Residues on Fruits and Vegetables

B. A. Porter, J. E. Fahey.

How to use a chemical to control insects on fruits and vegetables without harming the person who eats them remains a serious problem.

It concerns the chemists who develop insecticides, the growers who use them, food officials of State and Federal agencies, and home gardeners, who might not always treat the poisons with the respect due them. Many scientists, who realize that worms and insect debris in fruits and vegetables would lower their value to grower and consumer, have done a tremendous amount of research on ways to keep the foods free of dangerous contamination with insecticides.

It seemed that the problem was settled in 1880, when in reporting the first official tests of arsenicals, the investigator, A. J. Cook, of Michigan, took into account the possible effect of the insecticide on the consumer. Materials then available were paris green and London purple. The results of the analyses were taken to mean that there was no danger that injurious quantities of poison could reach the consumer when those insecticides were used. Further reassurance was given 11 years later by another official worker, C. P. Gillette, of the Iowa Agricultural Experiment Station. He also studied the matter carefully and announced that a person would have to eat at one sitting 30 cabbages that had been dusted with paris green to get enough poison to hurt him. Evidently he assumed that the insecticide would be evenly distributed and the loose leaves trimmed off. In those early days, spraying and dusting were light compared with later practice, and the insecticides in use had poor sticking qualities. The conclusions of the early investigators were probably correct at the time.

But the problem since has become intensified. Our cropping areas have become more concentrated. Insect pests have become more abundant and hard to control. Spray and dust programs have included more and heavier applications. Fears increased that with the growing use of insecticides the fruits and vegetables on the consumer's table might have excessive residues on them. Men in the Department of Agriculture therefore, in surveys in 1915 to 1919, analyzed hundreds of samples of peaches, cherries, plums, apples, pears, grapes, cranberries, tomatoes, celery, and cucumbers for lead, arsenic and copper. The investigators concluded that only little spray residue remained on fruit or vegetables that had been sprayed according to standard recommendations. They reported, however, that excessive residues remained on fruits or vegetables that had been oversprayed or sprayed too close to harvesttime.

Another complication has arisen since then. At first the possible immediate effect of spray residues on consumers was chiefly considered. Less thought was given to the possible cumulative effect of taking in extremely small quantities of poison day after day. Such effects are hard to detect and easy to confuse with other conditions. Cases of sickness developing immediately after the poisons were taken in have been rare, if they have occurred at all. But since the 1920's the belief has grown that the gradual accumulation of poisons in the system might have unfavorable effects.

The problems of spray residue on vegetable crops have been met in several ways. Many insects that attack vegetable crops, such as the Mexican bean beetle and several kinds of cabbage caterpillars, can be controlled by the use of pyrethrum or materials that contain rotenone. Residues of both are considered unobjectionable. Both are also effective against most of the insects that attack small fruits like currants, raspberries, and cranberries. Commercial practices often eliminate residues of insecticides on vegetable crops. For instance, the part of the cabbage plant that has been exposed to the insecticide is often trimmed off in preparing the cabbages for the market. The use of insecticides can be limited to the early stages of the growth of the plant when the part to be eaten has not yet formed.

Many of the insects that affect tree fruits can be controlled without causing excessive spray residue. The plum curculio, which also feeds extensively on peach and apple, lays most of its eggs and causes the most damage in most localities in late spring or early summer. Insecticides can be used freely then, because only the smallest traces will remain on the fruit at picking time. Many insects attack tree fruits throughout the growing season, however. The most important of these has been the codling moth on apples and pears. Since this problem received major attention from the early 1920's until the early 1940's, we review its history during that period in some detail.

The codling moth is often called the appleworm. Many have had the experience of biting into an apple containing a worm or into an apple in which a worm had lived. Wormy apples rot quickly in storage or during shipment. Uncontrolled, the worms can ruin 50 to 90 percent of the crop. A crop that is even 50-percent wormy has little commercial value, since the cost of handling and sorting it is often more than the value of the part that can be salvaged. Such apples are fit only for immediate local use, or for low-grade byproducts.

Lead arsenate was the standard insecticide against the codling moth from early in the present century until 1945. As the worms became more and more abundant and hard to control, the number of spray applications, the strength of the spray mixture, and the number of gallons applied per tree steadily increased. The amount of lead and arsenic on the fruit at harvest time also increased steadily: Western pears in 1919 were condemned by the Boston Board of Health because of excessive residues of arsenic. A few years later British health authorities objected to shipments of American apples for the same reason. By 1925 it had become evident that the use of lead arsenate sprays had increased to the point where American apples and pears were carrying quantities of residues that were at least potentially dangerous to public health.

As soon as the serious nature of the problem became clear, the Department of Agriculture moved to carry out its responsibility for the enforcement of the Food and Drug Act. An administrative tolerance for arsenic (as of As₂O₃) of 0.025 grain per pound of fruit ( about 3.5 parts per million) was established, following a conference of health authorities. Efforts were made to reduce the residues below this figure, and by 1932 the amount of arsenic permitted on apples and pears under the administrative tolerances had been progressively reduced to 0.01 grain per pound of fruit ( about 1.4 parts per million).

During the 1920's attention was focused entirely on the arsenic portion of lead arsenate, on the assumption that if the arsenic were reduced to safe quantities, the lead would also be eliminated as a hazard. It was found later that the lead did not always weather off as rapidly as the arsenic and that it was not always so completely removed by washing. Since the early 1930's residues of lead arsenate have therefore been judged largely by their lead content.

Although both lead and arsenic have long been known to be serious poisons, precise information was lacking on the quantities of those materials that could be taken into the human body in the form of spray residues without harm. To shed light on this problem, the United States Public Health Service carried on an investigation from 1937 to 1940 around Wenatchee, Wash., a leading apple-growing section, where during the 1930's as much as 7 million pounds of lead arsenate was used annually. A careful study was made of 1,231 persons, many of whom worked or lived in or close to apple orchards and were thus extensively exposed to lead arsenate, both in their diet and their surroundings. The following is quoted from the report of the studies:

"Only six men and one woman had a combination of clinical and laboratory findings directly referable to the absorption of lead arsenate. Some physicians may interpret these cases as minimal lead arsenate intoxication. However, as regards lead, these cases do not come up to the criteria of the Committee on Lead Poisoning of the American Public Health Association for lead intoxication, incipient plumb-ism or lead poisoning. These subjects were all orchardists and ranged in age from 23 to 68 years."

Although such a study is less exact than experimentation on guinea pigs, mice, or other laboratory animals, it gives an approximate indication of what actually happens to human beings exposed regularly to lead arsenate. Many of the persons examined had undoubtedly taken much greater quantities of lead arsenate than the general consuming public. On the basis of the results of the study, the administrative tolerances for apples and pears were increased to 0.025 grain of arsenic (as arsenic trioxide) per pound and 0.05 grain of lead per pound of fruit (about 3.5 and 7 parts per million, respectively). That eased the situation greatly from the standpoint of the growers. It was then possible for most growers in the East and Midwest to market apples without washing them and for the growers in the Northwest to clean their fruit satisfactorily with mild washing programs.

During the 1920's and ever since, vigorous efforts have been made to solve the problem of spray residues on apples and pears. These efforts have taken two directions : The development of methods and equipment for removing the excessive residue after the fruit was harvested, and the development of effective insecticides or other methods of control less objectionable from the standpoint of residues.

Effective washing methods and machinery were promptly developed for removing most of the residues before the fruit is marketed. In the washing process the apples are passed through dilute hydrochloric acid and then through a spray of clean water to rinse off the acid and the poison. In extreme cases a double wash is used; the apples are passed through a dilute alkaline solution, rinsed, passed through a dilute acid, and then rinsed again. To aid in removal, the wash solutions are sometimes heated. The development of such equipment permitted the safe marketing of the apples, but the cost of installing, maintaining, and operating the machinery came at a bad time. Prices during the 193o's were abnormally low, and many growers were having difficulty in meeting their obligations. Washing the fruit added one more item of production cost. It was a real hardship to many growers, and some of them even lost their orchards.

The ultimate solution of the problem evidently should be sought in another direction. The old-time practice of trapping the worms in bands placed around the trunks of the trees was revived and improved by a chemical treatment of the bands. Traps baited with fermenting sugar solutions with the addition of attractive chemicals were developed. The possibilities in light traps, orchard clean-up, and parasites were explored. Many of those practices had control value, but they did not reduce infestations to a point where the spray program could be materially shortened.