Pumping frequency for the 17-μm ir emission from the cabbage looper moth sex scent (pheromone) Philip S. Callahan and Eugene Hamilton

U.S. Department of Agriculture, ARS, Insect Attractants, Behavior & Basic Biology Research Laboratory, Gaines­ville, Florida 32604. Received 24 January 1977. The sex scent (pheromone) of the cabbage looper moth,

Trichoplusia ni (Hiibner), (Z)-7-dodecen-l-ol acetate, when used in conjunction with a black light bulb in a moth trap in­creases the catch of male moths from six to ten times over that of the black light or pheromone scent used separately.1,2 The attraction of moths to candle flames and other light sources is a well-known phenomenon. Entomologists have assumed, that the increased attraction was caused by the light acting on the moth's eye and by the pheromone acting on the an­tenna. Our interpretation is that the near uv and/or blue light (3600-4500 Å) emitted by the black light bulb pumps the free floating sex scent and thus stimulates a higher fluorescent emission than would ordinarily be provided by the near uv and blue light present in the night sky from starlight and moon­light. Callahan3,4 postulated that the dielectric spines (e = 2.5) on the antenna of insects are open dielectric resonators to narrowband (laserlike) frequencies emitted by the mole-

1476 APPLIED OPTICS / Vol. 16, No. 6 / June 1977

Fig. 1. Emission device. A, oscillator for driving speaker; B, arti­ficial nose cell containing vibrating electret and speaker; C, emission

mirror for interferometer; D, power supply for tungsten filament.

Fig. 2. The pheromone (scent) is pulled through the tunnel and attracted to the vibrating Teflon electret, where the molecules are

vibrated at 50-60 Hz.

Fig. 3. Solid line, absorption of pheromone at 17.95 μm (557 cm-1); dotted line, stimulated emission at 17.95 μm (557 cm-1) when pher­

omone is irradiated with a 6-W BLB bulb and modulated.

cules of sex scents (pheromones) given off by the female moth to attract the male. Since the dielectric spines (called sensilla by entomologists) are arrayed in an approximately log-peri­odic fashion along the antenna of the moth, Callahan5 pre­dicted that the spines should resonate to a series of frequencies depending on the concentration of pheromone molecules that were flowing across the antenna spines. The spines are from about 30 μm to 50 μm long, so for a rod (c = 2.5) L/λo should be in the range of 1-6 wavelengths long.6,7 Accordingly, we searched the 16-28-μm water vapor window8 with a Digilab FTS 14 Fourier transform spectrophotometer for emissions from black light stimulated (Z)-7-dodecen-l-ol acetate (pheromone). In the region between 600 c m - 1 (16.67 μm) and 500 c m - 1 (20 μm) (ca. 2λ), we observed ir fluorescence emis­sions that increased in wavelength with an increase in con­centration (Callahan9). Concentration tuning is a well-known technique utilized for shifting organic die laser frequencies.

To define the optimum pumping frequency, we designed an artificial nose to contain the pheromone. This device (Patent 3,997,785) is shown in Figs. 1 and 2. The 500 μg of the pheromone scent is placed in the nose tunnel (Fig. 2), and air is blown across the scent at a wind speed of 0.5 m/sec. The scent passes over the vibrating Teflon electret sensilla (cilia) and is irradiated by the tungsten ir source. An oscillator, adjustable from 5 Hz to 250 Hz, is used to drive the speaker coil, to which the Teflon electret is attached. The molecules are attracted to the electret and vibrated on its surface in much the same manner as occurs on the surface of the antenna sensilla of the moth. The moth antenna sensilla are also el-ectrets.10

All insects vibrate their antennae when searching for host plant or sex scents.10 We modulated the flowing pheromone gas with the electret vibrator set at 50-60 Hz, the same an­tenna vibration (modulation) frequency as that of a flying cabbage looper male moth10 searching the air for the scent.

The ir emission from the device is given in Fig. 3. The top spectrum (solid line) shows the pheromone absorption line at 557 c m - 1 (17.95 μm) irradiated with the regular tungsten ir source of the Digilab FTS 14 Fourier transform spectro­photometer. The water absorption lines between 591.85 c m - 1

(3B) and 502.27 cm"1 (23B) (National Bureau of Standards11) are also shown. The bottom spectrum (dotted line) shows the pheromone emission at 557 c m - 1 (17.95 μm) when the pher­omone in the device is irradiated with light from a 6-W GE black light bulb and modulated at 50 Hz. Strong emission is evident. Also, SNR is enhanced, as shown by the (partial) disappearance of the water absorption bands on either side of the 17.95-μm emitted wavelength. Further experiments are in progress with the objective of lasing the sex pheromone and obtaining enough high energy output to selectively attract this species of moth. Such a selective wavelength emitter might be utilized to attract the cabbage looper or, conversely, to jam the attractive frequency from the pheromone and prevent mating. A frequency system would have tremendous advantage over insecticides in that the wavelength would not pollute the environment and could be turned off when not in use. The cabbage looper moth destroys millions of dollars worth of cabbage and other cole crops yearly.

References 1. T. J. Henneberry, A. F. Howland, and W. W. Wolf, J. Econ. En-

tomol. 59, 623 (1966). 2. J. R. McLaughlin, E. R. Mitchell, D. L. Chambers, and J. H.

Tumlinson, Environ. Entomol. 3, 677 (1974). 3. P. S. Callahan, Ann. Entomol. Soc. Am. 58, 727 (1965). 4. P. S. Callahan, Misc. Publ. Entomol. Soc. Am. 5, 315 (1967).

5. P. S. Callahan, in Proceedings of the 19th Annual Conference of Engineering in Medicine and Biology, San Francisco (1966), pp. 22-25, 157.

6. P. S. Callahan, Appl. Opt. 7, 1425 (1968). 7. D. G. Kiely, Dielectric Aerials (Methuen, London, 1953). 8. W. A. Traub and M. T. Stier, Appl. Opt. 15, 364 (1976). 9. P. S. Callahan, in 1st International Conference on Biomedical

Transducers (International Federation for Medical and Biolog­ical Engineering, Paris), (1975), Vol. 1 (Bionics), p. 133.

10. P. S. Callahan, Int. J. Insect Morphol. Embryol. 4, 381 (1975). 11. L. R. Blain, E. K. Plyler, and W. S. Benedict, J. Res. Nat. Bur.

Stand. 66A, 223 (1962).

June 1977 / Vol. 16, No. 6 / APPLIED OPTICS 1477