DDT, the first of the new, complex organic compounds, since 1945 has largely replaced lead arsenate in the control of codling moth. Less DDT is needed and in many areas fewer applications are necessary. As a result, residues from DDT programs are much less than the former residues from lead arsenate. But residues of DDT are hard to remove. If they are found to be too great, the problem will have to be met by adjustments in the spray program or by the substitution of less objectionable spray materials during the latter part of the growing season.

The codling moth is only one of many insect pests that the grower must control. Now that the codling moth has been reduced to a comparatively minor status, other pests have assumed greater importance. Some, such as the orchard mites or spider mites, have become more abundant and destructive following the use of DDT. To meet this situation the development of additional new pesticides is being pushed by Department of Agriculture and State workers and insecticide companies, who have had to conduct extensive research in several fields. The work of the chemists and entomologists in developing, testing, and evaluating insecticides is only the beginning. A chemical that shows promise must be evaluated as to residues, particularly whether the residues are likely to be a potential hazard if the material is used commercially. The pharmacologist must carry on experiments with the new product on laboratory animals. The determination of the dosages that are fatal or that cause obvious distress is the simplest part of the job. The potential danger of taking in tiny quantities of an insecticide day after day must also be determined. Any effect on consumers of spray residues would come about by repeated consumption of small quantities. As indicated earlier, the, quantities found on marketed fruits and vegetables are rarely, if ever, great enough to have any immediate effect. To secure information on cumulative effects, experiments must extend over many months, as much as 2 years.

The investigations have created some problems for the chemists. The amounts to be measured are so small that they are expressed as parts per million. During the interval between the last insecticide application and harvest the residue may be reduced in three ways by crop growth, by physical weathering, and by chemical breakdown of the insecticide so that the residues at harvest, expressed in parts per million, are lower than they were immediately after application of the spray or dust.

To determine such tiny quantities of insecticides, the chemist must develop highly sensitive methods of analysis. Such methods are rarely available for use with the new insecticides, and the development of the required highly accurate methods is often difficult.

Many of the new insecticides are chlorinated hydrocarbons. If it is definitely known what compound is present, all that has to be done is to determine the amount of organic chlorine in it and then compute from that figure the amount of the original compound that is present. For example, DDT contains 5o percent of chlorine. If DDT is known to be the only insecticide that has been used, the amount of chlorine present is determined. The resulting figure is then multiplied by 2, which gives the amount of DDT present. The problem is rarely that simple, however. Sometimes the analysis of untreated fruits or vegetables shows Considerable amounts of natural organic chlorine that must be allowed for.

Sometimes two chlorinated hydrocarbon insecticides are used on the same fruit or vegetable. It is then impossible to compute the amounts of each from the organic chlorine determination, and special methods have to be worked out to measure the different compounds separately. Some of the chemical determinations are made by comparing the color of a solution containing poison dissolved from the sprayed fruit with the color of a solution containing a known. quantity of the chemical. Some fruits or vegetables contain substances that produce colors similar to those being measured. These natural color changes interfere with the accuracy of the analysis, and must be eliminated or allowed for. The solution of these difficulties requires great ingenuity on the part of the chemist.

Although the establishment of tolerances as a result of hearings conducted by the Federal Security Agency in 1950 may have a stabilizing influence, the situation will never be a static one. Insect-control problems are changing continually. New insecticides will continue to appear and create new problems. The residue factor will always be an important consideration. We are confident that steady progress will be made, and the public will be insured of a safe, adequate, and continuous supply of fruits and vegetables of high quality.

B. A. PORTER is in charge of the division of fruit insect investigations in the Bureau of Entomology and Plant Quarantine. He joined the Department in 1917, and for many years conducted field investigations of various orchard-insect problems in Connecticut and Indiana. He is a graduate of Massachusetts Agricultural College.

J. E. FAHEY is a chemist in the Bureau of Entomology and Plant Quarantine. Since 1934 he has been in charge of the chemical work at the Department's fruit insect laboratory at Vincennes, Ind. He has made special studies of spray residues on fruits and other agricultural products. He was graduated from Oregon State College in 1928.