University of Tennessee graduate student Noa Davidai (left) and professor Gary McCracken watch freetail bats emerge from the Frio Cave near Uvalde, Texas. They study the range and value of bats, such as insect control for farmers. And ‘fecal rain’? That enriches soil, Dr. McCracken says. (Dallas Morning News, July 9, 2007, p. 1)
It’s easy to understand. Look at the on-line Dictionary.com definition of the Mexican free-tailed bat, for example:
Mexican free-tailed bat
any of several small, insect-eating bats of the genus Tadarida, of Mexico and the southwestern U.S., inhabiting limestone caves: residual DDT has reduced most populations.
Dictionary.com Unabridged (v 1.1)
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.
Was that difficult? It’s right there in the definition of the animal: DDT kills bats.
Bats eat mosquitoes, those things that carry malaria and other diseases. A Mexican free-tailed bat eats about 70% of its body weight in mosquitoes, every night.
This morning’s Dallas Morning News has a front page story, with great photo, on the value of bats in Texas, “Taking bats to the bank.”
Researchers have long known that bats in Texas caves dine on insect pests. But just how many bats there are and the value of their feeding had proved elusive until a five-year, $2.4 million National Science Foundation study by scientists from Boston University, the University of Tennessee, the U.S. Department of Agriculture and Texas Parks and Wildlife.
From sundown to sunup, the freetail bats consume a staggering 400 metric tons of insects a year in the Winter Garden, or 2 million pounds each night. They range over a radius of 75 miles and feed from ground level to 10,000 feet.
The bats help save $1.7 million annually by preventing crop damage and additional pesticide use in the eight-county Winter Garden, which produces $6 million in cotton each year, according to the report by the Boston University team.
“Most people think of bats as ugly or vile, but there is a real value they provide humankind,” said principal investigator Tom Kunz of Boston University. “The bats are a literal shield for this crop region. But until this project, no one developed a means to measure the specific economic value of bats to agriculture.”
From my experience with agriculture, that $1.7 million figure looks low, way low. Scientific studies like this tend to be very conservative, though, so we can say with great confidence that this is a floor figure.
DDT kills bats, and those bats who don’t die from eating DDT-laced insects often provide meals for predator birds, who then get a greater dose of the next-generation-killing chemical.
Studies have shown that the pesticide DDT often used by farmers in the 1950s and 1960s may also have led to the depletion of large numbers of Mexican free-tailed bats (Clark).The Carlsbad Caverns colony decreased steadily in size from nearly 20 million down to only a couple hundred thousand during the 1960s due to DDT use (Wilson 110).A study in 1974 documented levels of the toxin in fat stores the bats would accumulate before migration, and found that when those fat stores were metabolized during the long flight, DDT levels were high enough to kill many of the bats (Wilson 110).In addition, DDT ingested by mother bats was passed along to their young causing most of them to die before reaching maturity (Wilson 110).Clark’s follow up study in 2001 also showed levels of DDT in bat specimens from the 1950s and 1960s to be considerably higher than in specimens from later decades (Clark).These kinds of toxin levels would account for the dramatic decrease in the Carlsbad bat population.
All of this adds up to a conclusion that critics of Rachel Carson who make the wild claims that DDT is harmless, and that but for DDT mosquito control would have been achieved, and therefore malaria would be wiped out do not have a clue what they are talking about, and probably have some skullduggery in mind when they go after Rachel Carson. Ironically, overuse of DDT actually benefits mosquitoes in the U.S., killing the predators of mosquitoes and other crop and human pests, allowing the mosquitoes to breed and feed uninhibited.
In her book Silent Spring, Rachel Carson had noted a pesticide spill in Austin, Texas, which occurred in 1961 and virtually cleaned out all the fish in the Colorado River downstream — fish, of course, prey on mosquito larvae. DDT use in Texas, therefore, hammers mosquito abatement possibilities at both ends. Read the rest of this entry »