From the Viette's Views Gardening Blog
Blue-winged wasps, Scolia dubia, are one of the scoliid wasps; a group of fairly large solitary wasps that lay their eggs on the beetle larvae that feed on and damage the roots of your grass.
This particular wasp is fairly common throughout the United States. They are often seen in small groups flying low over the lawn. Finding these guys hovering over your lawn is both a good thing and a bad thing. The bad news is that if you see a lot of them, it means that you probably have a healthy population of white grubs in your lawn. The good news is that these wasps are out to parasitize the grubs and can actually help control them in you lawn.
The female wasps search out these grubs and lay their eggs on them – one egg per grub. They dig into the ground under the grass or follow the tunnels made by the grubs. When they locate a grub, they sting it to paralyze it and then lay a single egg on its body. When the egg hatches, the wasp larva feeds on the grub, eventually killing it.
Female scoliid wasps will lay up to two eggs per day over about a two month period. That translates to a lot of dead grubs. The adults are active from June to October but the peak of their breeding activity seems to be in the mid to late summer, which, not coincidentally, corresponds to the hatch of their preferred hosts; green June beetle and Japanese beetle grubs.
As an added benefit, the adult wasps feed on the nectar of many different flowers and so aid in pollination. They aren’t the most important pollinators around but in these days of dwindling bee populations, every little bit helps.
All-in-all, these are pretty useful wasps to have around. They are not aggressive and usually won’t sting you unless you happen to be a grub. The fact is that they can help you by killing some of those damaging white grubs in your lawn. Good biological control without chemicals! In fact, these beneficial wasps have sometimes been brought in and released in areas that have severe grub infestations.