This situation presents several interesting problems that I intend to ramble on about endlessly. First is the issue of "treating" sand boxes with any kind of insecticide. This is generally a very touchy thing to do, even if the reality is that many insecticide choices would not pose any risk to the children who play there. The second reality is that the parents of these children might firmly believe that some future health problem of those children is tied to their exposure to what you applied, so if at all possible it is a better idea to avoid applying any kinds of pesticides directly to the places where children spend this much time. If something is required discuss it with the client and suggest the use of some of the "natural", plant-derived products that they may be more comfortable with.
A second issue is with regard to these bees themselves. There are many kinds of solitary bees and wasps that burrow in sandy soils and other soils to prepare chambers for their offspring to develop in. The wasps are all predators that gather caterpillars, crickets, spiders, and other unwanted bugs and leave them as food for their larvae. The bees will be pollinators that gather pollen and nectar and prepare it as a food cache for their larvae. ALL of these insects are highly beneficial, and a landscape and a natural environment are far better off with them than without them.
Along with this is the fact that as "solitary" bees and wasps they are not creating a colony and therefore have no instinct to rush out to defend that colony. The likelihood of being stung by any of these insects is extremely low, and would happen only if the adult female wasp or bee felt directly threatened, such as being caught within clothing or captured in a hand and now possibly stinging in an effort to escape. The children who are using this sand box could probably play there with these bees whirling around them with little concern of being stung.
Now, that's a wonderful lesson on Nature that probably means nothing to this customer, who is still going to fear that his grandchildren will be stung. After all, these female bees do have stingers and are capable of stinging. But........maybe explaining all of this will give them a better appreciation for the bees and their benefit and for a short period of time they may be willing to avoid this sandy area until the bees complete their work and are gone. This is a very seasonal, short term phenomenon. We should at least make the effort to educate homeowners to understand the bugs around them so that we can preserve those that need to stay alive.
The bees are attracted to this sandy area because it is easy to dig in. If the sand were completely dry the bees might move on, as tunnels would collapse as they dig. You might cover the sand with dark plastic for awhile and physically prevent the bees from working there. You could treat each individual hole with an insecticide, preferably in the evening when the adult bees may be resting in them. But, spraying over the whole area with an insecticide would probably have minimal effect and could become that worry for the parents later on that their children had been "exposed" to a poison.
Try the education route first to see if they are responsive to learning some fun facts about bugs, and perhaps begin to enjoy the bees rather than fear them.
Mr. Pest Control
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