Scolia dubia on goldenrod / David Hill / platycryptus on flickr.com/CC BY 4.0
What is it doing? Well they are selecting mates and mating, prior to laying eggs on grubs in the landscape.
We have seen these wasps in considerable number at Penn State's SEAREC (Southeast Agricultural Research and Extension Center) in Manheim, PA in the morning hours, but as temperatures rise--and they certainly have in the past few weeks--in the afternoons, this swirling mass of Hymenoptera disappears from the in-ground perennial areas. Then they can be found enjoying nectar on such plants as Solidago (Golden Rod) and Pycnanthemum (Mountain Mint) in the herbaceous plantings at the SEAREC research station.
Blue winged wasp (scolia dubia) feeding on Mountain mint, Pycnanthemum muticum. Photo: Sandy Feather, Penn State Extension
The female digger wasps burrow into the soil and attach an egg to a grub, often Japanese Beetles, or June Bugs, which then will consume the grub as the wasp larvae, grows and matures. If you have been battling beetles that are attacking your plants in late June through early August, then rest assured you have some allies if you see Scolia dubia in your garden or production area. As predator and parasitoid, populations usually peak after the prey population has grown significantly. Numerous Scolia in the vicinity indicate there must be a significant population of prey insects available.
The wasps will generally not sting unless threatened, and I have walked calmly through these swirling masses, which let me pass unmolested. Interestingly enough, these wasps are a native species, and in the USA range from the St. Laurence River down to Florida, and west to Arizona, so they cover a significant portion of our United States. Scolia dubia is easy to identify, with a black head and thorax, and a rust red colored abdomen. Two yellow spots are found on the first red abdominal segment.
While we have seen these wasps in past years at SEAREC, the sheer number of them this year is impressive. We have had some interesting new Goldenrod cultivars in the Penn State Flower Trials in the past few seasons, and the Scolia dubia wasps seem to enjoy nectar from the cultivars and from the straight species. A real survey as to preferences of these wasps on Goldenrod has yet to be conducted, but it would be safe to conclude that Scolia is well fed on all of these Solidago they are seen to visit. The nectar quality may differ from selection to selection of Goldenrod, and more research is required to determine what each form contributes to the health of the wasps.
Scolia dubia on goldenrod. Photo: Sinclair Adam, Penn State Extension
While we all are awaiting that information, we can happily plant the forms of Goldenrod that the wasps use, and this will certainly enhance your local population of Scolia. An increase in the wasps will certainly result in a decrease in Japanese Beetles next season, which would be a desirable outcome for gardens and landscapes. Scolia dubia is certainly an asset to horticulture!