The croaking sounds of frogs after rain, so common a decade ago, is now drowned by the whine of mosquitoes, and new research suggests that the decline in tadpole stocks [can also contribute in rise of mosquito numbers and lead to increase spread of diseases such as dengue.]
The research reveals that tadpoles feed on mosquito eggs – in particular dengue mosquito eggs that act as vehicles to transmit the disease through seasons. The new study also reveals a fortuitous cycle in which egg-laying mosquitoes are attracted to water in which tadpoles live, which then gives the amphibian the opportunity to become a predator of the eggs and deplete future mosquito stocks.
Aedes aegypti – the dengue mosquito – not only lays eggs in water-filled discarded plastic containers, tyres, etc. but also in natural sites such as tree holes, marshy areas, ponds and temporary pools that are used by frogs for breeding. This means the amphibians should be recognised as a prime resource against the mosquito menace.
The Peradeniya University team that has conducted the research includes Gayan Bowatte, Piyumali Perera, Gayani Senevirathne, Suyama Meegaskumbura and Madhava Meegaskumbura. Dr. Madhava Meegaskumbura, an amphibian expert, said this was the first evidence that showed that tadpoles feed on mosquito eggs and play an important ecological role in mosquito control.
Biological control of mosquitoes is valued for its low ecological impact and reduced side-effects on humans, and fish are used as a biological weapon against mosquito larvae. But fish, especially when introduced, can cause ecological damage by becoming a threat to native creatures, including frogs.
Furthermore, fish need interconnected waterways to spread and are often not found in the isolated pools, tree holes, rock-pools, ponds and most temporary water bodies that are ideal breeding grounds for mosquitoes. Thus amphibians play a vital role in these areas, point out the researchers.
The transmission of dengue virus from female mosquitoes to their eggs and the resistance of the eggs to drought conditions make them excellent vehicles of disease propagation through seasons. The number of dengue virus-laden eggs that survive unfavourable seasons determines the population size and incidence of the disease during the subsequent rainy season, so effectively dealing with mosquito eggs is key to controlling the propagation of the disease.
The five tadpole species used in this study are representative of several aquatic habitats: Polypedates cruciger tadpoles are typically found in open or closed shallow pools and ponds; Bufo melanostictus tadpoles in small and large pools and ponds, even where there are fish; Euphlyctis cyanophlictis in many types of shallow ephemeral pools and small streams; Hoplobatruchus crassus tadpoles in large pools including ephemeral pools, and Ramanella obscura tadpoles in tree holes and small pools, well isolated from streams. The presence of tadpoles in all these types of aquatic habitats, which are not often suitable for fish life, makes tadpoles very versatile in dealing with dengue mosquito eggs.
Furthermore, the breeding seasons of all frogs studied and the dengue seasons coincide, leading to maximum interaction. All five frog species studied by the University of Peradeniya team are commonly found amid human habitation, and many of them lay a large number of eggs. Bufo melanostictus, for example, lays thousands of eggs while Polypedates cruciger lays hundreds of eggs in foamy masses above water tanks into which tadpoles fall to undergo further development.
Dr. Madhava Meegaskumbura said it was essential that the role of tadpoles be evaluated in the management of water bodies for mosquito-borne disease prevention.
published on SundayTimes on 24.11.2013 http://www.sundaytimes.lk/131124/news/ninja-tadpoles-against-the-dengue-menace-74312.html