On warm summer evenings, as the hum of insects fills the air, it’s common to hear someone exclaim, “We could use some bats to handle these bugs!” Across the globe, insect-eating bats are celebrated as some of nature’s most effective pest controllers, and the bats under the Congress Avenue Bridge in Austin, Texas are no exception. Every summer, over 140,000 people from around the world gather to witness the breathtaking flight of more than a million Mexican free-tailed bats(1). With so many bats in the air, it’s natural to wonder exactly what kinds of pesty insects they’re feasting on each night. To answer that question, funded by a grant from Merlin Tuttle’s Bat Conservation, I collected guano samples, using DNA barcoding to reveal the full extent of their nightly diet.
DNA barcoding allows us to identify species just by looking at their DNA. When a bat munches on an insect, traces of that insect’s DNA end up in the bat’s feces. We collected guano samples from under the Congress Avenue Bridge, and then in the lab, we amplified specific regions of the insect’s DNA. We compared our findings against huge international databases filled with DNA sequences from all kinds of animals to determine exactly which insects the bats had consumed.
This technique has become popular over the last 20 years, but before genetic analysis, scientists had to manually dissect fecal pellets under a microscope. That method had two major pitfalls. First, it required visual identification of a species, which is challenging when all that remains in a fecal sample are a few tiny scales(2,3). Second, soft-bodied insects like mosquitoes might be completely digested by the bat, leaving no recognizable traces and biasing results toward insects with hard shells or scales, like beetles and moths(3,4). This is why DNA barcoding, which relies on genetic material that isn’t destroyed during digestion, is such an innovative and accurate way to identify an animal’s diet
I discovered that the Congress Avenue Bridge bats have quite a diverse menu, feasting on at least 61 different species of insects and arthropods. Their diet includes 20 types of true bugs (e.g. plant hoppers, seed bugs), 17 types of beetles, and various species of moths, flies, ants, and crickets. Some of the more familiar insects on their menu include meadow katydids and lemon moths.
Seventeen of the insect species we identified are considered pests. These include nocturnal armyworms, which are infamous for tearing through crops like corn, barley, oats, rice, rye, and wheat(5). I also found the diamondback moth, a major threat to crops (including: broccoli, Brussels sprouts, cabbage, cauliflower, and more). It is especially worrisome because it’s resistant to many insecticides(6) but not to the voracious feeding of bats. Additionally, we discovered that the bats dine on strawberry seed bugs and rice water weevils, both of which can cause serious damage to wild and agricultural strawberries and rice(7,8). Plus, we found the Beech Blight Aphid, a pest that can wreak havoc on birch trees(9). While the bats’ culinary predilections might not sound all that appetizing to us, they result in the bats delivering important ecological services.
For example, our study revealed that the bats are eating some of the most pesty , and sometimes dangerous, summer insects – mosquitos! One of the mosquito species, Aedes vexans, is a global carrier of diseases like West Nile virus and Rift Valley fever virus(10). The other, Psorophora columbiae, can transmit dog heartworm and Venezuelan equine encephalitis virus(11). These mosquitoes are aggressive biters and love the waterways around Austin, Texas, making the bats’ role in controlling them even more important.
This study highlights just how important bats are for keeping our ecosystems healthy, especially in cities like Austin. By munching on a wide variety of insects, from crop-destroying pests to disease-carrying mosquitoes, these bats help us out more than we might realize — saving us from relying on harmful chemicals.
While rearing young, the Congress Avenue bats can consume up to 73% of their bodyweight in a single night(12). That’s almost 7 tons of insects per night*!
Learning about what these bats eat gives us a better appreciation for their role in our environment and reminds us why it’s so crucial to protect them. After all, when we protect bats, we’re also protecting our food, our health, and the natural balance of our urban landscapes. My findings highlight the importance of conservation support from organizations like Merlin Tuttle’s Bat Conservation, whose scholarship enabled me to conduct this first investigation of the famous Congress Avenue free-tailed bats.
*Feeding rate was calculated conservatively for a single night, given that amounts eaten change with reproductive status and weather. Since not all bats in the bridge are nursing mothers, we based our calculations on 750,000 bats vs the generally cited maximum of 1.5 million. Mexican free-tailed bats weigh on average 11.5g and eat 73% their body weight (0.73). Details of remaining assumptions are based on Kunz et al. (1995). Calculation: ( (11.5g x 0.73) = 8.4g x 750,000 bats = 6,296,250g / 1000 kg = 6,296kg x 2.2lbs / 1kg = 13,581 lbs x 1 U.S ton / 2,000 lbs = 6.93 U.S tons).
About the author: Dr. Brandi Christiano is a field biologist specializing in bat ecology. During her doctoral work at The University of Texas at Austin, she conducted behavioral experiments at The Smithsonian Tropical Research Institute in the Panamanian rainforest, exploring diet choice and social cues of fruit-eating bats. Currently, she is launching her conservation career by developing habitat and diet restoration strategies for bat populations in Pennsylvania.
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2024 © Merlin Tuttle’s Bat Conservation. All rights reserved.
Madelline Mathis has a degree in environmental studies from Rollins College and a passion for wildlife conservation. She is an outstanding nature photographer who has worked extensively with Merlin and other MTBC staff studying and photographing bats in Mozambique, Cuba, Costa Rica, and Texas. Following college graduation, she was employed as an environmental specialist for the Florida Department of Environmental Protection. She subsequently founded the Florida chapter of the International DarkSky Association and currently serves on the board of DarkSky Texas. She also serves on the board of Houston Wilderness and was appointed to the Austin Water Resource Community Planning Task Force.
Michael Lazari Karapetian has over twenty years of investment management experience. He has a degree in business management, is a certified NBA agent, and gained early experience as a money manager for the Bank of America where he established model portfolios for high-net-worth clients. In 2003 he founded Lazari Capital Management, Inc. and Lazari Asset Management, Inc. He is President and CIO of both and manages over a half a billion in assets. In his personal time he champions philanthropic causes. He serves on the board of Moravian College and has a strong affinity for wildlife, both funding and volunteering on behalf of endangered species.
