BECAUSE THE DISAGREEABLE flavor and odor of BHC are readily imparted to food products it is unsafe to apply it to the tops of food plants after the above-ground edible parts have developed appreciably; and it should never be put in the soil before planting any crop, the edible part of which develops below the soil surface. This characteristic of BHC has sharply limited its use in the growing of food crops.

BHC is used rather heavily, early in the season, for controlling certain fruit insects such as plum curculio; 3 to 6 pounds of the gamma isomer or equivalent per acre in a mature orchard. In the form of lindane this represents but 3 to 6 pounds, but in the form of the less expensive technical BHC it represents 25 to 50 pounds. About 40 to 30 pounds per acre of technical BHC are used very extensively to control cotton insects and 5 to 10 pounds per acre are extensively used in the soil to control soil-borne pests of various grain and other farm crops.

From the rates of disappearance of BHC from the soils that we have observed it seems probable that soil applications of 5 to 10 pounds of technical grade (or 1 to 2 pounds of lindane) at intervals of 1 or 2 years will rarely if ever develop residues that impair the growth or yield of crops. These small amounts, however, will probably contaminate foods that develop below the soil surface.

As much as 5o pounds of BHC, per acre added directly to the soil year after year will almost certainly build up an amount in 5 years or less that will be definitely harmful to several important crops. BHC is somewhat volatile, however, loses its plant and insect toxicity in the soil much less slowly than DDT, and is believed to "weather away" and decompose appreciably after it is applied to the foliage of plants. It has, therefore, appeared unlikely that as large a pro- portion of the amount applied will actually reach the soil as occurs with DDT. Experiments only 2 years in progress (1951) tend to confirm this view. While BHC actually accumulates in the soil following heavy applications to the foliage of crops, such accumulation appears to be substantially less than with DDT with which it is being compared.

At heavy rates, however, it accumulates, and it is toxic to plants.

CHLORDANE IS RARELY used at rates in excess of 10 pounds per acre per crop, generally at 6 pounds or less. Ten and 20-pound applications directly to the soil have shown considerable loss of insecticidal value after 1 year. Chemical analysis of plots so treated indicated that only 4 to 5 pounds of chlordane remained a year after application. Thus, although chlordane does seem to be only moderately persistent, some of a normal application persists more than a year, indicating that if used repeatedly at intervals of about a year a slow accumulation of a residue may be expected. These statements on persistence are admittedly based on meager evidence and may have to be modified later. Nevertheless, chlordane seems to present a definitely less immediate and potentially serious problem than does DDT.

As indicated on preceding pages, toxaphene, and parathion and other phosphorus compounds appear to involve no hazard through the accumulating of residues in the soil.

Methoxychlor, TDE, aldrin, dieldrin, heptachlor, and other new synthetic insecticides have appeared so recently that too little has been learned about their plant toxicity and persistence in the soil to permit specific statements about them as this is written (1952). Early results, however, indicate that methoxyclor, TDE, and DDT are rather similar in persistence and that DDT is more toxic than the first two. Chlordane, aldrin, and dieldrin appear persistent but somewhat less so than DDT.

Since it has been shown that some of these remarkably efficient and economical insecticides may and do accumulate to an undesirable degree in the soil under certain conditions, we may confidently look to the research chemist and to the chemical manufacturer for still newer compounds that will have the advantages of those now in use but without the disadvantages of too great stability and toxicity to plants in the soil. Since a few good insecticides are already known that appear now to present no soil-residue problem, others doubtless will be produced. It is true that those now known to be relatively unstable and less toxic to plants may not be as effective or economical for controlling certain farm pests as some of the more persistent ones are. There is no reason, however, to suppose that it will always be so. Highly effective non-accumulative insecticides will surely be developed for use where desirable. Their further development, production, and use to replace the too persistent ones should be urged.

VICTOR R. BOSWELL is head of the division of vegetable crops and diseases at the Plant Industry Station at Beltsville, Md. He grew up on a small farm in southwest Missouri and studied horticulture at the University of Missouri and the University, of Maryland. After 6 years of teaching and research experience in Maryland he entered the Bureau of Plant Industry of the Department of Agriculture in 1928, with responsibility for the vegetable crop investigations in that Bureau, which responsibility he still holds. He became assistant head of the division of fruit and vegetable crops and diseases in 1941. Dr. Boswell has written many publications on a wide range of problems relating to the production, growth, development, yield, and quality of vegetable crops. He was assigned to the War Department in 1945-46 as a member of the National Resources Section of SCAP in Tokyo where he was responsible for the production branch in the agriculture division. He was president of the American Society for Horticultural Science in 1939.