Let me begin with a statement that I am not normally a garden pest killer. I prefer to get rid of the pests' attraction to my yard as the method of elimination. That has proven to be the most successful solution over time. However, this instance wasn't that easy.
Several years ago, we noticed a nest of Cicada Killer Wasps in our yard. I wasn't too alarmed because my research had revealed that these ground dwelling insects were relatively harmless and almost certain not to sting under any circumstance. That didn't make the wasp any less intimidating though. With a length of over two inches, menacing coloration, and active swarming, they can put the scare in just about anyone.
The good news is that the wasps have a relative short life-span and are only around for a few weeks a year at the end of July / beginning of August. They propagate by laying their eggs into the bodies of Cicadas that they capture and kill, so you are most likely to see them during the same time that Cicadas are present. Since the wasps are attracted by Cicadas and the favorable soil conditions of our yard, there was no way of making this site less attractive to them (i.e. We weren't willing to cover the yard in several inches of mulch or attempt to eliminate the Cicadas!).
The bad news is that they apparently found the conditions our yard favorable for nesting and over the course of a several years had multiplied to a good-sized colony. This year dozens of nests appeared throughout the yard; we decided that something had to be done about them.
I found several commercial insecticides that would do the trick, but we were concerned about using these types of toxic chemicals around our family's pets.
We read about physically killing them... literally swatting them out of the air with a tennis racquet and smashing them by foot. Uh. No thank you.
One home brew method captured our attention. It required the application of household ammonia into the underground nest; something that would be harmless to our pets in the manner in which it would be used. It would also be a relatively inexpensive solution compared to the costly commercial insecticide sprays and powders. We decided that it was worth a try.
The idea is to identify the Cicada Killer Wasp nests during the day while the insects are active. That was easy. The wasps actively swarm low to the ground and emerge from the soil from a distinctive hole (about 1/2 inch in diameter) with a pile of excavated earth beneath the entrance to the underground nest. We flagged each of these nests with a colorful plastic knife; something that would be easy to see under low-light conditions.
Later that day, in the evening when temperatures had dropped and the wasps were back to their nests and inactive, we returned with our materials. We came armed with a flashlight, small funnel and a bottle of household ammonia (scented is okay). At each nest, we inserted the funnel and poured about one (1) to two (2) cups of ammonia into the hole. Then upon removing the funnel, we covered the entrance hole with some of the excavated soil and tamped it firmly in place. This process was repeated at each nest location identified earlier in the day.
There are a number of wasp nests in our yard, and since the larvae don't all hatch on the same date the inhabitants continue to emerge over time. That means there have been new nests appearing almost each day. However, all of the nests that we have treated with our solution have remained sealed and inactive. Success!
We fully expect that we will need to repeat this process again for the next few years before we completely rid our yard of these pests, but based upon our effort to-date, the solution has worked. There is a noticeably smaller infestation of the Cicada Killer Wasps in our yard then what there has been over the past few years. Hopefully, this process will work for you as well.