|Figure 1. Typical Cerceris fumipennis nest entrance.|
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 (Fig. 7). 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 (Fig. 8). 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.
|Figure 2 . Diagram of a Cerceris fumipennis nest. A, B, C and D indicate the location of completed cells; dotted lines indicate the sections of the main burrow that were backfilled upon the completion of each cell.|
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 (Fig. 9). 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.
|Figure 3. Cerceris fumipennis egg oviposited along the mesosternum of a Dicerca beetle.|
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.