significant relationship between plant powder concentration and insect mortality. The plant powders and synthetic insecticide reduced adult F1 insects by more than 75% compared to the untreated control. Tephrosia vogelii was most repellent to S. zeamais at 7.5–10.0% (w/ w), repelling 87.5% of the insects, followed by T. vogelii at 2.5% w/w and L. camara at 10% w/w which repelled 65.0 and 62.5% of insects respectively. The implications of these results are discussed in the context of smallscale farmer usage of these plants for stored product protection.RésuméLes propriétés insecticides et répulsives de Lantana camara et Tephrosia vogelii ont été évaluées contre Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) sur du maïs stocké en grain. Le matériel végétal de chaque espèce, réduit en poudre, a été appliqué à cinq concentrations (1,0, 2,5, 5,0, 7,5 et 10,0%), et comparé à un insecticide synthétique (Actellic Super™ 2% en poudre) et à un témoin non-traité. L’efficacité des traitements a été évaluée en mesurant la mortalité des adultes (âgés de cinq à huit jours), l’émergence de la descendance des F,, et l’effet répulsif sur les adultes. Après 21 jours, la mortalité provoquée par L. camara est comprise entre 82,7 et 90,0%, celle causée par T. vogelii entre 85,0 et 93,7%. Pour les deux plantes, les temps moyens d’exposition léthale, entraînant une mortalité de 50% (LT), varient de cinq à six jours (7,5 et 10,0% m/m) et de 7 à 8 jours (2,5 et 5,0% m/m). Une analyse de régression Probit a démontré une relation significative entre la mortalité des insectes et la concentration des poudres végétales utilisées. L’application de ces poudres végétales, comme l’insecticide synthétique, entraine une réduction de-plus de 75% de la descendance en F1, par rapport au témoin non-traité. Tephrosia vogelii s’avère être le plus répulsif contre S. zeamais, aux concentrations de 7,5 et 10,0% (m/m), repoussant 87,5% des insectes. Puis viennent T. vogelii à 2,5% m/m, repoussant 65,0% des insectes et L. camara à 10,0% m/m, repoussant 62,5%. La signification et les conséquences de ces résultats sont présentées dans le contexte de l’utilisation de ces plantes pour la protection des denrées stockées par les petits agriculteurs, dans les pays en voie développement.