Feeding Habits of Bats

PRINCIPAL INVESTIGATORS:
R. Long, UC Cooperative Extension, Yolo County
T. Simpson
W. Reil

ABSTRACT

Feeding habits of bats were evaluated from 2 colonies in 1995; one from Mexican free-tails and the second from little brown bats in Yolo County, CA. Results indicate that the Mexican free-tailed and little brown bats primarily eat, flies (Diptera), water boatmen (Corixids) and moths. Mexican free-tailed bats also eat lots of beetles, a group lacking in the little brown bat diet. Field observations indicate that Mexican free-tailed bats will forage through pear orchards, including in and around the trees. This information, combined with bat feeding habits as well as data on low codling moth pressure in pears in Solano County, CA, suggests that bats may have some effect on suppressing this moth pest in pears late in the season.

Introduction

Reports in the literature cite examples where a colony of 150 big brown bats in the mid-west will eat 38,000 cucumber beetles, 16,000 June bugs, 19,000 stinkbugs and 50,000 leafhoppers as well as thousands of moths each season. This indicates that bats are important predators of night-time flying insects and could have some impact on reducing agriculture pests in some areas.

As a result of the predatory nature of bats, some growers are building bat houses and attracting colonies to their farms, to try to get some biocontrol of their pests. One grower maintains that he has been able to reduce pesticide use on his farm from 13 to 2 sprays per year after attracting a colony of 600+ bats to his farm. However, most of the data out there on the impact of bats on agricultural pests is anecdotal and has yet to be substantiated.

The objective of our research was to determine what bats are feeding on in the Sacramento Valley and assess their potential impact on agricultural pests. Although we looked at the impact of bats on several cropping systems, our work focused on codling moth in pear orchards.

Materials and Methods

Feeding habits of the Mexican free-tail (Tadarida) and little brown bats (Myotis) was assessed by looking through bat guano pellets. Since bats can't digest insect exoskeletons, much of what passes through are undigested parts of legs, antennae, and wings, which can often be identified to insect order and sometimes to family.

The frequency with which the bats feed on different insects was determined by counting the number of pellets that contain a particular insect, and dividing that number by the total number of pellets. For example, if we found 90 pellets out of 100 with moths, that equals a 90% frequency. Percent volume was determined by assessing how much of the pellet was made up of the different insects.

The bat guano samples were collected from 2 different colonies every 10 days to 2 weeks from early April when the bats arrived in our area, until early October when they started migrating out. One colony was Mexican free-tails that were living in the expansion joints in the I-80 overpass just south of Davis in Solano Co. The second colony were little brown bats that were living in some old grain sacks in a grower's barn in Zamora in Yolo Co. Both colonies were in rural areas and surrounded by agricultural lands.

To assess the impact of bats on codling moth in pears, we spent five evenings observing bat foraging activity in pear orchards using a bat detector. This small hand held electronic device is like a small radio, but picks up bat echolocations and converts them to audible sounds, enabling one to monitor bat feeding activity. We also sampled 100 fruit from 17 orchards close to and far away from a large colony (ca. 30,000) of Mexican free-tailed bats, to assess their potential impact on suppressing codling moth in pears.

Results and Discussion

The results of our research indicate that Mexican free-tailed bats primarily feed on water boatmen (Corixids), moths, beetles and flies (Table 1 and Fig. 1). The little brown bats feed primarily on water boatmen and moths, and flies (Table 2 and Fig. 2). Preference changed during the season such that both species of bats fed more on moths than other insects later in the summer. As has been shown in other studies, bats prefer larger prey items such as moths and mayflies, as it's more energy efficient for them to select and eat larger prey. This is especially true when the bats are pregnant or lactating and need a high energy source.

Both species of bats also foraged on true bugs and brown lacewings, but this made up a small portion of their diet. Many of our beneficial insects have evolved ways of avoiding predation by bats; green lacewings, for example have "bat-detectors" on their wings, and drop to the ground when they detect bat echolocations. Brown lacewings, by contrast, have a peak activity in our area that occurs during late winter when the bats are not around.

In our observations on bat activity in pear orchards at dusk using a bat detector, we found that bats will actively forage in and around trees while searching for prey. Most of the bat activity that we picked up was in an orchard that was adjacent to a colony of 30,000+ bats that was living in an abandoned store. Foraging activity by the bats in pear orchards was greatly reduced the further away one got from this colony.

There was a significant trend in percent fruit infestation by codling moth in the pear orchards whereby the further one got from the bat colony, the higher the damage to the pears (Fig. 3). These results are preliminary, but suggest that bats may have some effect on moth pests in pears, late in the season when they feed more heavily on moths.

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