Bat-Friendly Bridges Help Farmers

Merlin Tuttle


Dave Wyatt (holding light) and Gregg Erickson inspecting an old wooden causeway for bats in 1996. Thanks to their efforts, the California Department of Transportation worked diligently during replacement to ensure bats were accommodated. Today, area farmers are grateful.

Building bat roosts into highway bridges in farmlands can benefit farmers at little or no cost to taxpayers. Mark Bloschock, a supervising bridge design engineer at the Texas Department of Transportation, discovered the potential for bridges to help bats when he worked on Austin’s now famous Congress Avenue Bridge. As hundreds of thousands of bats unexpectedly moved in, he contacted me for advice. He soon discovered that, by simply making small adjustments in the spacing between box beams, large numbers of bats could be attracted where needed, and where they weren’t wanted, they could be discouraged by simply changing the spacing.

In 1998, when highway US 90 required two new bridges over Seco Creek, near D’Hanis, Texas, he wrote specifications that placed the box beams three quarters to 1.5 inches apart, hoping to attract Brazilian free-tailed bats (Tadarida brasiliensis) to this important agricultural area. The bats quickly moved in and soon exceeded half a million, today as many as two million.

For decades, Rachael Long, an agriculture consultant, has been a leader in educating area farmers regarding the values of bats and how best to meet their needs. Here, she inspects a colony in an abandoned warehouse.
I visited the Seco Creek bridges a few weeks ago, finding many thousands more free-tailed bats than could be accommodated in existing crevices. There is clearly a housing shortage and additional crop protection opportunity.



We first learned of Mark’s success when I received a phone call from Rodney Sams, an agricultural consultant specializing in pest management. He explained that he and Joel Curtis, a pecan orchard owner he worked for, were baffled. Their pheromone traps, used to evaluate needs for pesticides, normally held 80 to 100 moths each, but that spring they were down to just a handful. They had seen a large bat emergence from the nearby Seco Creek Bridges and wondered if they were the reason for the dramatic reduction in chemical needs.

I checked with Mark, and he said yes, that was where he had designed bat habitat into a pair of bridges.

Christopher Anderson, a San Antonio Express News writer, interviewed each of us for his story published on April 11, 2000. He reported “Sams strongly believes the voracious insect eaters [bats] are keeping Curtis’ pecans from being assaulted.”

Mark believes that box beam bridges, with suitable spacing, are preferred, not just alternative housing for bats. Luckily for the world of bats, he became a passionate advocate for helping bridge bats help people, and thanks in large measure to his enthusiasm, there are now important bat colonies safely living in bridges from Florida to California.

The latest reported success began in 1996 when David Wyatt and Gregg Erickson, of the California Department of Transportation, and Rachael Long, a farm advisor with the University of California Cooperative Extension Service, invited me to join them to investigate the needs of bats living in old bridges, including those that lived in an old wooden causeway that needed replacement. Thanks to their enthusiasm, Yolo County’s bridge bats now have safe homes in bat-friendly bridges, one of which has become a tourist attraction.

Mark Bloschock inspecting bats in a device used to count bats in bridges.
On August 16, 2018 the Atlas Obscura, in its online magazine, published an article titled, Batnadoes Can Protect California’s Crops, Bats are farmers’ new best friends by Anne Ewbank. In the article, local farmer, Mike DeWit swears that the 250,000 free-tailed bats from the Yolo Causeway play an “outsized role” in keeping his wild rice crops pest free. Organic farmers of the region are particularly intrigued, increasingly seeking advice on how to attract bats. For the sake of both bats and humans, we’re hoping the news will spread as each year we learn more about bat values and how to attract them.

Here’s a guide to Texas’ best bat watching bridges and Merlin’s photos showing the species that live there.

Take action on behalf of bats!

We encourage you to send praise and thanks to the author and editor of the Atlas Obscura article by filling out a feedback form. Of course sharing on social media is always a great way to help bats get the positive publicity they deserve. Bats need all the friends they can get!

Seco Creek bridge crevices overflowing with thousands of free-tailed bats. At approximately 20-year intervals vegetation will need to be trimmed to avoid blocking bat access.

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Mosquito Eating in Bats

By Merlin Tuttle

For decades, bat biologists have debated the extent to which bats prey on, and potentially reduce mosquito populations. However, recent research suggests bats may be eating far more mosquitoes than yet suspected. Amy Wray and associates (2018) relied on newly refined techniques that provide greater sensitivity.


In their paper, titled Incidence and taxonomic richness of mosquitoes in the diets of little brown and big brown bats, they reported that these common species eat a greater variety of mosquitoes, and catch them more frequently than previously suspected.

Just one little brown myotis can catch 1,000 mosquito-sized insects in a single hour.

They tested guano samples from 22 locations in a range of habitats, across the State of Wisconsin, from May 17 through July 29. Seventy-two percent of little brown myotis (Myotis lucifugus) samples contained mosquitoes representing 15 species, more than twice as many as big brown bats (Eptesicus fuscus). But even the larger big brown bats, previously thought to feed mostly on beetles and moths evidenced mosquito-eating in 33 percent of samples. Mosquito-eating remained constant throughout the active season in big brown bats but declined slightly in little browns.

A third of big brown bats in Wisconsin fed on mosquitoes.
Little brown myotis from Tennessee.

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Bats and Chocolate Production

By Merlin Tuttle

While conducting her Ph.D. thesis research, Bea Maas and her team (Maas et al. 2013) collected data that would surprise even her. When insect-eating bats and birds were excluded from cacao trees in Sulawesi, Indonesia, the crop yield fell by 31 percent. And when she compared losses due to night versus daytime exclusion, bats versus birds, she discovered that bats accounted for 22 percent of the prevented losses.

A control site in the same study (poles without netting).

To obtain such data, Bea selected 15 plantations where she enclosed 120 cacao trees in 60 exclosures (like huge, mesh cages) constructed of nylon mesh. There were four exclosure treatments per plantation, one daytime, one nighttime, one day and night, and one always left open as a control.

A bat/bird exclosure in Sarawak, Indonesia, built with bamboo poles and commercial nylon monofilament netting with a mesh size of 2 x 2 cm. These were opened and closed like curtains daily.


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An Organic Farmer’s Experience with Bat Houses

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View from above prior to roof attachment.  The sealed-in ceiling between the roof and roosting crevices is for enhanced thermal stability.

Nearly 20 years ago, Frank Bibin, a Georgian pecan grower contacted Merlin for advice on attracting bats to help control insect pests in his orchards. He has since gone organic and become an important advocate for building artificial bat roosts.  To learn of his results go to Pebble Hill Grove–About Bats.

Frank’s first small bat houses were put up in 1998. This video, filmed in Frank’s pecan orchard, tells his early story. It took two years to attract the first bats. Thereafter, numbers grew rapidly. Now that local bats are accustomed to using bat houses, new houses are normally occupied within 30-40 days.

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View from entry (bottom), showing 5 newly installed, 3/4″-wide roosting crevices. One of these houses shelters up to 400 bats.

Through years of testing, he has developed many new innovations. His bat houses and mounting accessories are achieving nearly 100 percent occupancy at many locations in the Southeast and are available for purchase at Frank’s website, Pebble Hill Grove–Bat Houses . Parks and nature centers are among his primary customers.  Though there are many suppliers of smaller bat houses, we know of none better than Frank’s when it comes to long-lasting roosts that attract larger numbers of bats. 

How Frank does it… (more…)

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Bat houses help rice farmers in Spain

checking a bat houseWe would like to highlight a recent study of exceptional importance to all who care about conserving bats. As one who has long promoted the potential benefits of attracting bats to artificial roosts, Merlin is especially pleased with the publication of a recent multi-year study documenting the successful attraction of thousands of bats to small, inexpensive bat houses, leading to well documented reduction of rice pests below threshold levels that require use of chemical pesticides.

The study titled, Pest control service provided by bats in Mediterranean rice paddies, appeared in the journal Mammalian Biology and is available for free download at

A bat house opened to permit checking by researchers

Soprano pipistrelle bats (Pipistrellus pygmaeus), a common bat widespread across continental Europe, east to western Asia Minor, the Caucasus and Siberia


The researchers report that properly located bat houses were readily occupied by soprano pipistrelle bats (Pipistrellus pygmaeus), and that as numbers increased damage from rice borer moths (Chilo supressalis) fell sufficiently to eliminate further need for chemical pesticides. They additionally note that the cost of putting up bat houses was 6-8 times less than that of relying on chemical treatments. These authors also provide an invaluable summary of current knowledge of bat values to agriculture. There are numerous opportunities to expand on this pioneering research which is urgently needed. We deeply appreciate the help of Adrià López-Baucells and Oriol Massana Valeriano in providing outstanding photographic documentation of this project.

Iberian rice field where research was conducted


Here’s contact info for ADRIÀ LÓPEZ BAUCELLS
PhD student on Bat Ecology and Conservation

Smithsonian Tropical Research Institute – Instituto Nacional de Pesquisas da Amazônia
Lisbon University – cE3c
Museu de Ciències Naturals de Granollers

PhD Project Site
Portuguese Research Group Site
Natural Science Museum of Granollers

Tel.: 351 914 413 804

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Khao Chong Phran’s Bat Economics

Poachers were killing huge numbers of Khao Chong Phran’s bats and selling them to restaurants until guards were hired to protect the bats. In Thailand bats were killed for the restaurant trade before a law made it illegal.

The Buddhist temple at Khao Chong Phran is said to have been built largely from guano fertilizer sales. When Merlin first visited the site in 1981, monks were alarmed by a precipitous drop in guano production and asked his advice on the problem. He discovered that poachers were killing large numbers of bats by setting nets over the cave entrance late at night when the monks weren’t looking. The bats were sold to restaurants as a food delicacy. After Merlin convinced the monks to hire a guard in 1981, bat guano sales increased from $12,500 U.S. annually to $89,000 within 10 years, and by 2002, annual sales had reached $135,000 U.S.  Recently, the guano producing bats had been in gradual decline despite 24-hour protection by a team of four guards, so Merlin was quite pleased to discover several evenings ago that the most likely cause of renewed decline was simple to remedy–remove gradually encroaching vegetation.


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