The siren type of sound generator is used to produce low-frequency sound waves in air. It is not so efficient as the magnetostriction or piezoelectric type, but it can handle a large amount of mechanical energy and the frequency can be changed by changing the speed of the rotor.

A number of exploratory tests of the use of ultrasonic waves on insects have been made at the Agricultural Research Center at Beltsville, Md. A piezoelectric type of generator, an Ultrasonerator, model SL 520, was used. It has four transducer assemblies, each designed to produce a fixed frequency. The frequencies available are 400, 700, 1,000, and 1,500 kc. We used the 400 kc. The maximum power input was 300 milliamperes (ma.) and 1,800 volts or 540 watts. Other tests were run at 200 milliamperes and 1,250 volts or about one-half power. Because of various losses, only about one-half the input power could be applied to the specimen. It is hard to expose test samples to the sound waves without shielding out a good part of the energy with the materials used to hold the samples.

Standing waves usually are set up in the enclosure in which the generator is operating. The position in which the sample is placed in relation to the standing waves is important. The crystal and other parts of the jar are housed in a battery jar containing about 12 liters of transformer oil. The quartz radiates the ultrasonic waves upward through the transformer oil into a very thin copper cup, 3 inches in diameter and 3 inches deep, which contains circulating water. The test samples were suspended in the water-cooling cup. In the piezo-electric and magnetostriction generators operating in liquid, heat is produced in the liquid by the conversion of electrical to mechanical vibrations as well as by the absorption of the sound waves. Since the effect of temperature is fairly well known in entomology, the first tests were made to learn the other effects of ultrasonics. That was done by maintaining the circulating water at a constant temperature. However, temperature seems to modify other effects of ultrasonics and should be later explored.

We tested newly hatched codling moth larvae. We tried several means of holding them. Paper extraction thimbles or filter paper shielded them too Much. Better results were had when the larvae were exposed in small glass vials containing water. In one series of the latter, when the temperature was maintained at about 17 C., and the power at 300 ma. and 1,800 volts with a 400-kc. frequency, the mortality varied from 100 percent at 40 seconds exposure to 25 percent at 10 seconds. When the temperature was increased to 20 C., the mortality varied from 70 percent at 5 seconds to 50 percent at 2 seconds. When the power was reduced to 160 ma. and 1,100 volts and a temperature of 20 C., the mortality was zero at 5 seconds exposure.

Codling moth larvae in apple plugs sealed with wax in test tubes were exposed to 400-kc. ultrasonic waves at 200 ma. and 1,250 volts, and at 300 ma. and 1,800 volts, and there was no mortality when exposure was 15, 30, 60, or 120 seconds. When the wax was not used, there was still no mortality when exposed for io minutes at 17 C. or 1 hour at 20 . Codling moth eggs, on paper and 1 to 4 days old, were exposed in the water testing medium to ultrasonic waves at 400 kc., 300 ma., and 1,800 volts with exposure times up to 120 seconds. About 30 percent failed to hatch, as compared with 14 percent of unexposed eggs.

Cabbage aphids, with the usual Waxy coating, were exposed on small sections of cabbage leaves. The wetting agent, sodium lauryl sulfate (1-10,000), we used did not entirely prevent the formation of air bubbles on the leaves. A frequency of 400 kc. and a power of 300 ma. and 1,750 volts were used. The mortality varied from 62 percent at 4 minutes exposure to 100 percent at 30 minutes. No wax was visible on the dead or dying aphids, but the surviving aphids had a normal waxy coating. Probably the latter were protected by air bubbles. There seems to be some direct correlation between the mortality and the apparent absence of wax on the aphids.

Bean aphids on bean leaves were similarly exposed. A direct correlation was found between the mortality and the protection of the aphid by air bubbles. All aphids that were thoroughly wet were killed, and many of them were bloated.

Two-spotted spider mites were also similarly exposed. They were more thoroughly wet than the aphids and more than go percent were killed at exposures of 4 to 30 minutes. Some of the dead mites were flattened out as a result of the ultrasonic waves.

Third-instar yellow-fever mosquito larvae were exposed to ultrasonic energy with a frequency of 400 kc. At a power of 300 ma. and 1,800 volts, the larvae were all killed at exposures of 7 seconds at 25 C. tog minutes at 420, any of the larvae were eviscerated after exposure of 3 minutes or more. With a power of 200 ma. and 1,250 volts the larvae had a mortality of 92 percent after 2.5 seconds exposure at 22.2 C. and 100 percent after 20 seconds exposure at 23.5 . When the power was further reduced to 100 ma. and 750 volts, varying the exposure from 2.5 to 20 seconds did not seem to affect the mortality, which was about 35 percent at 21.5 C.

In previous tests on adult yellow-.fever mosquitoes, in which low-frequency sound waves in air produced by a siren-type generator were used, there was no apparent effect on the mosquitoes at frequencies of 100 to 21,000 vibrations per second with an energy of 2 watts nor any ill effect at 13,000 and 21,000 vibrations per second at 100 watts energy. The mosquitoes were exposed in 20-mesh copper-screen cylindrical cages 2 inches in diameter and 7 inches long.

Various investigators have studied the sounds made by insects. Many of them can now be accurately reproduced. It has been suggested that the sounds could be used to attract insects so they can be destroyed.

RADIO WAVES have been used since about 1928 in a number of studies of insects. Most of the work has been with limited power and range of frequencies. The effect of the radio waves on the insect seems to be mainly due to heating. In experiments on bacteria, in which heat was removed as rapidly as it was generated and the temperature could be kept below the thermal death point, there was some evidence that high-intensity electric fields could kill without heat. Because heating would normally first kill the organisms from these high intensities, radio waves have been used only as a means of heating to the temperature required for the death of the insect. For practical purposes, heating with radio waves must be cheaper than the simpler heating methods, much faster, or less harmful to the commodity. In order to use and evaluate radio waves for insect control, some of their properties should be known.

Electromagnetic waves, which include radio waves, can be classified according to their wavelength. The audio waves useful in converting electrical to audible sound waves have a wavelength longer than 20,000 meters. The radio waves useful in transmitting energy over long distances have a wavelength of 20,000 meters to about I centimeter. Infrared rays have a wavelength from 1 centimeter to about 1 micron; visible light rays from 1 micron to 4,000 angstrom units, each color having its own wavelength band; ultra- violet from 4,000 to 300 angstrom units; X-rays from 300 to 1 angstrom units; and gamma and cosmic rays below 1 angstrom unit.

Electromagnetic waves are simply traveling fields in which the energy alternately varies between an electric and a magnetic field. These waves can be projected through a vacuum and, like other wave forms, have three major dimensions, frequency, velocity, and intensity.

The frequency is the cycles per second in which a field changes from an electric through a magnetic and back to an electric field. One kilocycle equals 1,000 cycles per second, and one megacycle equals 1 million cycles per second.

The velocity of electromagnetic waves in a vacuum is about 300 million meters per second, which is said to be the speed of light. The permeability and dielectric constant of the material through which the waves travel affect the velocity.

The wavelength is a function of frequency and velocity. In free space, the wavelength in meters can be computed by dividing 300 million by the frequency.

A field of high-frequency radio waves is more useful in heating poor conductors of electricity than in heating good conductors such as metals, which are affected mainly near the surface. If heating of good conductors by electricity is desired, it is more efficient to run alternating current directly through the conductor or to induce an alternating current in the conductor by surrounding it with an alternating current.

Heating nonconductors (dielectrics) in a field of high-frequency radio waves is usually done in an oven whose top and bottom are condenser plates. Oscillating tubes are used to activate one plate with a positive charge while the other is negatively charged and to reverse this charge with any frequency desired. With proper design, an efficient field can be established when specimens are inserted between the plates. To be efficient the frequency must be such that standing waves are not formed on the plates or in the specimen. Also, the plates must be shaped to prevent edge effects, and the specimens must establish a uniform dielectric constant between the plates.

The intensity of the field in radio waves is given in volts per centimeter. The permissible voltage across the electrodes in dielectric heating is limited by the dielectric strength of the Material, which sometimes changes with frequency and temperature. When moisture is present steam may be generated. If the conductivity of the material is such that arcing occurs when high voltages are applied, the arcing will char the specimen.