Why the Can Does Not Finish It
An aerosol sold for wasp nests delivers a knockdown insecticide over a few feet. Against the wasps flying at the entrance it works — they drop. Against the colony it does very little, because the queen and the brood are inside a structure the spray never reaches, and a colony with a live queen simply replaces the workers that were killed.
What it reliably does do is alarm the nest. A colony that has been sprayed is defensive for hours afterwards and the person who sprayed it is usually at the top of a ladder. The nest is then harder to approach, for us as well, for the rest of the day.
The Four That Make It Worse
- Sealing the entrance. Foam or filler traps nothing. Several thousand wasps chew a different route, and from a wall or roof void the shortest route is inward, into the room.
- Water. A hose from a distance does not reach the colony, does agitate it, and soaks the loft or the cavity as well.
- Fire and smoke. Smoke drives wasps down into the house rather than killing them, and a dry paper nest in a lit flue is a fire risk on its own.
- Knocking it down. A nest taken off a soffit with a broom releases the entire colony at once, at head height, with nowhere for anybody to go.
If You Have Already Tried Something
Stop, and leave the area alone for the rest of the day. Do not go back to look at it, do not try a second can, and above all do not seal it now that they are agitated. A colony settles back down within a few hours if it is left completely alone.
Then tell us what was used and where. A nest that has been sprayed or partially blocked behaves differently — the wasps may have opened a second exit, and the original entrance may no longer be the one that matters. Knowing that before we arrive saves working it out from scratch.
Why the Professional Version Works
Not because it is stronger. Because of where it goes and what carries it. The treatment is placed at the entrance in a form the wasps walk through, and they take it inside on their bodies and distribute it through the colony, including to the queen. The nest is killed from the inside by its own traffic.
Nobody gets a lecture for having tried. Half the nests we treat have had something fired at them first. It only matters that we know, so we approach the right hole.