When Q first uses his flamethrower, when he is flaming the wasps near the van, it later shows the decals burned. Later on in the movie, the decals on the van are no longer burned.
Kent, Dr. Brown, Q and Rafe destroy the swarm by luring them into a garage filled with gas from propane tanks stored for the cook-off. However, these tanks have safety valves that do not allow gas to escape unless the tank is attached to a grill.