Tree-hole breeding of Aedes aegypti in southern India: a preliminary report.

TREE-HOLE BREEDING OF AEDES AEGYPTI IN SOUTHERN INDIA 333 Tree-Hole Breeding of Aedes aegypti in Southern India: a Preliminary Report* by T. RAMACHANDRA RAO, K. N. PANNICKER & RACHEL REUBEN It has long been assumed that in India Aedes aegypti (L), the well-known vector of dengue and chikungunya viruses, does not usually breed in natural water containers such as tree-holes, plant axils, cut bamboos, etc. Barraud (1934) stated: " It is unusual to find larvae of this species in tree holes ". Christophers (1960) in his review of the bionomics of Aedes aegypti did not cite any other reference to such a habit in India nor did Horsfall (1955) in his work on the bionomics of mosquitos. However, Gilotra et al. (1967) have recently recorded larvae of this species in bamboo stumps in Calcutta gardens and Kalra (1965) recorded them from hollow bases of uprooted coconut trees in Visakhapatnam. We have not seen any record of true tree-hole breeding in India of Aedes aegypti in recent years. Even in other Asian countries the breeding habits of Aedes aegypti are known to be similar to those in India, i.e., mainly restricted to man-made containers both inside and outside human dwellings. No truly feral breed- ing of the species has been reported in this part of the world and even the occasional instances of breed- ing in tree holes that have been encountered are regarded as not important (Scanlon, 1967). The only clear instance of the breeding of Aedes aegypti in tree-holes in South-East Asia is that recorded by Macdonald (1956) in Malaya, where he did find larvae of this species in 8 out of 31 tree-holes in towns and villages where the species was known to occur in the houses but not in the axils of plants such as banana, Colocasia, pineapple, etc. However, in East Africa such a phenomenon has been known for a long time, and recently one of us (T.R.R.) had occasion to become associated with the work of the East Africa Aedes Research Unit of WHO in Dar-es-Salaam, Tanzania, where tree-hole breeding of Aedes aegypti is a fairly regular feature of its biology. On his return to India in June 1969 * From the Virus Research Centre, Poona, India. The Virus Research Centre is maintained by the Indian Council of Medical Research and also receives a grant (01-007-1) from the US National Institutes of Health from PL 480 Funds, through the Indian Council of Medical Research. The Centre has been designated a WHO Regional Reference Centre for Arboviruses for South-East Asia. he initiated investigations in this regard at Vellore City, Tamil Nadu State, where intense investigations on Aedes aegypti have been in progress for a number of years in connexion with epidemiological studies of dengue and chikungunya viruses (Carey et al., 1966; Reuben, 1967). A large number of tree-holes in a variety of trees, such as mango (Mangifera indica), gulmohar (Poinciana regia), karanj (Pongamia pinnata) and kallal (Ficus sp.) were searched. Larvae and pupae were sometimes taken directly from the rainwater accumulated in the tree-holes and sometimes from water deliberately put into tree-holes by us a few days previously. Sometimes scrapings were also taken from dry tree-holes. They were brought to the laboratory and the eggs, the larvae and the pupae (if any) were reared to adults. From 48 such tree- holes, 2263 adults were obtained. The species of mosquito, their numbers, and the number of tree- holes from which each species was obtained are shown below: Species No. of specimens No, of tree-holesreared from which reared Aedes aegypti 1105 24 Aedes albopictus 957 47 Aedes vittatus 20 1 Aedes thomsoni 149 6 Aedes (D.) reginae 19 2 Culex fatigans 13 1 Total specimens 2 263 Total tree-holes positive 48 These collections were made mostly in Vellore City and a few of the neighbouring towns, such as Ranipet, Wallajah, Arcot and Chittoor, during the months of July, August and September 1969. In one rural locality where 3 larval collections were made from tree-holes, Aedes aegypti was found once. In this preliminary study no record was kept of the number of potential breeding-places searched or of the number of tree-holes actually containing water. Records were maintained only of tree-holes from which larvae or pupae were obtained. It can, however, be stated that all but a few of the tree-holes with water examined contained larvae. 2474B 334 NOTES Aedes aegypti was found in holes at heights ranging from 1 foot to about 30 feet (i.e., about 30 cm to about 9 m). Further, 3 holes in 3 different mango trees in Vellore City were studied more intensively with a view to seeing whether the larvae found there would complete their development successfully. In 2 of these holes, pupae of the species were found on several occasions. In one, in particular, in which searches were made daily except on Sundays between 8 August and 9 September, 187 pupae ofAedes aegypti and 11 pupae of Aedes albopictus were found. The adult Aedes aegypti reared from the tree-holes were mainly of the type form, but var. queenslandensis was not infrequent. The skin colour was brown. In 2 casual test collections in tree holes in gulmohar trees made in Madras City in October, one yielded 2 specimens of Aedes aegypti and 45 of Aedes albopictus, and the other 221 of Aedes aegypti and 19 of Aedes albopictus. These observations would suggest that the breeding of Aedes aegypti now noted in the tree-holes in southern India is not just a casual occurrence. It is too early to say whether the observations recorded here are a new and unusual feature of Aedes aegypti behaviour in southern India or whether the surveys carried out for the distribution of this species in India in the past have not been adequate to bring out this aspect of the behaviour. Further intensive studies are planned all over southern India. These findings are of considerable interest not only to workers on Aedes aegypti who are trying to understand the differences in the genetical make-up of Aedes aegypti in South-East Asia and East Africa associated with differing breeding habits but also to epidemiologists who are concerned with the dis- tribution and habits of the species in relation to the urgent problems of dengue haemorrhagic fever and chikungunya virus outbreaks in South-East Asia. REFERENCES Barraud, P. J. (1934) The Fauna ofBritish India, including Ceylon and Burma. Diptera, Vol. V. Family Culicidae. Tribes Megarhini and Culicini, London, Taylor and Francis Carey, D. E., Myers, R. M., Reuben, R. & Rodrigues, F. M. (1966) Amer. J. trop. Med. Hyg., 15, 580-587 Christophers, S. R. (1960) Aedes aegypti (L), the yellow fever mosquito; its life history, bionomics and structure, London, Cambridge University Press Gilotra, S. K., Roseboom, L. E. & Bhattacharya, N. C. (1967) Bull. Wld Hlth Org., 37, 437-446 Horsfall, W. R. (1955) Mosquitoes: their bionomics and relationship to disease, London, Constable Kalra, N. L. (1965) Bull. Indian Soc. Malar., 2, 101-115 Macdonald, W. W. (1956) Ann. trop. med. Parasit., 50, 399-414 Reuben, R. (1967) Indian J. med. Res., 55, 1-12 Scanlon, J. E. (1967) Jap. J. med. Sci. Biol., 20, 108 The Transportation of Mosquitos between International Airports by R. B. HIGHTON' & E. C. C. VAN SOMEREN1 A detailed account of the invasion of Anopheles gambiae into north-east Brazil and its final elimina- tion between the years 1930 and 1940 has been given by Soper & Wilson (1943). These workers, realizing the potential threat of An. gambiae being reimported from Africa by aircraft and ships, advocated a strict disinsection programme. In the period 1941-42, 7 An. gambiae were recovered from planes arriving from Africa. Recently, larvae of Culex cinerellus Edw., an African mosquito, and Aedes aegypti (L.) were reportedly found in an aircraft arriving in Kansas, USA, from overseas (Pippin, Thompson & Wilson, 1968). The larvae were found in a pool of ' Entomologist, Division of Insect-borne Diseases, Min- istry of Health, Nairobi, Kenya. water on a tarpaulin sheet used to cover cargo which had been picked up in Liberia. Symes, as early as 1933, showed concern about the transportation of mosquitos between the countries of Africa and records the collection of An. gambiae adults from aircraft arriving at Kisumu, Kenya, from Juba, Sudan, and Entebbe, Uganda (Symes, 1935). Appreciating the potential danger of the spread of yellow fever, Symes initiated the disinsec- tion of all aircraft and the adoption of elaborate measures against Ae. aegypti in Kenya (Symes, 1947). Nowadays, the spraying of aircraft to kill insects seems rarely to be carried out but anti- Ae. aegypti measures are still practised at all international airports in Kenya. 2474c