I'm interested in observations about cicada killer wasps (Sphecius speciosus) this season [see "No Killers in Sight as Dog-Day Cicadas Sing," July 6, 2018]. So, when Jeff Webeler (White Oak Gardens, Cincinnati) e-mailed this past Friday about a large numbers of wasps digging in sand backfill behind a retaining wall, I drove at more or less the speed limit to visit the site.
However, the wasps doing the digging surprised both me and Jeff. They were Bicyrtes quadrifasciatus, a species with no approved common name, but generally referred to as the Stink Bug Hunter for their preferred prey. They are an archenemy of non-native invasive Brown Marmorated Stink Bugs (BMSB) (Halyomorpha halys) which makes them a very good beneficial insect!
Stink bug hunters belong to the same family as cicada killers, Crabronidae. However, they are only about 3/4ths the size of cicada killers. Both wasps have conspicuous cream-colored abdominal markings on a black background. The stink bug hunters have four abdominal stripes which are referenced in their specific epithet: quadrifasciatus means "four striped."
Both the stink bug hunters and cicada killers are considered "solitary wasps" even though they typically nest in large numbers over relatively small areas with their collection of burrows referred to as "colonies." The colonies evolve because the females of both species have specific nesting requirements. It is also speculated that large numbers of females gathered in one location helps males to find them which increases the chances all of the females will be mated.
Cicada killers dig burrows in exposed, well-drained soil including native soils. However, they will also dig burrows in sand which can make them a serious nuisance pest in sand volleyball courts and golf course sand traps.
Stink bug hunters dig their burrows almost exclusively in sand which is why they are grouped with the Sand Wasps, tribe Bembicini. They are fast diggers and quickly disappear into their sand burrows. Jeff noted they looked like tiny dogs shoveling sand with their front legs backwards between their hind legs. A close examination revealed that their first and second pairs of legs are slightly curved inward to support digging as well as grasping their prey.
Stink bug hunter females may attack different types of True Bugs (suborder Heteroptera) including leaf-footed bugs (family Coreidae). However, it's their overarching predilection for stink bugs (family Pentatomidae) that makes them such an important enemy of BMSB.
The females tend to select stink bug nymphs, possibly owing to their smaller size compared to the adults. She grabs and stings the nymph to paralyze it, then she carries her prize back to her burrow and stuffs it inside. After she has provisioned her burrow with several helpless nymphs, she lays a single egg, leaves the burrow and seals the entrance. It's an ingenious way to get around the lack of refrigeration. The immobilized nymphs remains alive (perhaps horribly aware?) to serve as fresh meat for the soon-to-hatch, grub-like wasp larva.
A survey of native biocontrol agents of BMSB conducted in 2011 by Penn State researchers in fruit orchards and surrounding habitat revealed that stink bug hunters were a significant native enemy of this non-native stink bug. They found that 96% of the stink bugs found in the wasp's burrows were BMSB nymphs. While the survey was conducted during a "peak year" in BMSB numbers in PA and in locations with concentrated bug populations (orchards), the results still point to these sand wasps as being an important bio-ally.
This means that we should be mindful of these wasps and avoid destroying their sand burrows. While I've never seen them in sand volleyball courts or golf course sand traps, I wouldn't be surprised if they select these locations for nesting sites. As with cicada killers, spraying an insecticide should not be the first choice for management. "Grooming" the sand with a sand rake on a daily or even twice-daily bases as wasps are initiating their burrowing activity should convince the wasps they should choose another nesting location.
What about the risk of getting stung? Like cicada killers and the vast majority of solitary wasps, sand wasps were not aggressive. In fact, Jeff and I both noted that although they would occasionally buzz around us, they never displayed any type of threatening behavior despite our close-in observations.
Attracting these wasps to landscapes may be an important method for decreasing BMSB populations. The adults feed on nectar; they are pollinators. So, adding "pollinator plants" to a landscaping provides roundabout support for these bio-allies.