Mosquito-control work will increase from once to three times a week in parks in dengue-alert districts during the rainy season, the Leisure and Cultural Services Department said yesterday after a city-wide anti-mosquito operation. A dengue alert was issued two weeks ago for Tsing Yi and Fanling after the districts' ovitrap index reached 36.5 per cent and 26 per cent, respectively. The ovitraps catch the Aedes albopictus mosquitoes, which transmit dengue. More than 300 mosquito traps have been installed at 160 parks and public outdoor facilities. Chemicals such as larvicidal oil have been applied to kill mosquito larvae in drains. Yesterday's operation covered about 1,400 department leisure and cultural venues. The department's acting chief leisure manager, Benjamin Hung Tak-chuen, said: 'The upward trend of the ovitrap index is largely due to the warm and humid weather in summer. And we have taken measures to eliminate the mosquito breeding grounds in the parks.' In a written reply to legislators, Secretary for Health, Welfare and Food York Chow Yat-ngok warned that the dengue mosquito problem was more serious than last year. Dr Chow said 1,600 workers had been added to carry out the work.