Wednesday, April 4, 2018

Odd Little Weevils

Over the eons....ok, decades, but it seems like eons, I have honed my search image for cryptic insects that other people overlook. Last week I was rewarded for discerning a pebble-mimicking insect from debris on the sidewalk. I will not disclose the number of times I have scrutinized some little object that turned out to be an actual pebble, bird poop, or other inanimate thing. This particular creature turned out to be a common but seldom-seen "bison dung weevil," genus Thecesternus.

These beetles, also known as "bison snout beetles," get their name from having first been discovered as habitually seeking shelter under chunks of bison dung on the North American plains. The weevils are nocturnal, flightless, and need protection from the searing heat of the day. Back in the day, "buffalo chips" were the most plentiful answer to that problem.

There are seven species of Thecesternus collectively found in the central, eastern, and southwest U.S. north to Alberta in Canada. They are only about six millimeters in body length, have a very truncated "nose," and are expert in feigning death by drawing in their face, antennae, and, to some degree, legs when frightened by a potential predator. It was difficult to get images of this particular specimen with its antennae extended, so sensitive was it to motion, vibration, and apparently even the camera flash. This represents only the third specimen I have found in Colorado, and one of the other two was dead when I discovered it.

What little we know about these beetles is thanks to the evaluation of one species, T. hirsutus, as a potential biological control in Australia for Parthenium hysterophorus, variously known as Santa Maria, Santa Maria Feverfew, Whitetop Weed, Famine Weed, and Bhajpa Weed, among other aliases. Native to the New World tropics, this plant is known for causing respiratory allergies, contact dermatitis, and genetic mutations in both people and livestock. It is not without redeeming attributes, too, but it has no place in regions where it did not originate. Consequently, several insects have been employed to control it.

Thecesternus hirsutus spends the winter in the larval stage, underground. Larvae hatch from eggs laid in the soil by the adult female weevils in the fall, when autumn rains trigger growth in plant life. The C-shaped grubs then burrow deeper and begin feeding externally on the roots of the host plant. Their activity stimulates the formation of a gall that eventually reaches about ten millimeters in diameter. Each larva encloses itself in an earthen chamber around its feeding site for protection from soil-dwelling predators. The cell is initially rather fragile, but is reinforced internally by anal secretions the larva applies to the interior walls with its mouth. The resulting "room" is quite durable.

The larvae feed into the winter months, reaching maturity between December and February (remember we are talking northern Mexico). The larvae remain dormant until early April when they molt into the pupa stage. Adult beetles emerge in April or May. The beetles feed above ground over the summer before starting the cycle anew.

In the laboratory rearing of T. hirsutus, a few young larvae were found in spring, indicating that some adult females may oviposit (lay eggs) at that time, resulting in a partial second generation of grubs during the summer months when it is usually adults that are present. Both of the living adult specimens I have encountered were found in April here in El Paso County, Colorado.

T. hirsutus turned out to be a poor candidate for control of Parthenium hysterophorus, but the rearing of the weevils demonstrated how well adapted they are to unpredictable weather patterns and volatile changes in climate. This may be a genus of beetles worth examining more in-depth as models of flexibility in the face of global warming and its attendant yearly extremes of heat, drought, and deluge.

The first specimen I discovered in Black Forest, Colorado

Sources: Arnett, Ross H., Jr., Michael C. Thomas, Paul E. Skelley, and J. Howard Frank, eds. 2002. American Beetles (vol. 2). Boca Raton: CRC Press. 861 pp.
Jacques, H.E. 1951. How to Know the Beetles. Dubuque, Iowa: Wm. C. Brown Company Publishers. 372 pp.
McClay, A.S. and D.M. Anderson. 1985. "Biology and Immature Stages of Thecesternus hirsutus Pierce (Coleoptera: Curculionidae) in North-eastern Mexico," Proc. Entomol. Soc. Wash. 87: 207-215.
Patel, Seema. 2011. "Harmful and beneficial aspects of Parthenium hysterophorus: an update," 3 Biotech 1(1): 1-9.

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