Cerceris fumipennis, the only species of buprestid-hunting Crabronidae occurring in eastern North America, is found throughout the continental United States east of the Rockies: from Texas and Florida, north to Maine and Wyoming and is now known from more than twenty colonies of varying sizes in Canada. The wasps are most often found nesting in open areas of hard-packed sandy soil surrounded by woody habitat suitable for their prey – the emerald ash borer beetle. Ontario colonies are associated with somewhat disturbed sites compacted by human activity such, as baseball diamonds, parking areas, infrequently used roads, roadsides, foot paths and the soil around campfire pits.
Cerceris fumipennis is distinguished by five conspicuous characteristics:
- It is large, about the size of common yellowjacket wasps.
- It has dark smoky, blue/black wings (i.e. fumipennis)
- The wasp’s body is predominantly black except for a few yellow markings. It has a conspicuous, single broad
- creamy yellow abdominal band.
- Females have three creamy yellow patches between the eyes; while males are marked with two yellow triangles abutting their eyes.
Cerceris fumipennis is a solitary ground-nesting wasp. Each lone female constructs and attempts to maintain a single subterranean nest for the duration of the flight season. Her solitary nest is in close proximity to others, forming a neighbourhood or informal colony of nests. The nest’s entrance is easily visible, marked by a small circular mound of earth. This hole leads into a vertical, pen-sized burrow that descends for about 3 cm before bending to a 45 degree angle and continuing downwards for a further 9 cm, at which point it levels out and becomes clogged with a loose sand plug. Paralyzed buprestid beetles are often temporarily stored in this sand plug. A female wasp will typically attack a target beetle by alighting on it, climbing over it, and grabbing it by the thorax with her mandibles before inserting her stinger into the base of the beetle’s leg (in the membrane of the coxal joint, a gap in the buprestid’s armour) and injecting a paralytic venom. Once at the nest entrance or in the burrow, the female wasp will sometimes re-sting poorly paralyzed prey in the same joint. Buprestid beetles usually respond to disturbance by retracting their appendages and waiting for the danger to pass. This allows the wasp to carry a compact and motionless beetle into its nest.
Just beyond the loose sand plug is the current jelly bean-sized brood cell. The female wasp constructs and provisions only one cell at a time, so the number of completed cells surrounding a burrow provides an indication of how productive that wasp has been thus far in the flight season. Within minutes of placing the final paralyzed beetle into its subterranean cell the adult wasp lays a single hotdog-shaped egg along the beetle’s mesosternum. Like many crabronid and sphecid wasps, C. fumipennis females stock their cells with all necessary food before laying an egg (this is called “mass provisioning”). Prey beetles are paralyzed, not killed, ensuring that each beetle will remain fresh until the wasp larva can begin feeding upon it. After the egg is laid, the completed cell is detached from the burrow as the female wasp backfills the access with 3 – 6 cm of soil. This reduces exposure to parasitoids and kleptoparasites (food thieves). During the provisioning period, cells are often invaded by larvae of kleptoparasitic miltogrammine flies (“satellite flies”). Female flies intercept prey-laden wasps and deposit larvae or eggs on the paralyzed beetles. The voracious fly maggots out compete the developing wasp for the paralyzed beetles and the wasp larva dies of starvation. The voracious fly maggots out compete the developing wasp for the paralyzed beetles and the wasp larva dies of starvation.
Once one cell is completed the wasp begins work on the next cell by excavating in a new direction off the main burrow. Most cells (approximately 5 – 12, but up to 24) are constructed 7 – 20 cm below grade with the egg, larva and pupal stages all developing within the confines of the single nest. In Ontario, the period between oviposition and eclosion, which equals the duration of time spent in the brood cell, is about 10 months.
Emergence dates and speed of the life cycle vary across the wasp’s broad distribution. In Ontario, the flight season typically begins during the last week of June and continues until early September. Emergence dates and duration of the flight season can be influenced by droughts, which could postpone emergence or shorten the flight season.
After tunneling up to the soil surface male wasps never re-enter a burrow. Emerging females use their emergence tunnels as new nests to which they conduct daily orientation flights. Each orientation flight begins with the female flying in ever-increasing arcs around its just-exited nest, often facing the nest while flying sideways. It is during this period that she familiarizes herself with immediate landmarks around her nest and more distant landmarks that will guide her back to the colony. The preferred landmarks are three-dimensional objects with broken silhouettes; the wasps often orient using multiple landmarks at a time. When the female is away foraging, the nest entrance remains open. Each female wasp will collect an average of two buprestids a day but in addition to these successful flights the wasps will often return from forays without prey, only to travel off on another foray moments later. These seemingly unsuccessful flights may be a way for the wasp to practice its route back to the nest from various parts of its range. Reorienting itself over the landscape is particularly important as the wasp forages farther from its nest. Currently the maximum foraging range is estimated at 2 km with an estimated average flight distance of 750 metres from the nest. The fact that C. fumipennis diligently relearns and reorients to its surroundings augers well for the success of mobile colonies.
During the Ontario flight season the first 7 – 10 days see significant new nest construction, low nest fidelity and high nest usurpation. A wasp will continue to excavate and provision new cells as long as she can retain ownership of a burrow. At most colonies, provisioning begins 3 – 5 days after females emerge from their cells, and continues for a 3 – 6 week period. Productive provisioning for Ontario’s wasps starts at the beginning of July and then winds down near the second week of August, so Ontario’s C. fumipennis are most useful for buprestid survey and EAB biosurveillance during July and early August.
“Nest fidelity” refers to the number of consecutive days during a flight season that a female remains faithful to maintaining and provisioning a single nest (approximately 1 – 49 days). Loose, dry sand filling in the nest entrance presents a challenge. If a ”cave-in” occurs while the wasp is in the nest she can usually clear the sand block by pushing it out with her abdomen. If however, a wasp is trapped outside her nest by a cave-in of fine sand she may not be able to re-enter. Often this leads to “nest usurpation”.
“Nest usurpation” occurs when a nestless wasp takes possession of an unguarded nest or she wins a standoff with the occupant. The displaced female wasp will eventually find and occupy an empty hole or displace another smaller female. Nest usurpation can result in a chain reaction of displacement; such chain reactions seem to be most common during the early emergence period and prolonged dry spells. Nest usurpation in C. fumipennis may be an adaptation to avoid digging through the hard or unstable surface soil.
Monitoring for Emerald Ash Borer using Cerceris fumipennis nests
By marking the wasps and nests, a human monitor at a colony can quickly and easily track the flights and successes of various wasps. Each wasp can be easily and safely handled as they rarely (never in our experience) use their stinger in defence. Wasps can be individually marked with a small dot of non-toxic paint dabbed on the top of the thorax to facilitate the tracking of individual wasps and to allow the recording of foraging times and range. Nests can be marked and regulated using a “collar” made using a small 2 x 6 cm plastic or cardboard file card and a standard hole-punch. Holes are made on each end of the tab and it is secured over the nest entrance with a golf tee driven through one hole.
The collar’s hole is large enough to allow wasps without prey to pass through uninterrupted but is small enough to prevent a female returning with prey from squeezing through. In response the wasp (reluctant to release its prey) will buzz and claw at the collar’s opening, alerting a human monitor to the wasp’s return. The bright green EAB adult prey is so distinctive it can be visually identified and then the collar can be moved to one side to allow the wasp to pass into the nest with her prey. Once the female wasp has entered, the collar is repositioned over the entrance and is ready for the wasp’s subsequent exit and next successful return.
Prospects for the future
Biosurveillance requires the predator species to be available at all suspected areas of prey infestation. Naturally-established colonies of C. fumipennis, while widespread in eastern North America, are only capable of foraging over a limited area and it is unlikely that there are enough well-placed natural colonies to use this as a surveillance tool within Canada and the US. Consequently, the development of mobile transplant wasp colonies could vastly improve the value of C. fumipennis as an EAB surveillance tool. Research is currently being conducted in this area.
How to find Cerceris Fumipennis colonies
Cerceris fumipennis occurs in suitable habitats from Florida to southern Ontario. Follow these steps:
Locating Colony Habitat
- Narrow your search area by checking for specimens in your local university or museum collections. If the collection is not well-curated you might have to pick the C. fumipennis out of unidentified material, which is generally easy because the males and females both have distinctive facial and abdominal marks. These wasps are commonly collected on flowers some distance from their colonies, so collection records will give you a basic idea of where to start your search.
- Visit the locations on the specimen labels during the wasp’s flight season and look for suitable nest sites. Cerceris fumipennis becomes active around the end of June in Ontario; earlier farther south. The wasps seem to prefer flat open sites exposed to full sunlight for most of the day.
- The ground should be a hard-packed with relatively fine, sandy soil (exclude beaches and sand boxes). Sparse herbaceous vegetation is important so areas with a mixture of about 50% bare hard-packed sand and 50% short herbaceous vegetation are best. Focus on areas disturbed by humans: baseball diamonds, informal parking spots, infrequently used roads, sandy roadsides, foot paths and soil around fire pits or open campsites.
- The buprestids being gathered by C. fumipennis are primarily arboreal and it is unlikely that the wasps would nest far from the “grocery store”. Most known colonies are less than 200 m (200 yards) from a forested area.
- Avoid any freshly dumped mounds of soil or recently landscaped areas. The wasps overwinter approximately 15 cm below the soil surface and seem to build new nest chambers off the hole they emerged from earlier in the summer. For there to be a colony of suitable size the soil below 3 cm must have been left undisturbed for more than a year.
Finding nests at the site
After finding a promising colony habitat, you will need to locate the nests:
- Walk around any hard-packed, sparsely vegetated soil and look for nest entrances, which are often tucked beside a tuft of grass. Each digger wasp and bee creates their own telltale entrance. Some wasps cover up the openings but C. fumipennis makes a nice little round mound (approximately 4 cm in diameter) much like an ant mound. When you find mounds, check to see if they possess a round central entrance hole, which should travel straight down into the nest, not come in from a side location. The diameter of the hole should fit a golf pencil. A number of digger bees make circular mounds but the entrance holes are much smaller.
- Other insects are helpful indicators when trying to locate a C. fumipennis colony; all are taking advantage of similar soil and light conditions. Keep an eye out for digger wasps buzzing over the ground and excavating nests. Bee wolves, Tachytes wasps, other Cerceris species, digger bees and tiger beetles are found at many C. fumipennis colonies in Ontario.
- Dead buprestids lying around the nests or near the entrances are a good sign that you have found a colony, even if wasps are not active at the site. The female C. fumipennis are encumbered with some of the larger buprestid beetles. If the wasp feels threatened it will drop the large beetle. Without its prey, the urge to “get” a beetle seems to kick in and rather than picking up the dropped beetle, the wasp will head off to catch a new one.
Confirming nest occupancy
To determine if the nest holes you have found are occupied by our wasp, try two tricks:
- Look down the burrow hole to see if a female is looking out. Often females wait 2 cm below the nest entrance to guard against other females that may want to take their nest. If the wasp looking back at you is a female C. fumipennis she will have a black head with three creamy yellow square patches in a V shape in the middle of her face.
- If the hole is empty place a clear plastic cup over the entrance with a stone on top to prevent the cup from blowing away. Check the cup every five minutes to see if a female is flying around it or buzzing inside it. Do not leave the cups unattended for long periods, as wasps may over-heat and die on sunny days. By catching the female you can easily identify the species.
- Use cheap wooden golf tees and coloured collar tabs. Then if the entrance becomes obscured you will know exactly where the nest entrance should be. Writing the nest numbers on the top of the tee or collar will allow you to distinguish each nest.
- Use a GPS to record the geographical co-ordinates.
If the hole is empty place a clear plastic cup over the entrance with a stone on top to prevent the cup from blowing away. Check the cup every five minutes to see if a female is flying around it or buzzing inside it. Do not leave the cups unattended for long periods, as wasps may over-heat and die on sunny days. By catching the female you can easily identify the species.
This native wasp can be considered a natural emerald ash borer treatment. Finding the first colony will be the hard part but once you have found one colony you will begin to notice them elsewhere. If the wasps are present, you will see them. If they are not conspicuous, then search elsewhere.
Once you have found some colonies you may want to go back and revisit a few of your earlier sites. It is easy to overlook small colonies on days when they are not active such as after a heavy rain. Revisiting possible sites a week later is a good idea.
To optimize efforts you will want to work around the following schedule. In Ontario, the wasps are active from about June 28th to September 5th. There is only a single brood in Ontario but in southern Florida the wasp has two broods with the first one becoming active in early April. Observations in both Ontario and Florida suggest that the wasps rarely forage before 9:30am and most females stop around 6:00pm; females spend the night in their burrows. The wasps are more active on sunny days than cloudy days.