Development of the Sterile Insect Technique to control the dengue vector Aedes aegypti (Linnaeus) in Sri Lanka.

BACKGROUND: The Sterile Insect Technique (SIT) is presently being tested to control dengue in several countries. SIT aims to cause the decline of the target insect population through the release of a sufficient number of sterilized male insects. This induces sterility in the female population, as females that mate with sterilized males produce no offspring. Male insects are sterilized through the use of ionizing irradiation. This study aimed to evaluate variable parameters that may affect irradiation in mosquito pupae. METHODS: An Ae. aegypti colony was maintained under standard laboratory conditions. Male and female Ae. aegypti pupae were separated using a Fay and Morlan glass sorter and exposed to different doses of gamma radiation (40, 50, 60, 70 and 80 Gy) using a Co60 source. The effects of radiation on survival, flight ability and the reproductive capacity of Ae. aegypti were evaluated under laboratory conditions. In addition, mating competitiveness was evaluated for irradiated male Ae. aegypti mosquitoes to be used for future SIT programmes in Sri Lanka. RESULTS: Survival of irradiated pupae was reduced by irradiation in a dose-dependent manner but it was invariably greater than 90% in control, 40, 50, 60, 70 Gy in both male and female Ae. aegypti. Irradiation didn't show any significant adverse effects on flight ability of male and female mosquitoes, which consistently exceeded 90%. A similar number of eggs per female was observed between the non-irradiated groups and the irradiated groups for both irradiated males and females. Egg hatch rates were significantly lower when an irradiation dose above 50 Gy was used as compared to 40 Gy in both males and females. Irradiation at higher doses significantly reduced male and female survival when compared to the non-irradiated Ae. aegypti mosquitoes. Competitiveness index (C) scores of sterile and non-sterile males compared with non-irradiated male mosquitoes under laboratory and semi-field conditions were 0.56 and 0.51 respectively at 50 Gy. SIGNIFICATION: Based on the results obtained from the current study, a 50 Gy dose was selected as the optimal radiation dose for the production of sterile Ae. aegypti males for future SIT-based dengue control programmes aiming at the suppression of Ae. aegypti populations in Sri Lanka.

A comparison of the larvivorous habits of exotic Poecilia reticulata and native Aplocheilus parvus.

BACKGROUND: The exotic fish Poecilia reticulata is promoted in the tropics as a biological control agent for aquatic pathogenic carriers, such as mosquitoes. Such control measures are often adopted blindly, ignoring the potential of native species and the adverse effects of introduced species. The present study was conducted to assess the diet composition of two species of fish, the native Aplocheilus parvus and exotic P. reticulata, and to assess the availability of food items in their natural environment in four types of aquatic systems. Diet composition was estimated using 24 h gut contents analysis, in a clay quarry pit and a perennial reservoir for A. parvus, and in a man-made canal and a second-order natural stream for P. reticulata. Food items in these environments were quantified by analyzing water samples collected every 2 h. RESULTS: The diet of A. parvus in the clay quarry pit and reservoir consisted of adult or larval stages of Insecta, Maxillopoda and Malacostraca. In both habitats, A. parvus selectively fed on insect parts and insect larvae. The diet of P. reticulata consisted of filamentous algae, diatoms and detritus. The diet of A. parvus showed active selection of insectivore food items against their low availability. In contrast, the diet of P. reticulata showed consumption of food items in accordance with their availability in the environment. The highest mean number of food items in the gut for A. parvus was recorded around mid-day in the clay quarry pit, but no peak feeding time was identified in the perennial reservoir. For P. reticulata, peak feeding was recorded around mid-day in both the habitats. CONCLUSION: Irrespective of the type of environment and rate of occurrence, A. parvus preferred insect and insect larvae, whereas P. reticulata consumed the most readily available food items. The active selection of insects by A. parvus suggests they may have value as a biological control agent.

First record of Anopheles stephensi in Sri Lanka: a potential challenge for prevention of malaria reintroduction.

BACKGROUND: The major malaria vector in Sri Lanka is reported to be Anopheles culicifacies with Anopheles subpictus, Anopheles annularis, and Anopheles varuna considered as potential vectors. The occurrence of Anopheles stephensi, which is the key vector of urban malaria in India and the Middle East, had never been reported from Sri Lanka. METHODS: A series of entomological investigations were carried out by the Anti Malaria Campaign, Ministry of Health, Sri Lanka during December 2016 to April 2017 in two localities of the Mannar District in the Northern Province of the country. Adult mosquito collections were done through indoor and outdoor resting collections, animal and human biting collections and emergence traps. Potential mosquito breeding sites were investigated through larval surveys. The larvae and adults of An. stephensi were initially identified using morphological keys, and subsequently confirmed by sequencing the barcode region of the cytochrome c oxidase I (COI) gene. RESULTS: This is the first report of the presence of An. stephensi in the island of Mannar in the Northern Province of Sri Lanka. Anopheles stephensi (36.65%) was the most abundant anopheline species in the larval habitats in Mannar. It was found breeding together with An. culicifacies (20.7%), An. subpictus (13.5%) and An. varuna (28.13%). Anopheles stephensi was found to be abundantly breeding in built wells used for domestic purposes. Adult females of An. stephensi were observed in emergence trap collections (93.9%), human landing catches all night (79.2%), pyrethrum spray sheet collections (38.6%), outdoor collections (8.3%), donkey-baited trap collections (14.3), and cattle-baited net trap collections (0.7%). CONCLUSIONS: Sri Lanka was certified as malaria-free by the WHO in September 2016, however, this new finding may pose a serious challenge to the efforts of the Ministry of Health to prevent the re-introduction of malaria transmission in the country, considering the role that An. stephensi could play in urban and high vulnerability areas of Sri Lanka.

Species composition and population dynamics of malaria vectors in three previously ignored aquatic systems in Sri Lanka.

BACKGROUND: In 2015 alone there were an estimated 214 million new cases of malaria across the globe and 438,000 deaths were reported. Although indigenous malaria has not been reported in Sri Lanka since 2012, to date 247 imported cases of malaria have been identified. Knowledge of the locations, behaviour and vectorial capacity of potential malarial vectors is therefore needed to prevent future outbreaks. Attention is now being focused on some previously ignored habitats. METHODS: Active and abandoned granite and clay quarry pits, located in wet and intermediate zones, and agro wells located in the dry zone of Sri Lanka were mapped and sampled for 1 year, as potential mosquito breeding sites. Species composition and spatio-temporal variation in both malarial and other mosquito larvae were recorded. RESULTS: A total of 18 species of mosquito larvae were identified. Other than Anopheles culicifacies, the primary malaria vector, five species of potential malaria vectors (Anopheles vagus, Anopheles varuna, Anopheles nigerrimus, Anopheles peditaeniatus and Anopheles barbirostris) were found in all three aquatic systems. Additionally, Anopheles annularis was found in granite quarries and Anopheles subpictus and Anopheles pallidus in both types of quarry, but only during the initial sampling. Apart from potential malaria vectors, mosquito larvae such as Anopheles jamesii, Culex tritaeniorhynchus, Culex infula and Culex malayi were found in all three habitats at least once during the sampling period. Apart from potential malaria vectors and other mosquito larvae common to all three aquatic systems, Culex gelidus, Culex mimulus and Culex pseudo vishnui were detected in agro wells. Culex gelidus was also detected in granite quarry pits. Culex mimulus, Culex lutzia and Culex fuscocephala were detected in clay quarry pits. Accordingly, a total of 14, 13 and 15 mosquito species were identified in agro wells, granite and clay quarry pits, respectively. CONCLUSIONS: Although zero occurrence of indigenous malaria has been achieved in Sri Lanka, the current study emphasizes the potential for future epidemics. The presence of native flora and fauna in abandoned granite and clay quarry pits and the need to extract drinking water from agro wells demand bio-sensitive control methods in these three aquatic systems.