Provided is a water-based insecticidal composition to be vaporized and diffused by being heated for controlling mosquitoes having reduced susceptibility to a pyrethroid insecticidal component. The composition contains 0.1-3.0 mass % of the pyrethroid insecticidal component, 10-70 mass % of at least one glycol ether compound having a boiling point of 150-300° C. as a susceptibility reduction-countering auxiliary agent for the pyrethroid insecticidal component, and water.
