Silence as a Defense against Predatory Bats in Two Species of Calling Insects

-Males of both the lesser wax moth, Achroia grisella (F.) and the bush katydid (Insara covilleae Rehn and Habard), call mates with high-frequency sound audible to insectivorous bats. Both insects cease acoustical mate calling when they hear the cries of either an approaching bat or a similar artificially-produced sound. But, the lesser wax moth sometimes continues to call by producing pheromone, but not sound. Both insects resume acoustical calling either soon after a bat has passed or after the artificial sound is removed. Some moths, lacewings, and orthroptera can hear the echolocating cries of insectivorous bats and, while flying, take evasive action to avoid capture (Roeder, 1971; Miller and Olesen, 1979; Moiseff et al., 1978, respectively). Although these evasive responses generally consist of erratic flying or passively falling to the ground, some moths exposed to a lower threshold of sound intensity turn and fly away from the sound source (Roeder, 1962). Apparently to confuse the bats, some flying moths produce short bursts of high-frequency sound when echolocating bats approach (Dunning and Roeder, 1965; Fullard et al., 1979). Male lesser wax moths, Achroia grisella (F.) produce mate calling sound at about 100 kHz. Simulated male Achroia sounds of pulse repetition rates of about 43 Hz and higher, attract virgin females and cause them to wing fan. At 30 Hz and below, virgin females cease wing fanning (Spangler et al., 1984). This behavior suggests that the virgin females can locate males while maintaining the defense either of evasive flying or dropping when they hear the longer and more slowly repeated search cries of insectivorous bats. Females of the cricket Teleogryllus oceanicus turn toward male matecalling songs with carrier frequencies from 3 to 9 kHz. But females turn away when the same songs are played within frequency range of bat cries at 30 to 70 kHz. When flying, these crickets probably can avoid insectivorous bats (Moiseff et al., 1978). Recent evidence has shown that bats can locate and capture prey by orienting to sounds produced by their prey. For example, the bat Trachops cirrhosus locates its prey, the frog Physalaemus pustulosus, by homing in on the frog's male-produced mating call (Tuttle and Ryan, 1981). Sounds produced by cicadas may aid the bat Tonatia bidens to feed on them (Bonaccorso, 1979). Big brown bats (Eptesicus fuscus) detect and orient toward relatively low-frequency sounds produced by chorusing frogs or groups of stridulating insects (Buchler and Childs, 1981). The desert insectivorous bat, Antrosous pallidus, can locate and capture insect prey on the ground by the rustling sounds they produce (Bell, 1982). Bats are also known to remove prey from surfaces (Novick, 1977). At least 16 species of moths respond defensively in a number of ways to simulated bat sounds THE SOUTHWESTERN NATURALIST 29(4):481-488 NOVEMBER 14, 1984 This content downloaded from 207.46.13.193 on Thu, 08 Sep 2016 04:25:35 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist even when walking or remaining stationary on surfaces (Werner, 1981). Torpid peacock butterflies produce intense ultrasonic clicks that may repel bats attempting to prey upon them (Mohl and Miller, 1976). In the orthroptera, Wilson (1971) found that Tettigoniidae constituted about 25% of the diet of the bat Micronycteris hirsuta. Rentz (1975) netted bats with flightless katydid prey still in their mouths, establishing that the katydids were captured from stationary positions. Several Costa Rican Tettigoniidae have adaptive characteristics which can enable them to avoid predatory bats: elongate antennae, lowered thresholds for elicitation of defensive behavior and a severely curtailed signal emission time (Morris and Beier, 1982). Since at least some bats can locate and seize stationary prey, including insects, and since some stationary insects appear to have developed defenses against predatory bats, one must suspect that some insectivorous bats locate and capture acoustically-calling insects by their sound. I report here the results of tests on two insect species in different orders, the lesser wax moth (Achroia grisella (F.)), and a bush katydid (Insara covilleae Rehn and Habard). From a stationary position, males of both species call mates acoustically. Most of the calling song energy is above 20 kHz and within the hearing range of insectivorous bats, but both insects cease mate calling when a bat approaches. MATERIALS.-The Insara covilleae individuals used in this study were from wild populations. This insect is common in the regions of the southwestern USA where its host, the creosote bush, Larrea tridentata is plentiful. It eats the petiole and resin gland portion of the leaves (Schultz et al., 1977). Also, I have observed Insara feeding on creosote buds. In the vicinity of Tucson, Arizona, creosote typically has two blooming seasons, one in April and the second in JulyAugust. Creosote bush katydids may feed heavily on the buds during these periods as large numbers mature to adults near the end of each flowering period. In the study area, adult males could be found calling from early May into November and later, depending on the occurrence of freezing weather. A sharp dip in the population of calling males occurs during the typically hot periods of late June and July (Spangler, unpublished data). The lesser wax moth, Achroia grisella, was cultured from wild populations obtained from infested bee hives and maintained on a cereal/wheat germ/glycerin artificial diet provided in plastic shoe boxes. Additional wild individuals, when found, were added to the cultures. Both insects were subjected to the natural echolocating cries of free flying bats passing nearby. The sounds of both the subject insect and each passing bat were recorded to determine any correlation between bat echolocating cries and the production of calling sound by the insect. Insara covilleae was recorded undisturbed from calling positions on creosote, while single Achroia grisella males were put in 6.3 X 6.3 cm cylindrical cages and placed on a platform 5 m above ground level in an area of frequent bat activity. Although both insects began calling at dusk and typically continued calling throughout much of the night, the recording periods began at dusk and were limited to 1 h per night. Continuous sound for determining thresholds causing silence was generated by a portable oscillator driving a 60 kHz Massa MK III transducer'. Sound intensity produced by both the generator and the insects was measured using a Bruel and Kjaer 3135 microphone, a Nicolet 206 digital storage oscilloscope and a General Radio 1564 sound and vibration analyzer. The sound produced by the generator was 66.6 dB at 20 cm. All sound intensities was referenced to 0 dB = 20 ,t Pa. The frequency was verified by storing the sound received by the microphone on the oscilloscope, downloading it into a Cromemco Z-2D microcomputer which performed a fast'Mention of a trademark, proprietary product or vendor does not constitute a guarantee or warranty by the U.S.D.A. and does not imply its approval to the exclusion of other products or vendors that may also be suitable. vol. 29, no. 4 482 This content downloaded from 207.46.13.193 on Thu, 08 Sep 2016 04:25:35 UTC All use subject to http://about.jstor.