Military Characteristics of Insecticides FREDERICK W. WHITTEMORE, JR.
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Preventive Medicine Division, Office of The Surgeon General, Department of the Army, Washington, D. C.
A high degree of insecticidal activity per unit weight, effectiveness against many species of insects, prolonged residual effect combined with rapid knockdown, compatibility with various vehicles, and availability under wartime conditions are among the most important military characteristics of insecticides. No present insecticide is entirely satisfactory.
The properties of insecticides are, i n military parlance, called characteristics. M a n y of these characteristics are of importance i n both military and civilian usage, but because of the vast difference between military and civilian operations, certain properties of insecticides become extremely important to the armed services, and i n many instances are more important to the military economy than to the civilian economy. Possibly the most important single military characteristic is a high degree of insecticidal activity per unit weight. The necessity for this requirement may be more fully appreciated when i t is realized that approximately 3 tons of equipment and supplies per individual soldier are necessary during the assault stages of an invasion, and 0.75 ton of supplies and equipment per man is necessary to maintain one combat soldier for one month i n the field. W i t h this vast amount of equipment and supplies for each i n d i vidual soldier, the weight of each specific item needed for his support must be critically scrutinized. Prior to the discovery of D D T , with its high degree of insecticidal activity, it was necessary to include far larger weights of insecticides than at present. Although further improvements may be made i n the direction of increasing the toxicity of insecticides per unit weight of the pure chemical, i t is not believed that this requirement can be changed appreciably. Future research should be directed, not toward finding insecticides of higher insecticidal activity per unit weight, but toward finding insecticides that fulfill more adequately the other military characteristics discussed below.
Effectiveness Another extremely important military characteristic is effectiveness against many different species of insects without the development of resistant strains. E v e r y insecticide that must be added to the military list of supplies geometrically increases the difficulties of procurement and distribution. A t the present time, nineteen different insecticides and insect repellents and four different rodenticides are issued by the A r m y Quartermaster. These figures do not include the different formulations of insect repellents issued under the same stock number. The three basic insect repellents are dimethyl phthlate, Indalone, and Rutgers 612. These repellents are issued either alone or i n various combinations, further complicating the supply situation because of the variation i n efficiency of these substances against different species of mosquitoes i n different parts of the world. Although all insecticide containers are adequately marked when they are filled at the factory, extreme difficulty has been encountered i n identifying many insecticides following 215
AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
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ADVANCES IN CHEMISTRY SERIES
long storage in the field under adverse conditions. E v e n under the best conditions the supply problem may become very complicated in overseas areas because of the necessity of utilizing unskilled foreign labor i n supply dumps. In many instances this may result in the storage in the same stockpile of items that differ from each other only i n percentage composition. The results of this situation can be readily foreseen. A requisition for a specific item is occasionally filled with an item which, at first glance, appears to be the one requested but, actually, was formulated for a different purpose and is either inadequate or dangerous to use for the intended purpose. A specific example of the hazards inherent in this situation may be found i n the case of insecticide space spray composed of 1% D D T , 0 . 1 % pyrethrins, or 2.5% thiocyanate in deodorized kerosene and 5 % residual-effect D D T , both of which are issued in 5-gallon steel drums. Obviously, if a requisition for residual-effect D D T were to be filled with space spray, the application of the solution as a residualeffect compound would be of little or no value. Under some conditions, when stocks have been exposed to such adverse weather conditions that all gross identifying marks have been removed from the containers, the assumption has been made by the untrained native laborers that all unidentifiable cans of the same size contained the same material. Were it possible to have just one insecticide for all military purposes, such a situation could easily be avoided. It is also necessary, from the military point of view, that the insecticides supplied to troops i n the field be readily convertible into end-use items. It should not be necessary to go through an involved procedure in preparing insecticides for field utilization. For example, the earliest lots of D D T that were received i n the N o r t h African theater were of the consistency of beeswax, and were extremely difficult to get into solution. It was necessary to process all this material through meat grinders, of the hamburg variety, requisitioned from the civilian economy before this material could be satisfactorily dissolved i n Diesel oil. Such a situation complicates field operations unnecessarily, and should be avoided in the development of future insecticides.
Residual Effect and Knockdown Another extremely important military characteristic is prolonged residual effect combined with rapid * 'knockdown. " Although, at the present time, we have some chemicals which give a comparatively long residual effect, and other chemicals which give a relatively quick knockdown, the military still require more prolonged residual effect and more rapid knockdown in their insecticides. Although D D T has a prolonged residual effect, and is extremely valuable in the control of such insects as mosquitoes and sand flies of the Phlebotomies type, improvements are needed in the speed of knockdown, particularly of mosquitoes. There is an appreciable time interval after the mosquito first comes i n contact with D D T before its biting reflexes are no longer inhibited. This factor is not so important with insects such as the Phlebotomus-type sand fly, because this group of insects makes extremely short flights of 2 to 3 yards and then stops to rest. Residual D D T applied to breeding areas and buildings will adequately prevent sand flies from entering buildings and biting personnel. M o s quitoes, insects capable of sustained flight, may conceivably enter a building, come i n contact once or twice with D D T , and still bite and infect a person with malaria before they are killed by the D D T . A t the present time we have no chemical that combines both quick knockdown and prolonged residual toxicity for use against all insects of medical importance. Still another requirement is that the concentrated form of the insecticide readily combines with various types of vehicles. A t the present time, pure D D T can be dissolved only in organic solvents, and it has been necessary to provide a 5 0 % water-dispersible D D T powder when water was to be used as the vehicle. W i t h the advent of 9 0 % water-dispersible D D T , it may be possible to utilize this material i n the preparation of both water suspensions and organic solutions of the chemical. The ideal chemical i n this respect would be one that could be shipped as 100% dust, and could be readily diluted with inert dust, water, and organic solvents i n the field. AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
WHITTEMORE—MILITARY CHARACTERISTICS OF INSECTICIDES
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Another extremely important military requirement that is not usually associated with civilian requirements is stability i n prolonged storage under adverse conditions. For example, the first lots offlyspray that were shipped to N o r t h Africa, i n 1942, contained pyrethrum as the principal active ingredient. I t was not possible to store this m a terial under cover and much of it remained under constant exposure to the intense tropical sun, undoubtedly deteriorating very rapidly. Covered storage facilities are the exception rather than the rule i n a combat theater of operations, and all insecticide compounds furnished to the military forces should withstand this type of storage.
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Availability The last, but by no means the least, important of the military characteristics is availability under wartime conditions. Materials are classified as strategic, critical, and noncritical i n this respect: A strategic material is one that must be imported into the United States; a critical material is one that is available i n the United States but, because of either limited plant facilities or excessive demands, becomes nonavailable; a noncritical m a terial is one that is readily available i n . sufficient amounts under wartime conditions. A n insecticide to be of value to the military forces must be readily available under wartime conditions, must not be dependent upon the importation of certain essential materials, and must not impose a burden upon plant facilities i n the United States. The only military characteristic which has been met adequately at the present time is that of a high degree of insecticidal activity per unit weight. The other requirements are not being met adequately by the insecticides i n commercial production at this time and a vast amount of research is necessary before the armed forces can be furnished with an insecticide which is entirely satisfactory for military needs.
AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
