We thank Mr Chin Kee Thou for his feedback ("Spray insecticide into gully traps, not onto the covers"; Forum Online, Aug 5).

What Mr Chin had observed being carried out at the void decks of Housing Board apartment blocks was in fact, oiling.

Insecticidal oil, which is biodegradable, is dispensed into the gully traps to prevent mosquitoes from breeding there.

The gully trap should first be opened before oil is dispensed into it so that its cover will not be stained with oil.

In some instances, the gully trap cover may be stuck or welded shut.

The pest control officer should then use a compression sprayer with a suitably sized nozzle to spray into the hole of the gully cover.

We will remind the town councils and their pest control operators on the proper oiling methods.

Tony Teo

Director, Environmental Public Health Operations

National Environment Agency