Imagine one’s surprise at finding wasps suddenly appearing inside their home in the middle of winter. It is not impossible, as queen yellowjackets and some female paper wasps hibernate, overwintering inside houses and other human structures. They may stir if they find their way inside to heated rooms. However, other kinds of insects can be mistaken for wasps. I recently had occasion to identify one such masquerader.
One of the keepers at the Cheyenne Mountain Zoo here in Colorado Springs discovered wasp-like insects emerging from woody “perching material” used for birds in the African Rift Valley exhibit. To her credit she recognized them for what they really were: beetles.
These insects are black with yellow stripes, superficially resembling the markings of yellowjackets. They also run very rapidly, which enhances their resemblance to stinging insects. Many species of longhorned wood-boring beetles in the family Cerambycidae are convincing wasp mimics, and indeed that is what these specimens turned out to be. It was a bit more challenging to determine exactly which genus and species.
It helps to know what a beetle’s “host plant” is when trying to identify it. This only applies to herbivorous species, of course, but it is surprising how many vegetarian beetles feed on only a handful of different plant species. A variety of dead limbs from native hardwoods and conifers are used by zoo personnel for structural objects in exhibits, or as enrichment items to make life in captivity more humane and enjoyable for the animals. So, I knew the beetles were of local origin rather than something imported accidentally from overseas.
From my experiences in Oregon, I had my suspicions as to what genus this beetle belonged to: Neoclytus. There are 25 species in North America, so the next step was determining which ones are found in Colorado. Using a publication on Colorado Cerambycidae (Heffern, 1998), I learned that there are five, maybe six species of Neoclytus known from the Centennial State.
I was able to go online and look up images of the common Colorado Neoclytus species, and the only one to match the specimens given to me by the zookeeper was the Banded Ash Borer, Neoclytus caprea.
Although ash (Fraxinus spp.) is the favored host, the Banded Ash Borer is known to bore in a variety of dead and dying trees. Netleaf Hackberry (Celtis reticulara), hickory (Carya spp.), elm (Ulmnus spp.), oak (Quercus spp.), mesquite (Prosopis spp.), walnut (Juglans spp.), eucalyptus (Eucalyptus spp.), and grape (Vitis spp.) are all hosts to the wood-boring larval stage of this beetle. Trees weakened by drought, disease, fire, and attacks from other insects are especially vulnerable. Rarely are healthy trees attacked.
Adult beetles normally emerge in early spring. After mating, females lay eggs in bark crevices of the host tree. The larvae that hatch from the eggs tunnel just beneath the bark before boring into the sapwood where they feed for most of the summer. The larvae pupate in autumn and overwinter in that stage.
The adult beetles measure 8-17 millimeters, size being a function of the amount of nutrition received in the larval stage. This is a widespread species, found throughout the U.S. except for the Pacific coast. It also ranges into eastern Canada. Look for adults outdoors from March to June. They emerge on the early end of that spectrum in more southerly latitudes.
Ash logs felled in winter are especially prone to attack by the beetles in the following spring. Infested logs brought indoors for firewood may spawn an early emergence of the beetles due to the artificial warmth and extended “daylight” presented by indoor stimuli.
Milling of infested timber may actually prolong the life cycle of wood-boring beetle larvae that survive the process. Dry wood may offer less nutritional value to the grubs trapped within it, meaning that a normal yearly life cycle may be extended to several years in the case of the Banded Ash Borer.
The next time you find a swarm of “wasps” in your home, check the firewood first, and see if it might not be beetles like the Banded Ash Borer, the Hickory Borer, or a related species. Relax, they are at most a nuisance.
Sources: Heffern, D. J. 1998. “A survey of the Cerambycidae (Coleoptera), or longhorned beetles of Colorado,” Contributions of the C. P. Gillette Museum of Arthropod Diversity. Colorado State University: Department of Bioagricultural Sciences and Pest Management. 32 pp.
Karren, J. B. and Alan H. Roe. 2000. “Banded Ash Borer,” Fact Sheet 11, Utah State University Extension.
Yanega, Douglas. 1996. Field Guide to Northeastern Longhorned Beetles (Coleoptera: Cerambycidae). Champaign, Illinois: Illinois Natural History Survey Manual 6. 174 pp.