Diesel always works better than gasoline. Doesn't evaporate as quickly, sticks to their waxy exoskeleton and wicks right down their tracheoles causing a nearly instant death.
I imagine if you know a welder, a light dose of argon down the hole at night would wipe them out. Argon is heavier than air so it would just fill up the hole and drown them, basically. A CO2/argon mix would work the same. That's the typical gas combo used for mig welders.
If you want to make sure the argon trick works, cut a 2 liter bottle in half, burn a hole in the side of the bottom half, tape the tube to it (or hot glue if you want a more permanent device) and slam it over the hole, pushing it into the ground. After that, start your gas feed.
Also, do this late at night, like midnight. Insects are dependent on external warmth and will be least active at night.
Oh, and make sure there isn't a second escape hole
