I do see that the long term impact of poison exposure would be more effective as the liquid spray tho

I reckon bugs are capable of inhaling the poison, but they are much more likely affected by the fire, heat, or lack of oxygen first. Considering how smoke is a common danger in fires but requires more time/quantity to impact someone, I imagine there are different elements to recall when the amount of oxygen needed is so much smaller than that of human body proportions.
Lol I'm not looking up gas chamber studies on insects, so I'll leave it all to speculation.