Posted by: barn owl | July 25, 2008

Zotz! Bats Save the Winter Garden

We associate tales of Dracula, Nosferatu, and other vampires with Eastern Europe and the gothic fiction of Anne Rice, but the Pre-Columbian cultures of Central and South America also had tales of human-bat hybrids and shape-shifters. Bats were associated with death and the Underworld, and bat-men and bat-gods, such as the Mayan vampire Camazotz, were depicted presiding over funeral ceremonies and blood sacrifice rituals. Bat imagery features prominently in pectoral ornaments, headdresses, and Mayan glyphs, and one Incan ruler, Atahualpa, is said to have worn a bat-skin cape (rather like LĂșthien and her bat-fell, in J.R.R. Tolkien’s Silmarillion). Bats, particularly the free-tailed species, are common here in South Texas, where they have access to many limestone caves and sinkholes in the surrounding hills and canyons. The sight of thousands of bats emerging from a cave at dusk must have impressed the indigenous people who inhabited this area before the Spaniards arrived, just as it thrills modern-day visitors to the Hill Country.

Just southwest of San Antonio and the Edwards Plateau, lie the Winter Garden counties of Texas, where crops such as sweet onions, spinach, corn, and strawberries thrive, and cotton is king. The cotton plant, specifically the most productive early-set fruit, is vulnerable to damage by the cotton bollworm, also known as the corn earworm (Helicoverpa zea). This year, with biofuel subsidies for corn production, I’ve noticed many fewer farms planted in cotton, which was so plentiful previously that fields would look as if they’d had light snowfall, immediately after harvest in August and September. I suppose it makes no difference to H. zea, and both cotton and corn are pesticide- and water-intensive crops, so the environment suffers with either.

cornfield

With biofuel subsidies, corn pwns cotton this year

However, according to a recent National Science Foundation-funded study, the Hill Country environment provides its own 100% organic, non-toxic pest control service, in the form of locally-breeding bats such as the Brazilian free-tailed, Tadarida brasiliensis. The estimated 100 million bats that emerge each night from caves, sinkholes, and bridges across south-central Texas preferentially consume moth species whose larvae destroy corn, cotton, cabbage, and tobacco plants. By mapping Brazilian free-tailed bat colonies near the Winter Garden, and observing foraging behavior over cotton fields at night, Cleveland and colleagues (2006) estimated that each bat performs about $0.02 worth of pest control service each night during the month of June. This estimate was based on the average number of adult bollworm moths consumed each night by an adult bat, the population dynamics of H. zea, and the adjusted market price of cotton per boll. In contrast, the financial, public health, and environmental costs of the pyrethroid insecticides applied to cotton to control the bollworm are quite high. The hungry little free-tailed bats of the Texas Winter Garden could prevent one, and perhaps two, early growing season pyrethroid applications to cotton crops, worth $100,000-$200,000 in avoided cost of pesticide use alone. The researchers plan to expand the study area within the state of Texas, as well as to Oklahoma and other Midwestern states.

References:

Cleveland, C.J., Betke, M., Federico, P. et al. (2006) Economic value of the pest control service provided by Brazilian free-tailed bats in south-central Texas. Front. Ecol. Environ. 4(5), 238-243.


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