Radiation dose-fractionation in adult Aedes aegypti mosquitoes.

Balancing process efficiency and adult sterile male biological quality is one of the challenges in the success of the sterile insect technique (SIT) against insect pest populations. For the SIT against mosquitoes, many stress factors need to be taken into consideration when producing sterile males that require high biological quality to remain competitive once released in the field. Pressures of mass rearing, sex sorting, irradiation treatments, packing, transport and release including handling procedures for each step, add to the overall stress budget of the sterile male post-release. Optimizing the irradiation step to achieve maximum sterility while keeping off-target somatic damage to a minimum can significantly improve male mating competitiveness. It is therefore worth examining various protocols that have been found to be effective in other insect species, such as dose fractionation. A fully sterilizing dose of 70 Gy was administered to Aedes aegypti males as one acute dose or fractionated into either two equal doses of 35 Gy, or one low dose of 10 Gy followed by a second dose of 60 Gy. The two doses were separated by either 1- or 2-day intervals. Longevity, flight ability, and mating competitiveness tests were performed to identify beneficial effects of the various treatments. Positive effects of fractionating dose were seen in terms of male longevity and mating competitiveness. Although applying split doses generally improved male quality parameters, the benefits may not outweigh the added labor in SIT programmes for the management of mosquito vectors.

Title: Fractionnement de la dose d'irradiation chez les moustiques Aedes aegypti adultes. Abstract: Équilibrer l'efficacité du processus et la qualité biologique des mâles adultes stériles est l'un des défis du succès de la technique des insectes stériles (TIS) contre les populations d'insectes nuisibles. Pour la TIS contre les moustiques, de nombreux facteurs de stress sont à prendre en compte lors de la production de mâles stériles qui nécessitent une haute qualité biologique pour rester compétitifs une fois relâchés au champ. Les pressions de l'élevage en masse, du triage par sexe, des traitements d'irradiation, de l'emballage, du transport et de la libération, y compris les procédures de manipulation pour chaque étape, s'ajoutent au budget de stress global du mâle stérile après la libération. L'optimisation de l'étape d'irradiation pour atteindre une stérilité maximale tout en minimisant les dommages somatiques hors cible peut améliorer considérablement la compétitivité de l'accouplement des mâles et il est donc important d'examiner divers protocoles qui se sont révélés efficaces chez d'autres espèces d'insectes, comme le fractionnement de dose. Une dose entièrement stérilisante de 70 Gy a été administrée aux mâles Aedes aegypti en une dose unique ou fractionnée en deux doses égales de 35 Gy, ou une faible dose de 10 Gy suivie d'une seconde dose de 60 Gy. Les deux doses étaient séparées par des intervalles de 1 ou 2 jours. Des tests de longévité, d'aptitude au vol et de compétitivité à l'accouplement ont été réalisés pour identifier les effets bénéfiques des différents traitements. Des effets positifs de la dose de fractionnement ont été observés en termes de longévité des mâles et de compétitivité à l'accouplement. Bien que l'application de doses fractionnées améliore généralement les paramètres de qualité des mâles, les avantages peuvent ne pas compenser le travail supplémentaire dans les programmes TIS pour la gestion des moustiques vecteurs.

Aedes aegypti larval development and pupal production in the FAO/IAEA mass-rearing rack and factors influencing sex sorting efficiency.

The production of a large number of mosquitoes of high biological qualities and reliable sex sorting before release are key challenges when applying the sterile insect technique as part of an area-wide integrated pest management approach. There is a need to fully evaluate the production capacity of the equipment developed in order to plan and maintain a daily production level for large-scale operational release activities. This study aimed to evaluate the potential use of the FAO/IAEA larval rearing unit for Aedes aegypti and the subsequent female contamination rate after sex sorting with a Fay-Morlan glass separator. Trays from each rack were tilted and their contents sorted either for each individual tray or after mixing the content of all trays from the rack. The pupal production and the female contamination rate were estimated with respect to day of collection, position of the tray, type of pupae collection, and sorting operator. Results showed significant daily variability of pupal production and female contamination rate, with a high male pupal production level achieved on the second day of collection and estimated female contamination of male pupae reached around 1%. Neither tray position nor type of pupae collection affected the pupal production and female contamination rate. However, the operator had a significant effect on the female contamination rate. These results highlight the need to optimize pupal production at early days of collection and to develop a more effective and automated method of sex separation.

TITLE: Développement larvaire et production de pupes d'Aedes aegypti dans le système d'élevage de masse de la FAO/AIEA et facteurs influençant l'efficacité de la séparation des sexes. ABSTRACT: La production d'un grand nombre de moustiques de haute qualité biologique et le tri des sexes avant les lâchers sont des défis clés lors de l'application de la technique des insectes stériles, dans le cadre d'une approche de lutte intégrée contre les ravageurs à l'échelle d'une zone. Il est nécessaire d'évaluer pleinement la capacité de production des équipements développés afin de planifier et de maintenir un niveau de production quotidien pour les activités de libération opérationnelle à grande échelle. Cette étude visait à évaluer l'utilisation potentielle de l'unité d'élevage larvaire FAO/AIEA pour Aedes aegypti et le taux de contamination par des femelles après le tri sexuel avec un séparateur en verre Fay­Morlan. Les plateaux de chaque rack ont été inclinés et leur contenu trié soit pour chaque plateau, soit après avoir mélangé le contenu de tous les plateaux du rack. La production de pupes et le taux de contamination par des femelles ont été estimés en fonction du jour de collecte, de la position du bac, du type de collecte des pupes et de l'opérateur du tri. Les résultats ont montré une variabilité quotidienne significative de la production de pupes et du taux de contamination par des femelles, avec un niveau élevé de production de pupes mâles atteint le deuxième jour de collecte et la contamination estimée des pupes mâles par des femelles a atteint environ 1 %. Ni la position du plateau ni le type de collecte des pupes n'ont affecté la production de pupes et le taux de contamination par des femelles. Cependant, l'opérateur avait un effet significatif sur le taux de contamination par les femelles. Ces résultats mettent en évidence la nécessité d'optimiser la production des pupes dès les premiers jours de la collecte et de développer une méthode de séparation des sexes plus efficace et automatisée.

A rapid quality control test to foster the development of the sterile insect technique against Anopheles arabiensis.

BACKGROUND: With the fight against malaria reportedly stalling there is an urgent demand for alternative and sustainable control measures. As the sterile insect technique (SIT) edges closer to becoming a viable complementary tool in mosquito control, it will be necessary to find standardized techniques of assessing male quality throughout the production system and post-irradiation handling. Flight ability is known to be a direct marker of insect quality. A new version of the reference International Atomic Energy Agency/Food and Agricultural Organization (IAEA/FAO) flight test device (FTD), modified to measure the flight ability and in turn quality of male Anopheles arabiensis within a 2-h period via a series of verification experiments is presented. METHODS: Anopheles arabiensis juveniles were mass reared in a rack and tray system. 7500 male pupae were sexed under a stereomicroscope (2500 per treatment). Stress treatments included irradiation (with 50, 90, 120 or 160 Gy, using a Gammacell 220), chilling (at 0, 4, 8 and 10 °C) and compaction weight (5, 15, 25, and 50 g). Controls did not undergo any stress treatment. Three days post-emergence, adult males were subjected to either chilling or compaction (or were previously irradiated at pupal stage), after which two repeats (100 males) from each treatment and control group were placed in a FTD to measure flight ability. Additionally, one male was caged with 10 virgin females for 4 days to assess mating capacity (five repeats). Survival was monitored daily for a period of 15 days on remaining adults (two repeats). RESULTS: Flight ability results accurately predicted male quality following irradiation, with the first significant difference occurring at an irradiation dose of 90 Gy, a result which was reflected in both survival and insemination rates. A weight of 5 g or more significantly reduced flight ability and insemination rate, with survival appearing less sensitive and not significantly impacted until a weight of 15 g was imposed. Flight ability was significantly reduced after treatments at 4 °C with the insemination rate more sensitive to chilling with survival again less sensitive (8 and 0 °C, respectively). CONCLUSIONS: The reported results conclude that the output of a short flight ability test, adapted from the previously tested Aedes FTD, is an accurate indicator of male mosquito quality and could be a useful tool for the development of the SIT against An. arabiensis.

Longevity of mass-reared, irradiated and packed male Anopheles arabiensis and Aedes aegypti under simulated environmental field conditions.

BACKGROUND: To ensure the success of a mosquito control programme that integrates the sterile insect technique (SIT), it is highly desirable to release sterile males with a maximal lifespan to increase release effectiveness. Understanding sterile male survival under field conditions is thus critical for determining the number of males to be released. Our study aimed to investigate the effect of mass rearing, irradiation, chilling, packing and release time on irradiated male mosquito longevity. METHODS: Anopheles arabiensis and Aedes aegypti immature stages were mass-reared using a rack and tray system. Batches of 50 males irradiated at the pupal stage were immobilised, packed into canisters and chilled for 6 hours at 6 °C. Mosquitoes were then transferred either in the early morning or early evening into climate chambers set to simulate the weather conditions, typical of the beginning of the rainy season in Khartoum, Sudan and Juazeiro, Brazil for An. arabiensis and Ae. aegypti, respectively. The longevity of experimental males was assessed and compared to mass-reared control males subjected either to simulated field or laboratory conditions. RESULTS: The combined irradiation, chilling and packing treatments significantly reduced the longevity of both An. arabiensis and Ae. aegypti under simulated field conditions (P < 0.001). However, packing alone did not significantly reduce longevity of Ae. aegypti (P = 0.38) but did in An. arabiensis (P < 0.001). Overall, the longevity of mass reared, irradiated and packed males was significantly reduced, with the median survival time (days) lower following an early morning introduction (4.62 ± 0.20) compared to an evening (7.34 ± 0.35) in An. arabiensis (P < 0.001). However, there was no significant difference in longevity between morning (9.07 ± 0.54) and evening (7.76 ± 0.50) in Ae. aegypti (P = 0.14). CONCLUSIONS: Our study showed that sterile mass-reared males have a reduced lifespan in comparison to laboratory-maintained controls under simulated field conditions, and that An. arabiensis appeared to be more sensitive to the handling process and release time than Ae. aegypti. Longevity and release time are important parameters to be considered for a successful area-wide integrated vector control programme with a SIT component.