Wednesday, May 4, 2011

Wasp Wednesday: Steniolia eremica

The sand wasps are among my favorite insects. They are incredibly industrious, colorful, and kill pesky insects like flies for a living. Members of the genus Steniolia are particularly ornate and I recently encountered a species new to me. I was hiking in Florida Canyon (pronounced Floor-EE-duh) in the Santa Rita Mountains of southern Arizona with Margarethe Brummermann on April 21 when we came upon some blooming thistles (Cirsium sp.). Several insects were visiting the blossoms, including males of Steniolia eremica.

Ordinarily, these insects are highly vigilant and easily startled into flight. Spellbound by the nectar-laden thistles, these dudes were easily approached. Male sand wasps are larger than the females, and these were substantial insects by sand wasp standards, maybe 22 millimeters in length, or more.

This species was described fairly recently, in 1964, by James E. Gillaspy, the leading authority on the genus. He considered S. eremica to be less abundant than other species that share its range, from central Nevada south through Arizona, southeast California, and into Mexico. The adult wasps have been collected on the wing from March to August.

Not much is known about the biology of this species, though one female specimen in a collection is pinned with a robber fly (family Asilidae). The assumption is that the fly was prey of the wasp, but considering how powerful asilids can be, I wonder if the reverse was not the case. Female Steniolia in general hunt flies, paralyzing them with a sting, and then take them back to a burrow where the wasp’s larval offspring await their next meal.

Besides my own personal observation of the males on thistle flowers, other nectar sources for the wasps include Palafoxia (Spanish Needles), Petalonyx (sandpaper plant), Melilotus (sweetclover), and Chilopsis (desert-willow). Steniolia have extra-long “tongues” to probe for nectar. The galea and glossa in particular are elongated and not retractable. These straw-like segments are tucked between the legs of the wasp when not in use.

Earlier in the “Wasp Wednesday” series I wrote about another species, Steniolia elegans. The basic biology is the same among all fifteen species. More information can be had in the form of revisions of the genus and documentation of California species.

Sources: Bohart, R. M. and James E. Gillaspy. 1985. “California Sand Wasps of the Subtribe Stictiellina,” Bulletin of the California Insect Survey volume 27. Berkeley: University of California Press. 89 pp.
Gillaspy, James E. 1964. “A Revisionary Study of the Genus Steniolia.” Transactions of the American Entomological Society. LXXXIX: 1-117.

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