A drain attachment that lets water in but keeps bugs trapped is under testing Fears of an explosion in the number of mosquitoes carrying potentially fatal dengue fever have prompted authorities to trial a new device aimed at stopping them from breeding in roadside drains. The Highways Department is testing an American-designed contraption that will allow water and solid waste into the drain system while preventing mosquitoes, other insects and odours from escaping. Some people had become concerned that existing roadside drains, which allow water and waste to gather at the bottom of a concrete sump, called a gulley, provide a perfect breeding place for the dengue-carrying mosquito, Aedes Albopictus. A spokeswoman for the Highways Department, which supplies and maintains the roadside gulleys, said they were designed to keep larger bits of waste out of the storm water system. 'We have to provide a filter in the gulley because we don't want to cause a blockage,' she said. 'Otherwise it could lead to flooding.' The department's chief concern was that the system still needed to be able to, well, drain. The new device, called a gulley inlet trap, slots into the top of existing gulleys and is currently being trialled in Cheung Chau, Sha Tin and Causeway Bay. A metal flap in the centre of the device opens up only if there is sufficient pressure on it. Once the water or waste has passed into the gulley, gravity pulls the flap shut again, trapping whatever nasties might be inside. 'The benefit is that it can prevent mosquitoes breeding and stop them and odours from escaping into the street,' the spokeswoman said. She pointed out that while the Highways Department built and maintained the gulleys, the responsibility for keeping them clean and free of mosquitoes fell on the Food and Environmental Hygiene Department, except on highways. Anything to do with the drainpipes themselves was the responsibility of the Drainage Services Department. Responsibility for drains in other places, such as country parks and buildings, fell to yet more departments. 'It sounds complicated but actually we work quite closely with them,' the spokeswoman said. The FEHD said it had no idea whether mosquitoes breeding in drains was a problem in Hong Kong, but it was nonetheless taking measures to prevent it, such as treating drains with larvicidal oil. Each of the new devices cost $1,000, and the Highways Department was weighing very carefully whether or not to fit them throughout Hong Kong. While she did not have an exact figure for how many would be needed, the spokeswoman said 'tens of thousands' would be needed for the city.