A Little Halloween Love for Bats
Migrating between the southwestern U.S. and Mexico, they have saved cotton farmers millions of dollars in crop damage and insecticide costs. But that appears to be changing.

By Raymond Sanchez, NASA Space Grant intern, University Relations - Communications
Oct. 29, 2014


Mexican free-tailed bats exiting Bracken Bat Cave in Texas, summer home to the largest bat colony in the world.
Mexican free-tailed bats exiting Bracken Bat Cave in Texas, summer home to the largest bat colony in the world. (Photo: Ann Froschauer/U.S. Fish and Wildlife Service)

Bats are the quintessential creatures of the night. From ancient mythology to modern pop culture, the winged mammals have long captured our imaginations and inhabited our deepest nightmares. 

But bats have a vital role to play in the success of local economies as free pest-control providers, according to research by University of Arizona scientist Laura López-Hoffman, assistant professor in the School of Natural Resources and the Environment, part of UA's College of Agriculture and Life Sciences.

Contrary to what Halloween movies might lead you to believe, only three out of about 1,240 known bat species feed on blood. Most dine on insects, and among them is the Mexican free-tailed bat, which migrates between the southwestern United States and northern Mexico. This species alone, it turns out, has saved cotton farmers across the region millions of dollars in crop damage and insecticide costs by voraciously consuming the six-legged pests.

"Our research investigates how this and other migratory species of bats consume pests on cotton," López-Hoffman said. "Along with other measures of pest control, these animals can help prevent serious damage to cotton crops."

López-Hoffman's approach focuses on ecosystem services, which refers to the ways that ecosystems can improve human wellbeing and society.

The bats' appetite not only saves the farmers valuable time and money, it secures their place under the watchful eye of conservationists. While the bats certainly should be protected for their intrinsic value as a unique member of a diverse ecosystem, economic incentives to preserve a species tend to catch the attention of policymakers.

However, a study led by López-Hoffman published earlier this year in the journal PLOS ONE suggests that the economic value of the bats is on a rapid decline. By counting the number of insects that individual bats consume every night and determining the locations of bat roosts relative to cotton fields, the researchers were able to tease out an estimate of the bats' economic value over an 18-year period.

According to the study, the ecosystem service value of the bats dropped from nearly $24 million in 1990 to only $4.88 million in 2008. The precipitous decline is due in large part to the introduction of genetically modified Bacillus thuringiensis (Bt) cotton, which is engineered to produce its own pesticides. This means that the bats have fewer pests to consume and less economic value to offer farmers.

"The problem with this approach is that trying to determine the value of an ecosystem in terms of benefits to humans is complex," López-Hoffman said. "The bats aren't immune to market fluctuations and technological substitutes."

López-Hoffman explained that the ecosystem services approach is controversial because of the difficulty in capturing all of the elements of an ecosystem in a way that truly assesses its value.

"When you're making monetary arguments for the value of an ecosystem, you have to be mindful of all the factors involved," she said. "Still, the idea is to determine ways of incentivizing people to protect these ecosystems for the general good."

In fact, there may still be a silver lining for the nocturnal exterminators. Since its introduction, there has been evidence to suggest that many pests are evolving a resistance to Bt cotton's toxins. Thanks to highly successful integrated pest management strategies, resistance of pink bollworm, one of the most devastating cotton pests, has not been observed in Arizona. In general, pesticides effectively target about 90 percent of pests, while the other 10 percent build up resistance, making them fair game for the bats.

Additionally, engineering new genetically modified organism crops to keep pace with insect resistance is costly. Luckily for farmers, the bats do it free of charge.

"These bats are generalist predators, meaning that they feed on many different types of insects," said López-Hoffman, who is working with a graduate student at the University of Sinaloa in Mexico investigating the effects the bats have on corn, bell peppers and other small crops. "They're likely feeding on pests that plague a wide range of different crops."

López-Hoffman emphasized that while the economic value of species such as the Mexican free-tailed bat might not be able to compete with the complexities of emerging technologies and dynamic markets, it does provide more ammunition for the argument in favor of conservation.

Ultimately, the bats' greatest value may lie in their ability to captivate and inspire us. All over the Southwest, hundreds of thousands of tourists each year flock to watch the bats begin their migrations. Those who are familiar with the region find the sight is awe-inspiring.

"If there's a way that our work can be used to help protect these species, then there's value in that," López-Hoffman said.

Extra info

Learn more about what bats can do for you, and what you can do for bats, during the first National Bat Week from Oct. 26 - Nov. 1.