The verb here is 「焚{た}く」 meaning "to burn" as in "to burn incense". The kind of insecticide we are talking about actually diffuses a ton of smoke.
Watch this short video and you will know exactly why you could not stay in your house for at least a few hours after setting off some types of 殺虫剤{さっちゅうざい}.
https://www.youtube.com/watch?v=3nDXTyB6MW8
Luckily, the video title contains our verb in question -- 「バルサンを焚{た}いてみました。」. バルサン is the name of the insecticide.
