Insecticides for Vegetables

by W. H. WHITE

THE perfect insecticide for vegetables must meet several definite specifications. It must leave no residue or deposit on the fruit or edible leaves of the plant to endanger the health of the consumer a point that is especially important in controlling insects,on leafy vegetables like cabbage, broccoli, kale, lettuce, celery, and asparagus, and almost as important in the case of vegetables that bear edible pods or fruits. The residues on leafy vegetables are not readily removed by washing. The insecticide for vegetables must be poisonous to a number of different kinds of insects and have no effect upon plant growth, whatever the climate. Temperature and humidity should not affect its toxicity to the insect or its physical qualities. It should be compatible with fungicides.

A material or combination that meets fully all these specifications has not yet been discovered, although many hundreds have been tested. But insecticides containing pyrethrins and rotenone as the principal toxic agents, developed during the past 15 years, have given satisfactory control of some important insect pests and have practically eliminated the hazard of residues.

The ground flowers of the plant Pyrethrum cinerariaefolium is the principal raw product from which pyrethrum insecticides are prepared. Before the war, Japan was the principal source of the raw product, but during the war supplies were obtained largely from Kenya Colony, Africa. The two toxic ingredients of the flower buds are known chemically as pyrethrins I and II. The raw product from Kenya Colony contains, on an average, 1.3 percent of total pyrethrins.

Rotenone insecticides are prepared from the ground roots of two kinds of plants, Derris elliptica (derris) from the Malay States and Dutch East Indies, and Lonchocarpus species (cube, timbo, barbasco) from South America. The ground roots of these plants contain, besides rotenone, other ingredients toxic to insects, but rotenone is considered the most important, and is used as a basis for the preparation of rotenone insecticides.

In the late 1920's and early 1930's the condemnation by food officials of vegetable products, such as celery and cabbage, because of undesirable insecticide residues emphasized the necessity of developing insecticides less hazardous to man than the arsenical compounds and of providing schedules for applying insecticides that would reduce or eliminate the spray-residue problem for certain types of vegetables.

In general, the problem was approached from two angles: To study the growth of the plant to determine the latest period in its development that an arsenical insecticide could be applied without involving a risk of contaminating the part of the plant that is used as food; to develop the use of pyrethrum and rotenone products and extend the use of nicotine insecticides for leaf-feeding insects.

The problem of residues was particularly acute in areas where arsenicals were used for the control of cabbage caterpillars, especially where the cabbage is marketed with several loose leaves around the head. Therefore, the investigations initiated in 1932 were concentrated on the control of insects affecting cabbage and related Cole crops. Plant-growth studies on the Wakefield variety of cabbage grown near Charleston, S. C., which is marketed with four outer loose leaves, revealed that objectionable residues will follow the use of arsenicals applied 30 days before harvesttime, or after the head begins to form. On cauliflower similar residues remained if the application was made after the curd or head had begun to form.