Grand Theft Caterpillar (caught on video)

Parasitoid wasps, those species in which the female incapacitates its host, and often caches one or more victims in a burrow, mud nest, or natural cavity, are not above stealing prey from others of their own kind, rather than doing an honest day's hunting. Such was the case I captured recently on video, while watching a Podalonia cutworm hunter in my Colorado Springs neighborhood.

Walking a path through degraded shortgrass prairie habitat, I suddenly noticed this wasp, with her prey, in the middle of the trail. She may have been feeding on the caterpillar's hemolymph (blood), or just making sure the larva was properly paralyzed.

She was nearly motionless for some time, so it came as a bit of a shock when it became apparent she was at the very entrance of a burrow she had excavated before going hunting. Here, in the video clip below, she finishes opening the tunnel, then drags her prize underground. Notice she is a "puller," carrying "armloads" of soil out of the nest rather than scratching it out behind her in a fountain of sand like sand wasps do.

Emerging from the tunnel after burying the cutworm, the female wasp suddenly took flight. I heard a loud buzzing and noticed she was engaged in a literal knock-down drag-out battle with another female Podalonia. The tangle of wings, legs, and bodies persisted a surprisingly long time before the two separated.

I could not tell who the victor was, until a wasp dragged the caterpillar back out of the nest burrow. Clearly, the winner was the usurper, and she was now claiming her spoils.

What is revealing about the complex instincts of hunting wasps is how their internal "program" demands they follow a strict sequence of behaviors, regardless of circumstance. So, the wasp repeatedly stung the already-paralyzed caterpillar, as it would if it had hunted the larva instead of pilfering it from the other wasp.

This drama was playing out in mid afternoon, and I had to get back home to eat something, feed and walk the dog, and whatever other chores I had been neglecting. Consequently, I could not determine whether the thief had not yet excavated her own burrow. She deposited the caterpillar and scurried seemingly randomly before beginning to dig. Perhaps she was opening her own burrow, or digging a new one. She tried digging in another, nearby spot as well, but I had to leave before the whole situation played out completely.

When I left, the original wasp had returned, and the paralyzed caterpillar at the center of the robbery was, ironically, sitting equidistant from both wasps. For all I know, the honest wasp was able to reclaim her property.

Podalonia version of detente

Every day in the world of "bugwatching" is an adventure, and your likelihood of witnessing such captivating episodes increases the more time you are outdoors looking. Sure, there is a fair degree of luck involved, but I guarantee you will see and hear many amazing behaviors if you are the least bit alert.

Wasp Wednesday: Podalonia

A friend recently asked if I might be able to identify a wasp from images taken by a third person. That person, Jeffry B. Mitton, was then kind enough to grant permission to use his images to complement my own photographic documentation of the “cutworm wasps,” genus Podalonia.

These slender, thread-waisted wasps are frequently mistaken for other members of the family Sphecidae. Females look like small Sphex species, while males are easily confused with Ammophila wasps. Podalonia and Ammophila both belong to the subfamily Ammophilinae in fact. There are nineteen species known in North America, but the genus also occurs in Central America, Eurasia, and Africa. Identification of the species hinges on minute characters like the presence or absence of arolia (tiny pads between the claws on each “foot”), so images of live, wild wasps rarely suffice for species determination.

Cutworm wasps are named for the habit of the solitary females to hunt for soil-dwelling caterpillars (“cutworms”), paralyze them, and store them as food for the wasp’s offspring. The wasp digs her burrow after she procures the prey, which is the opposite behavior sequence of most wasps in the Sphecidae family. Only one caterpillar goes in the bottom of each burrow, and a single egg is laid upon it. The opening of the tunnel is then sealed and the wasp begins seeking another victim to repeat the process. I have yet to photograph one of these wasps with a caterpillar, but that is what Jeff Mitton did. Unfortunately, I do not know how to save those images from the e-mail to my computer. So, here is an image from Kim Moore instead.

Another phenomenon I have not witnessed is the nightly aggregation of some species in sheltered spaces. Ironically, I was on a field trip with an entomology class from El Camino College on the weekend of May 21-22 when one of the students discovered a gathering of female Podalonia under bark on a Ponderosa Pine stump. This was at the James Reserve in the San Jacinto Mountains of southern California. Several specimens of the wasps were collected before I learned of the find, and a return to the stump revealed no specimens present. Males also aggregate, as confirmed by this image. Several hundred individuals can pack together in such clusters. This behavior is thought to correlate with inclement weather, and possibly overwintering in the case of females, but perhaps it is a more common occurrence.

Mating is something these wasps seem to do quite frequently, judging by my own images. Males are on top. Both genders enjoy sipping flower nectar, and sometimes they satisfy their thirst and sex drive at the same time.

Podalonia are most abundant in the western United States, but you should be on the lookout for them almost anywhere. They are certainly welcome allies in the garden and agricultural field where they can dispatch a good number of pest caterpillars. Meanwhile, you can find videos of some of the European species in action on YouTube.com and Wildlife Insight.

Sources: Kurczewski, Frank E., Mark F. O’Brien, and Margery G. Spoffford. 1992. “Nesting Behavior of Podalonia robusta (Cresson) (Hymenoptera: Sphecidae),” J. Hym. Res. 1(1): pp. 235-239.
Murray, William Donald. 1940. “Podalonia (Hymenoptera: Sphecidae) of North and Central America,” Entomologica Americana Vol. XX, Nos. 1 & 2: pp. 1-82.
O’Brien, Mark F. and Frank E. Kurczewski. 1982. “Ethology and Overwintering of Podalonia luctuosa (Hymenoptera: Sphecidae),” The Great Lakes Entomologist Vol. 15, No. 4: pp. 261-275.