Studies on the efficacy of Toxorhynchites larvae and three larvivorous fish species for the control of Aedes larval populations in water-storage tanks in the Matale district of Sri Lanka.

A study was conducted to compare the feeding efficacy of Toxorhynchites larvae (L3 & L4) and three larvivorous fish species on Aedes larvae. Ground-level cement water-storage tanks (20%–80%) and water-storing barrels (8.33%–54.55%) formed the majority of Aedes-positive outdoor containers. Ae. albopictus, Ae. macdougali and Ae. vittatus were recorded in water-storage tanks, with Ae. macdougali being dominant. In the laboratory, the consumption rate (time to devour 10 Ae. albopictus L3 larvae in a vessel of 78.57 cm2 of surface area) for Toxorhynchites was significantly lower (mean time of 330 minutes) than for any of the tested fish species, Poecilia reticulata (Guppy), Puntius bimaculatus (Ipilli Kadaya) and Rasbora caveri (Dandiya), which needed 16.67, 27.33 and 24 minutes respectively. There were no significant differences (P=0.062) between the consumption rates of the three fish species. A field study was carried out to determine the feeding efficacy of Toxorhynchites larvae, P. reticulata, P. bimaculatus and R. caveri on Aedes larval populations in outdoor cement tanks by noting the percentage reduction of Aedes larvae per 100 cm2 surface area after one week. Toxorhynchites larvae caused a 20%–83.33% reduction with 1–8 larvae per tank. A complete reduction (100%) was achieved with P. bimaculatus and R. caveri with 1–3 fish per tank. P. reticulata showed similar results, but with 90% reduction being achieved once with two fish per tank. There was a higher possibility of losing Tx. larvae than the fish species during the removal of water by the householders. The efficiency of the three fish species for consuming Aedes larvae was greater than that with Tx. larvae. It appears feasible to use Puntius bimaculatus, Rasbora caveri and Poecilia reticulata for controlling Aedes breeding in outdoor cement water-storage tanks in Sri Lanka.

Spatiotemporal distribution of insecticide resistance in Anopheles culicifacies and Anopheles subpictus in Sri Lanka.

The malaria situation in Sri Lanka worsened during the 1990s with the emergence and spread of resistance to the drugs and insecticides used for control. Chloroquine resistance has increased rapidly over this period, but adverse changes in malaria transmission are more closely associated with insecticide use rather than drug resistance. Insecticide susceptibility tests were routinely carried out in key anopheline vectors across the country for more than a decade. These sentinel data were combined with data collected by other research programmes and used to map the spatial and temporal trends of insecticide resistance in the main vectors, Anopheles culicifacies and A. subpictus, and to examine the relationship between insecticide resistance, changes in national spraying regimens and malaria prevalence. Both species had widespread resistance to malathion, the insecticide of choice in the early 1990s. Both species were initially susceptible to the organophosphate and pyrethroid insecticides used operationally from 1993, but some resistance has now been selected. The levels of malathion and fenitrothion resistance in A. subpictus were higher in some ecological regions than others, which may be related to the distribution of sibling species, agricultural pesticide exposure and/or environmental factors. The study highlights that the emergence and spread of insecticide resistance is a constant threat and that active surveillance systems are vital in identifying key vectors and evidence of resistance.

Control of malaria vectors with the insect growth regulator pyriproxyfen in a gem-mining area in Sri Lanka.

The study was conducted in eight adjacent villages in central Sri Lanka where there are many shallow pits dug by gem miners that fill with water. These become breeding places of the main malarial vector Anopheles culicifacies, and of the second most important vector Anopheles subpictus, but not of Anopheles varuna, the third most important vector. With the help of local volunteers, data on the adult populations of these three species was collected by various standard methods, and data on the incidence of malaria cases was collected by two clinics set up for the project and through the existing hospitals. Prevalence of malaria infection in symptom-less people was investigated by mass blood surveys. On the basis of a year's pre-intervention data the villages were stratified into four with high levels of malaria transmission and four with lower transmission. Within each stratum two villages were randomly assigned for mosquito control by treating all the gem pits, as well as river bed pools, with a granular formulation of the insect growth regulator pyriproxyfen at a target dose of 0.01 mg a.i./litre. The intervention caused significant reductions in the adult populations of An. culicifacies and An. subpictus. Similarly, incidence of malaria was reduced in the intervention villages to about 24% (95% c.l. 20-29%) of that in the controls. Prevalence of parasitaemia also declined significantly. It is concluded that in this situation where, with active community participation, the breeding sites of the main vectors could be located; vector control by a highly active and persistent insect growth regulator can be a very effective means of malaria control.