Above, some California grcivers are in- stalling bat houses on their farms to try to attract bats for pest control. Below, Mexican free-tailed bats, a common species in the Sacramento Valley, the underside of a bat box. Tzung-Su Ding o Steve Heydon Food habits of Mexican free-tailed Bats are predators of night-time flying pests, and a number of California bats and Yuma myotis bats were insects, including many that are agri- growers are installing bat houses on examined in the Sacramento Val- cultural pests (Tuttle 1988). For ex- their farms to try to attract bats for ley. Analyses of guano samples ample, big brown bats in the Midwest pest control (Tuttle and Hensley indicated that Mexican free-tailed favor cucumber beetles, June bugs, 1993). Nonetheless, there are few data on and Yuma myotis bats fed on stinkbugs and leafhoppers (Whitaker the food habits of bats in California; moths, water boatmen, beetles, 1995). Other species of bats primarily therefore, we do not knuw their poten- flies, midges, mosquitoes and feed on moths, the larvae of which tend to be crop pests. For example, tial impact on pests in our area. We plant bugs. The diet of Yuma McCracken (1996) found that Mexican need to assess the diet of bats on a lo- myotis bats tended to be diverse free-tailed bats feed extensively on mi- cal level, to understand their role as early in the season, but more grating populations of corn earworm predators in agricultural systems, be- uniform later on when they ate moths, a major pest of corn, tomatoes cause food habits vary with locality, more moths. In contrast, Mexican and beans. time of year, species of bat, time of free-tailed bats primarily fed on In terms of numbers of insects con- feeding and available insect prey. midges, flies and mosquitoes sumed, a typical colony of 150 bats The purpose of our research was to early in the season, then became will eat more than a million insects determine the food habits of Mexican more generalized feeders later each season (Whitaker 1995) and the free-tailed bats, Tadarida brasiliensis on. These data demonstrate that 20 million bats in Bracken Cave, Texas, and Yuma myotis bats, Myotis bats are beneficial as they feed eat about 250 tons of insects nightly. It yumanensis, in the Sacramento Valley. on insects that can be pests of seems clear that bats potentially can These two species commonly form agriculture. help protect growers from agricultural colonies in our area. e 8 CALIFORNIA AGRICULTURE, VOLUME 52, NUMBER 1 Guano sampling moths (Lepidoptera),and a We determined food habits of few brown lacewings Mexican free-tailed-and Yuma myotis (Hemerobiidae). Early in the season 0 Brown lacewings bats in Yo10 County in 1995 by exam- 0 Plant bugs ining their fecal pellets. This method is from April to June, Beetles 0 Water boatmen a reliable way to assess what bats are Yuma myotis bats fed Moths 0 Flies, midges, feeding on, as all flying insects contain on a diversity of insects mosquitoes chitin, a substance mostly indigestible including water boat- to bats. As a result, insect parts such as men, moths, flies, mos- legs, wings and antennae can usually quitoes, midges, beetles, B o( I I I I I be identified to order, family and plant bugs, moths and a April May June July Aug. Sept. sometimes species. This technique is few brown lacewings much less destructive than earlier (fig. 2). By July, they Fig. 1. Mexican free-tailed bat diet, as determined by methods that involved killing bats and started feeding on more the percent volume of insects found in guano pellets. looking at stomach contents. The dis- moths, which accounted a,00 0 advantage to this method is that a few for 70% of their diet by 90 aE soft-bodied insects may be too di- September. 80 gested to identify in the samples. The frequency of in- g 70 0 Brown lacewings A colony of Mexican free-tailed bats sects found in the Mexi- 5 60 Plant bugs Beetles currently inhabits expansion joints in a can free-tailed and Yuma ,: 50 0 Water boatmen 40 0 Flies, midges, freeway overpass located directly myotis bat diets show -5 mosquitoes above a perennial stream that is sur- that they often select 30 Moths rounded by irrigated farmland. This is similar prey (figs. 3 and 2L 20 a large maternity colony with several 4). One exception is with 1 lo Lo thousand individuals. The Yuma water boatmen; Yuma April May June July Aug. Sept. myotis bats live in the folds of burlap myotis bats primarily fed sacks that hang in a rancher's barn, on these insects in April Fig. 2. Yuma myotis bat diet, as determined by the near irrigated and dryland farms. This whereas Mexican free- percent volume of insects found in guano pellets. colony is also a maternity colony, in- tailed bats fed on them in cluding several hundred individuals. July. This difference in We collected approximately 20 fecal feeding habits is probably due to the The Shannon-Weaver diversity pellets from drop cloths placed below temporal availability of these insects. index was used to measure and each colony about every 2 weeks from Water boatmen are strong flyers and compare the variability of prey April, when the bats arrived, until begin to disperse from water holes as found in the Mexican free-tailed September when they migrated out of they dry. This occurred earlier in the and Yuma myotis bat diet, based the valley for the winter. Insofar as season near the Yuma myotis bat on percent volume (fig. 5). The possible, we identified each food item roost, as this colony is located in a higher the diversity index, the and visually estimated its percentage dryland farming area. more general their diet becomes. volume in each pellet. Data were sum- marized by calculating the average percent volume of each insect group for each sample date. The frequency that bats fed on each insect group was determined by the number of pellets that contained a particular insect di- vided by the total number of pellets. Seasonal diet changes In April and May, about 85% of the 50 Mexican free-tailed bat's diet consisted of flies, mosquitoes (Cyclicidae) and 50- midges (Chironornidae) (fig. 1). From I 1 100 ' July until September, they became Flies, / more generalized in their feeding hab- 50 midges. its as they fed on plant bugs including 01 I I 1 0 ' mosquitoes I leafhoppers (Hemiptera), beetles in- cluding a few ground beetles Fig. 3. Frequency of insects found in the Fig. 4. Frequency of insects found in the (Carabidae), water boatmen (Corixidae), Mexican free-tailed bat diet. Yuma myotis bat diet. CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 1998 9 tion and abundance of available prey posed to pulsed sounds. This may be (Kunz et al. 1995; Whitaker 1996). why some researchers saw a 50% re- However, assessment of prey available duction in damage to corn plots by to Mexican free-tailed bats is difficult corn borers when they broadcast bat- as these bats can forage up to 30 miles like ultrasound over test plots. These from their roosts and at altitudes up- moths are sensitive to bat echolocation wards of 2,400 feet. calls, and probably began to avoid the Yuma myotis bats usually roost and plots. forage near flowing water, and prima- Although bats are beneficial, they rily feed on aquatic insects such as are often maligned. In some cases, im- caddisflies, Trichoptera, and mayflies, portant roosting habitat has been de- Ephemeroptera (Brigham et al. 1992). stroyed and whole colonies elimi- They have also been shown to feed on nated. Bats are slow to reproduce, Dipterans and moths as we found in with most California species having our samples. We probably didn’t find only one young per year; they are also any caddisflies or mayflies in our vulnerable as many form colonies that samples as this colony is located about can be easily disturbed or destroyed. 8 miles from the nearest stream, pre- As a result, we need to protect current sumably beyond the colony’s normal colonies and develop ways to attract foraging range. more bats to farms. Although bats alone Both species of bats also fed on will not control agricultural pests, they brown lacewings and ground beetles, should be encouraged along with other indicating they will feed on beneficial naturally occurring predators, such as There are 26 bat species in California. Dif- insects. However, these insects made owls, raptors, and beneficial insects, to ferent species of bats have different feed- help diversify ecosystems and improve ing preferences. up a small percentage of their diet. Many beneficial insects are diurnal or biocontrol of pests. they have evolved ways to avoid pre- The diet of Mexican free-tailed bats dation by bats. For example, some R.F. Long is UC Cooperative Extension was more specific during April and adult green lacewings, Chrysopidae, Farm Advisor, Yolo and Solano counties; May, then became more general by which are most active at night, have T. Simpson is graduate student, Entomol- June. Conversely, the Yuma myotis bat ”bat-detectors” on their wings and ogy, UC Davis, T. Ding is graduate stu- diet tended to be more general early in drop to the ground when they detect dent, Agronomy, UC Davis, S. Heydon is the season, then more specific later in bat echolocations. They also emit a Senior Museum Scientist, Bokart Mu- the season. All the insects that these noxious odor, which makes them dis- seum, Entomology, UC Davis, and W.