Radiant Energy and Insects

Alfred H. Yeomans.

Radiant energy can be applied in uncounted ways to kill insects. To prevent damage in storage and the transportation of pests into quarantined zones, it has been tested on fruits, potatoes, grains, wood, textiles, and perhaps other commodities. It has been used to kill larvae of mosquitoes in water. The aim is to kill the insect without harming the material on which it lives and to do it economically.

Radiant energy includes electrical energy of various wavelengths such as radio, infrared, visible and ultraviolet light, X-rays, and gamma rays. It includes sound waves of various wavelengths, such as audible and ultrasonic. It includes also the energy from various atomic particles such as neutrons, alpha, and electrons.

Its action depends on the structure of matter. All matter, including insects, is made up of combinations of some of the 92 basic elements. Each basic element comprises a particular type of electric solar system called an atom. The systems themselves are composed of elementary particles, some of which have no electric charge or mass. Two of the particles, the protons (which have a positive electric charge) and the neutrons (which have no electric charge) comprise the basic mass of the atomic nucleus. The number of protons determines the type of atom formed. The nucleus is surrounded at a relatively great distance by negatively charged electrons having a definite pattern. Normally the number of electrons equals the number of protons in the nucleus. Atoms can be combined chemically into various patterns of electrical solar systems, or molecules, that form the various materials as we know them. Living organisms comprise such complicated patterns of these molecules that we do not know everything about their make-up. Nothing is known of the substance that gives life to the combination of molecules forming living organisms.

The energy contained in one atom is tremendous for its size. Some types of atoms can be broken apart and their energy released, such as the explosion of the atomic bomb. The electric solar system of the atom may also be changed less drastically. Radiant energy in various forms and intensities is the means employed to do so. Radiant energy may be used to increase the natural vibrations of the atoms and molecules. The result is increased temperatures of the material. It may be used to cause chemical combinations of atoms that are reluctant to combine. Molecules also may be struck with enough energy to break off electrons and leave fragments called ions.

The molecular structure of an insect is so complex that when radiant energy is applied it is difficult to determine which factors or combinations of factors cause its death. Each type of radiant energy results in a predominant action on the molecule, however. When the energy is intense enough, the insect is killed because it is torn apart physically.

THE USE OF HIGH-FREQUENCY or ultrasonic sound waves, other than the audible ones, is a relatively new science. The first work on it was done in about 1900. Until the First World War it remained a laboratory study, in which small tuning forks, sparks, and special whistles were used to produce the waves. During the war, a narrow beam of high-frequency sound was used to detect submarines. Since then it has been used for underwater signaling, testing for flaws in materials, and removing smoke. It has been used experimentally in television, medicine, biology, and metallurgy. It helps in the agitation of solutions and in making some chemical reactions.

The discovery that ultrasonic signals sent through water killed fish and destroyed other marine life led to research on the effect of ultrasonics on biological organisms. Many types of organisms have been exposed to various frequencies and intensities. Fish and frogs were easily killed, and some insects and bacteria were destroyed. Some of the effects of ultrasonics on biological organisms have been clearly explained. Others have been explained only partly. The biological effects may be due to heat generated by the sound waves or, when the energy is intense, to the shattering or tearing apart of the organism. Less apparent effects are probably due to chemical changes.

The use of sound waves for most insect-control purposes is impractical because of the inefficiency of low-frequency waves and the difficulty in transmitting high-frequency waves through air. The high-frequency equipment available in 1952 could be used only in laboratory tests. Even when a specimen can be exposed in liquid, the high reflecting and absorbing qualities of materials shielding the insect make this method of controlling all but exposed insects impractical.

Sound is a mechanical force produced by rapid vibration in some medium, such as air or water. It travels in the form of waves. Sound waves have three major dimensions, frequency, velocity, and intensity.

The frequency is expressed in cycles or vibrations per second. One kilocycle (kc.) is 1,000 cycles per second. Frequency and pitch are identical in most respects; the notes of the musical scale are defined in terms of frequency. The human ear registers sound from about 16 to 20,000 vibrations per second. Beyond that is ultrasonic sound. The highest frequency so far attained is 500 million cycles.

The velocity of sound waves is determined by the medium in which they travel. Sound travels about 1,000 feet per second in air and about 4,800 feet per second in water, depending mainly on the temperature.

The wavelength can be determined by dividing the velocity by the frequency. In air, at a frequency of 1,126 cycles per second, the wavelength is about 1 foot. At 1,000 kilocycles, the wavelength in air is about 0.0344 inch and in water about 0.145 inch. The cathode-ray oscilloscope is used for making sound waves visible so that they can be studied.

The intensity is the amount of energy in the sound wave. A piano key struck violently or softly produces the same number of vibrations per second but with different intensities. Since it is the energy that does the work, the most effective sound machine is the one that can put the most power into the vibrations it emits. Intensities at low frequencies are measured in decibels and otherwise in watts per square centimeter. There are several instruments for measuring the energy in a sound wave, but it is difficult to obtain a high degree of accuracy in the measurements.

The greatest damage to insects is caused by sound waves having a frequency that produces the maximum absorption in the insect but the minimum in the surrounding materials. The amount of absorption has been found to increase with the square of the frequency of the sound wave, and with the viscosity and heat conduction of the material. The absorption of energy in air is quite high compared to that in water.

L. Bergmann, the German physicist, gives the following values for the distance in air and water in which the sound intensity is reduced to one-half.

The absorption in solids depends on the grain or fiber structure of the material. Such fibrous materials as cotton or glass wool have high absorption values.

The generation of heat at the boundary surface of two substances traversed by ultrasonics is especially strong.

When sound waves travel from one type of medium into another, part of the energy is reflected back into the first medium. The amount reflected depends on the density of each material and the velocity of sound. In an air-solid boundary, practically 100 percent of the energy is reflected.

When a sound wave meets an obstacle, the amount of reflection depends on the size and shape of the object. If the object is small compared to the wavelength, some of the wave tends to bend around the object. When sound waves strike an object at an angle, a certain amount of the wave is refracted and the rest is reflected. In liquids and solids, when the angle of incidence is greater than about 15 degrees, all of the wave is reflected.

Thin plates may or may not conduct sound waves, depending on their dimensions and physical properties.

The hard shell of some insects, such as adult roaches, has high reflecting qualities that are difficult to overcome.

When the sound energy is intense enough to cause shattering of the cells of the insect, the maximum bursting action perhaps is obtained by wavelengths shorter than the cell but long enough to produce natural resonance. That would cause the maximum in pressure on different parts of the cell at the same time. Gas bubbles may form which burst with tremendous pressure and disrupt the organism.

One of the earliest practical ultrasonic generators was based on the discovery by Pierre and Jacques Curie in 1880 that a specially cut quartz crystal, when subjected to pressure and tension, will develop electric charges on its crystal faces. Later it was found that this is reversible and that the crystal will expand and contract, thus producing sound waves when an alternating voltage is applied to the surface. It is possible to amplify the vibrations greatly by cutting the crystals properly and backing it in such a way that resonance or natural vibrations amplify the waves. The crystal can also be cut concave to focus the waves. A concave crystal has been found to be more efficient than a flat one. At the focal point of converging sound waves, the energy is as much as 150 times greater than at other points. The crystal ultrasonic generator is called the piezoelectric type and is widely used to produce frequencies above zoo kilocycles. With special crystal rods they can also be made to produce lower frequencies. Because of the high frequency, this type of generator is not used to generate ultrasonics in air or other gases. The part of the generator that converts the electrical energy to sound waves, the transducer, is submerged in transformer oil, from which the sound waves are transferred to the testing medium.