Wasp vs. Wasp

My wife and I went down to Fountain Creek Regional Park in Fountain, Colorado recently (the evening of June 2, 2013), and I happened upon an interesting scene. About five or six female mason wasps, Symmorphus sp., were nesting in beetle borings in a cottonwood log by the side of the trail. About four cuckoo wasps, Chrysis sp., were hanging out around the holes the wasps were using as nests. Heidi said I must have spent 45 minutes taking pictures of them all, but it was worth it because I got to observe what was happening.

The above image is one of only two really nice shots I got of the female Symmorphus mason wasps. They would alight near their nest hole, and dive in very quickly. If I approached too quickly with the camera, the wasp would fly off before entering. If I waited too long, I would get a shot of the wasp’s abdomen disappearing, or a seemingly empty hole. Waiting for the wasp to emerge was basically futile, as she would exit much more quickly than was possible to focus.

You have to have an appreciation of the size of the players and the beetle borings to know what I was up against. The borings are probably the exit holes of deathwatch beetles, family Anobiidae, which bore in dead, solid wood as larvae. The diameter of each hole is about the same as the end of a retracted ballpoint pen (about 3 millimeters). Yes, both the mason wasps and cuckoo wasps could fit through those holes.

”Science has strict rules against anthropomorphism, but if you ask me, those chrysidids were cunning and calculating.”

Meanwhile, the bright metallic green cuckoo wasps were not as easily disturbed as the mason wasps. They might run randomly around between holes, but most of the time they sat quietly in the vicinity of a group of holes. Science has strict rules against anthropomorphism, but if you ask me, those chrysidids were cunning and calculating. They were waiting for the wasps to show them which holes were active nests worthy of their attention.

Cuckoo wasps are parasitic in the nests of other solitary wasps. They can’t sting, but the last few segments of the abdomen of the female telescopes into an egg-laying organ that can reach into the bottom of one of those nest tunnels. The cuckoo wasp lays an egg in the nest of its host, and the larva that hatches generally consumes the meal intended for the host offspring.

In the case of Symmorphus, females catch and paralyze leaf beetle larvae that are free-living, or are leaf-miners (living between the layers of a leaf). At least one species of Symmorphus attacks leaf-mining moth caterpillars on occasion. The larval victims are packed into a cell along the length of the beetle boring, and the mother wasp lays her own egg on the last victim. She then creates a partition of mud or chewed wood fibers, and begins a new cell. The process is repeated such that several cells usually occupy one tunnel.

The mason wasps, for their part, were also quite cognizant of the cuckoo wasps. I witnessed at least one altercation whereby a female mason wasp fended off a too-eager chrysidid at the entrance to her nest. Cuckoo wasps, while they can’t sting, have a dense, pitted exoskeleton that effectively repels the bites and stings of their hosts. They can even roll into a ball as a last resort, becoming virtually impregnable.

Without collecting specimens of the wasps, which I did not do, I cannot conclude the species I was observing. Process of elimination narrows the possibilities to Symmorphus cristatus or S. canadensis for the mason wasps. The cuckoo wasps could represent one or more species in the Chrysis coerulans species group. Scientists are still trying to sort out how many species there actually are in that complex.

After getting up from my baseball catcher’s stance, I could feel how my legs had stiffened. The pain was worth the gain, though, and I encourage my readers to endure for the sake of personal and collective knowledge. Field observations are not the scientific priority they once were in the fledgling years of ethology (study of animal behavior). It could easily be you who makes history with a new host record for a predator or parasite, or records some other hitherto unknown association between species.

Sources: Bohart, R.M. and Lynn S. Kimsey. 1982. “A Synopsis of the Chrysididae in America North of Mexico,” Mem. Am. Entomol. Inst. 33: 1-266
Buck, M. Marshall, S.A., and Cheung, D.K.B. 2008. “Identification Atlas of the Vespidae (Hymenoptera: Aculeata) of the Northeastern Nearctic Region,” Canadian Journal of Arthropod Identification No. 5: 492 pp.
Krombein, Karl V. 1967. Trap-nesting Wasps and Bees: Life Histories, Nests, and Associates. Washington, DC: Smithsonian Press. 570 pp.

Wasp Wednesday: Symmorphus

Sometimes I think I could do “Wasp Wednesday” for years just focusing on the subfamily Eumeninae (potter and mason wasps) of the Vespidae (social paper wasps, hornets, yellowjackets; and the solitary mason wasps). The diversity is so great that it seems impossible to exhaust them as subject matter. It helps that they are also amazing in terms of behavior; and common enough that virtually anyone can find them. Such is the case with the genus Symmorphus.

I had to double-check to make sure that the male wasp I recently photographed at Fountain Creek Regional Park in Colorado was indeed a species of Symmorphus. Eumenid wasps are challenging to identify even to genus. I find that most specimens of Symmorphus are small (roughly 6-10 mm), and slender compared to most other mason wasps. This is especially true of the males.

The defining characters of the genus are harder to see, however. The first dorsal (top) segment of the abdomen has a horizontal ridge (“carina” in scientific terms) about halfway down its length. That same segment also has a shallow vertical groove. Males have the last segment of the antennae simple, whereas in other genera of eumenids this last segment may be hooked or coiled.

Four species of Symmorphus occur in North America: S. albomarginatus ranges from Alaska to California, New Mexico, Quebec, and much of the eastern U.S. S. canadensis, our smallest species, is transcontinental except for Florida, Texas, New Mexico, and Arizona. S. cristatus is equally widespread, from Canada southward. S. projectus is strictly western, from the Pacific Northwest to Montana, Wyoming, and southward.

Mason wasps are our friends, for they prey on larval insects that can defoliate trees and shrubs if not culled by predators. The female wasps hunt the caterpillars and larvae and store them as paralyzed victims for their offspring to eat. Symmorphus wasps are specialists on the larvae of leaf beetles, weevils, or leaf-mining moth caterpillars.

The female wasp uses a pre-existing cavity in wood, or a hollow twig or stem for her nest. She partitions the tunnel into several cells placing a curtain of mud between each cell. She starts from the back and works forward. She first lays an egg in the cell, then begins hunting prey. Symmorphus are remarkably efficient in provisioning their nests. One 7-cell nest of S. canadensis was completed in about three days (Krombein, 1967).

The number of prey larvae per cell depends on the size of the prey, and in part on the gender of the wasp’s offspring. Female eumenid wasps can control the gender of each egg they lay. Larvae of female wasps generally require more food than male offspring.

Symmorphus often leaves an empty (“vestibular”) cell upon completion of the nest. This dummy cell may be an attempt to fool parasitic insects into thinking they have found an entirely empty nest. Some nests also have “intercalary” cells that are also vacant, between a series of provisioned cells. Despite this, nests are frequently parasitized by cuckoo wasps (family Chrysididae), eulophid wasps (genus Melittobia, family Eulophidae), a torymid wasp in the genus Monodontomerus, and satellite flies (genus Amobia, family Sarcophagidae). The Grain Itch Mite, Pyemotes ventricosus can infest nests; and scavengers like humpbacked flies (family Phoridae) and carpet beetles (family Dermestidae) may destroy wasp eggs in the process of consuming the stored prey intended for the wasp’s offspring.

Symmorphus is one of those wasps for which you can provide housing in the form of holes drilled in wooden blocks and placed in sheltered situations like the stack of firewood, or under an eave on your home or garage. They favor holes 3-4 millimetes in diameter, and probably no more than six inches deep. Try it. They will help keep your trees free of leaf beetle larvae in exchange for free rent.

Also look for these wasps around aphid colonies. Like most eumenids, and other wasps, they like the sweet, liquid waste secreted by aphids. Called “honeydew,” this sticky substance is like manna from heaven to many kinds of insects.

Sources: Buck, Matthias, Stephen A. Marshall, and David K.B. Cheung. 2008. “Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic Region,” Canadian Journal of Arthropod Identification No. 5. Biological Survey of Canada. 492 pp. (PDF)
Budriene, Anna. 2003. “Prey of Symmorphus Wasps (Hymenoptera: Eumeninae) in Lithuania,” Acta Zoologica Lituanica 13(3): 306-310
Cumming, Jeffrey M. 1989. “Classification and Evolution of the Eumenine Wasp Genus Symmorphus Wesmael (Hymenoptera: Vespidae),” Mem. Entomol. Soc. Can. 121(148): 1-168.
Krombein, Karl V. and Paul D. Hurd, Jr., et al. 1979. Catalog of Hymenoptera in America North of Mexico (vol. 2). Washington, DC: Smithsonian Institution Press. 1199-2209.Krombein, Karl V. 1967. Trap-nesting Wasps and Bees: Life Histories, Nests, and Associates. Washington, DC: Smithsonian Press. 570 pp.