Technical benzene hexachloride is a mixture of several isomers, the gamma isomer being the most toxic to insects. The practically pure isomer is known as lindane. A handicap to the general use of benzene hexachloride on fruit is its tendency to cause off-flavor condition when applied too close to harvest. Lindane is less likely to cause off-flavor in fruit than technical benzene hexachloride but may not overcome this fault altogether.
The organic phosphate insecticides, like DDT, were first found of value in Europe and were introduced into the United States after the close of World War II. Parathion in particular shows great promise for the control of many insect pests. Although these compounds are very poisonous and must be handled strictly in accordance with the manufacturers' recommendations, a recent announcement by Arnold J. Lehman, of the Food and Drug Administration, indicates that their residues are not likely to be harmful. He has stated that "parathion is not stored in the tissues to an appreciable extent—it is rapidly destroyed by the tissues of the body which in turn is an added mechanism for the prevention of tissue accumulation." Residues of hexaethyl tetraphosphate and tetraethyl pyrophosphate persist for only a short time and residues of parathion drop to a low level within 10 to 14 days after application. This information, however, does not make it unnecessary for the user to observe strictly all warnings and precautions issued by the manufacturers of parathion and of other organic phosphates. Serious effects and deaths have occurred though excessive exposures to parathion.
General Information Regarding the Use of the New Organic Insecticides
~Handling the insecticides.~ All the new organic insecticides, the organic phosphates in particular, are to some degree toxic not only to many insects but to man and animals as well. Even the most toxic ones can be used, however, without harmful effects on the operator, provided all the cautions issued by the manufacturer are properly followed. Special care must be taken in handling concentrated insecticides preparatory to making diluted spray or dust applications.
~Spray concentrations.~ DDT has been used more extensively than any of the other newer insecticides and for this reason there is considerable information relative to the spray concentrations known to be effective against insects susceptible to it. For spray purposes DDT is generally employed at the rate of 1-1/2 to 4 pounds of 50 percent wettable powder per 100 gallons of water.
Parathion is being used at 1/2 to 1-1/2 pounds of 15 percent wettable powder per 100 gallons of water for mites and up to 2 pounds to 100 gallons of water for insects more resistant to it. The occurrence of injury to the foliage and fruit of some varieties of apples when this insecticide is used is under investigation.
Benzene hexachloride (10 percent gamma isomer, wettable) is being used at 2 to 4 pounds, and sometimes less depending upon the insect, per 100 gallons of water. Wettable mixtures containing 25 percent of lindane (approximately pure gamma isomer) are used at dosages which would give an equivalent quantity of the gamma isomer in the diluted spray.
Chlordane is usually employed at the rate of 2 to 3 pounds of 50 percent wettable powder and toxaphene at 2 to 4 pounds of 40 percent wettable powder per 100 gallons.
These insecticides are also being sold for use as dusts, either ready to use or in a more concentrated form which can be reduced to dusting strength through the addition of inert material.
~Spray Residues.~ Spray residues are not important on nut crops, but on fruits it is important to time the insecticide applications so that harmful residues are avoided. Animals should not be allowed to graze vegetation beneath trees recently treated. Instructions on the packaged insecticide should be followed.