Riverside County
University of California
Riverside County

Bats pack a punch for pest control

A colony of big brown bats.
The arrival of spring heralds the return of bats to California’s Central Valley. Every year, hundreds of thousands migrate to this area. Some come from local areas where they hibernate; others species travel over 1,000 miles from their southern overwintering grounds. How bats find their way home is still a mystery, but studies on bat migration suggest they use a combination of factors such as the earth’s magnetic field, stars and landscapes.  Many bat species return to where they were born, and then have their own young pups, just like salmon finding their way up rivers, to spawn where they hatched.

California is home to 25 species of bats. All are insectivorous, with the exception of two species in the Southern California desert that feed on pollen and nectar. Here in the Central Valley there are seven common species of bats including red, hoary, pallid, big brown, and Mexican freetailed bats, as well as California myotis, and Yuma myotis, sometimes referred to as little brown bats.

Farmers welcome bats to their farms every year because they are voracious predators of pests including stinkbugs, cucumber beetles, and moths such as cutworms and armyworms that plague crops. Mosquitoes, midges and flies are also a favorite prey. Studies by UC Cooperative Extension in Yolo County and the UC Davis Department of Wildlife, Fish, & Conservation Biology are looking at the impact of bats on codling moth control in walnuts, a serious exotic pest that costs the walnut industry tens of thousands of dollars every year to control. Current data suggest that orchards with colonies of bats have higher bat activity than those without bats. 

Bathouses get morning sun on the side of a red barn.
Bat colonies can be attracted to farms with bat boxes, which are similar to birdhouses, but the opening is on the bottom where bats fly in and out. These boxes need to be placed on structures, such as barns, at least 10 feet off the ground where they get morning sun and afternoon shade. Proximity to water is important as bat houses within a quarter mile of open water have higher occupancy than those more distant. Houses on poles are seldom used and those in trees almost never. Houses on poles likely cool down too much at night (pups are born hairless) and those in trees are vulnerable to predators such as possums and raccoons. The Yolo County Resource Conservation District in Woodland builds and sells bat houses for those interested in installing them on their farms.

Sometimes bats roost where they are not wanted. If this occurs, excluding them from the area is the best way to remove them. However, this should not be done between the end of May and early August when bats have their young. Generally they have one offspring per year, with many weeks before the pups can fly. Insectivorous bats generally live for at least 10 to 15 years. Contact the UC Cooperative Extension Service in Yolo County for more information on excluding bats from buildings. If one is found in your house, simply turn out the lights and open the doors or windows and it will fly right out (they’re as scared as you are!). 

Injured and orphaned bats are occasionally found on the ground. Never touch a bat with bare hands as they can carry rabies. If one is found on the ground, use gloves or a dustpan and broom to gently pick it up and put it in an area where predators such as cats or dogs cannot get to it. The bat will usually fly away at some point. If a bat is obviously injured or sick, contact the California bat rescue unit and someone will come get the bat to try to nurse it back to health to release it back in the wild.

© Merlin D. Tuttle, Bat Conservation International, www.batcon.org
Bat numbers are in serious decline, so all the protection they can get is welcome. White nose syndrome, a fungus that infects hibernating bats, has killed millions of bats back East. This disease is spreading across the U.S. like a tsunami. Wind turbines are also having a devastating impact on bats, killing tens of thousands every year. Some die from hitting the blades, but most are killed when they fly into a low-pressure zone caused by the rotating blades (about an acre around the industrial wind turbines) and their ear drums or lungs burst. Birds can take the change in air pressure, but bats are more sensitive. 

Migration is the most dangerous time for bats because they become concentrated as they move, making them more vulnerable to dangers, such as wind turbines. Since little is known about the migratory movement of bats, it is crucial that we develop a better understanding of their seasonal whereabouts so hazards are minimized. Wind turbines especially should not be placed in their migratory pathways. 

For those interested in learning more about bats, contact the Yolo Basin Foundation. Every summer they lead tours to watch several hundred thousand bats leaving their roost at dusk under the Sacramento causeway to forage for insects, helping farmers with natural pest control.

Posted on Wednesday, March 21, 2012 at 10:35 AM


ANR publications has a great pub on bird and bat boxes.  
Songbird, Bat and Owl Boxes  
Publication Number: 21636  
Inventory Type: Paperback  
Language: English  
ISBN-13: 978-1-60107-485-0  
Copyright Date: 2008  
Length: 51 pp

Posted by Adina Merenlender on February 12, 2013 at 11:02 AM

What a great article I am part owner in a nuisance wildlife removal company and we do a fair amount of bat work and info like this is what i like to pass along. Most of my customers want no harm to the bats when they see how beneficial they are.

Posted by Richard Federation on April 7, 2014 at 11:33 AM

We have bats in the mariposa county that appear to fly during the day. New bat droppings every morning but no bats. We can see them in the night hanging in our eves. What type might they be?

Posted by Janet Brown on August 22, 2014 at 10:24 AM

We have bats in the mariposa county that appear to fly during the day. New bat droppings every morning but no bats. We can see them in the night hanging in our eves. What type might they be?

Posted by Janet Brown on August 22, 2014 at 10:24 AM

Though it is unusual, bats are occasionally out during the day, feeding on insects. Sounds like you have a night roost where bats gather and rest after a feeding bout. Early in the morning they return to their day roosts, hence disappear from sight, but with guano left behind. Could be several types; if you see insect parts such as heads of crickets left behind, that's the pallid bat. Other species common to our area include Mexican free-tailed bats, Myotis (often called little brown bats), and big brown bats. Thanks for your interest in bats!

Posted by Rachael Long on August 22, 2014 at 4:40 PM

I work at a charter school on a seven acre property in walnut, orange orchard territory, about a 1/4 mile from water. We located south of Fresno. How many bat boxes could we put up? I have two donated, the living space is 14 inches by 18 inches in each one. I have access to more donated boxes. We are very warm, May into October. We can have temps over a hundred in those months, so would morning sun still be good?

Posted by Jeff Alexander on November 13, 2014 at 9:07 AM

I don't recommend putting up a bat house at a school because bats can carry rabies, but please give me a call or send me an email and let's talk about other possibilities, 530-666-8734 or rflong@ucanr.edu.

Posted by Rachael Freeman Long on November 13, 2014 at 7:17 PM

Hi, I live up in the Santa Cruz Mnts and bats seem to come into our house nightly. My neighbor has a bat house and I think that's why I'm seeing more in my house.How can I keep them out and awAy from the property? I'm afraid to go to slerp at night because they come out the and I have a child sleeping in the same room. Please help!

Posted by Nan on July 1, 2017 at 2:29 AM

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