that occur through runoff than to reduce the high levels that result from direct application
and accidents.
c.
Soil. The fallout of pesticides following sprayed applications in the direct
treatment of soil has led to a buildup in the soil of various amounts of pesticides. Soil
residues may, therefore, be a cause for concern since they reach man by a number of
routes -- uptake from soil by consumable crops, leaching into water supplies,
volatilization into the air, and by direct contact with the soil. The magnitude of the
problem is directly related to the amount supplied to the soil and the rates of pesticide
degradation in the soil. Other related factors include the type and formulation of the
pesticide, the soil type, the soil moisture and temperature, the leaching of pesticides
from the soil by water, and wind erosion. It is necessary to understand these factors to
properly evaluate the contamination hazard in applying a given pesticide to a given soil.
d. Food. In addition to air and water, food offers a medium or route for
pesticide contamination. Crops of all kinds are particularly vulnerable to pesticide
contamination, for it is upon the crops that the target insects feed. Forage and cereal
crops, fruits, and vegetables become contaminated not only through direct application,
but also systemically through the roots of the plant. The degree of contamination
depends upon several variables -- the type of plant, the type of pesticide, the degree of
soil contamination, and the time lapse between application of the pesticide and
harvesting of the crop. As would be expected, residues from the chlorinated
hydrocarbons exceed those of other types of pesticides. This is partly because of their
wider usage, but it is also because plants have the ability to metabolize the
organophosphates in their cells. The problem in animal food supplies is essentially that
of the food chain discussed in paragraph 3-13. Most of our meat animals are grain and
forage eaters. Concentrations of highly residual pesticides present in grain and forage
feeds build up in the tissues -- particularly in fat -- of these animals. Such residues are
also found in animal products such as milk and eggs. Pesticide residues in milk have
been traced to the absorption of chemicals through the skin by cows that had been
sprayed for the control of flies, screwworms, and other pests.
3-15. PESTICIDE RESISTANCE
A major concern in the employment of pesticides is that of the development in
some species of pests of resistance to certain pesticides. Cockroaches, mosquitoes,
and houseflies, for example, have developed a resistance to chlordane. The problem in
pesticide-resistant insects is similar to that of antibiotic-resistant strains of
microorganisms. The resistance often becomes genetic, resulting in successive
generations of resistant organisms. This resistance frequently leads to the use of more
toxic materials or of increased concentrations of the same material. In either case, the
result is an added environmental hazard.
MD0173
3-17