higher percentage of the population, until a highly resistant strain of pest evolves.
When the conditions are right, the pesticide kills a large percentage of the pest popu-
lation, the pest completes several life cycles per year, and little movement from
untreated populations occurs. Then resistance can develop very rapidly.
Resistance to one pesticide often confers resistance, or faster development of resis-
tance, to a whole family of related pesticides. Alternating different pesticides or apply-
ing a mixture of chemicals can sometimes delay the development of resistance, but it
can also promote the development of super-resistant pests, called superpests, which
are resistant to multiple pesticides. Superpests have already developed and threaten a
number of crops throughout the world.^34
Fate of Pesticides in the Environment
Ideally, a pesticide stays in the treated area long enough to produce the desired
effect and then breaks down into harmless materials. Three primary modes of degra-
dation occur in soils:
¥biological—breakdown by micro-organisms
¥chemical—breakdown by chemical reactions, such as hydrolysis (soluble decom-
position) and oxidation¥photochemical—breakdown by ultraviolet or visible lightThe rate at which a chemical degrades is expressed as the half-life, which is the
amount of time it takes for half of the pesticide to be converted into something else, or
until its concentration is half of its initial level. The half-life of a pesticide depends on
the soil type, its formulation, and environmental conditions such as temperature and
moisture levels. Other processes that influence the fate of the chemical include plant
absorption, soil adhesion, leaching, and vaporization. If pesticides migrate from their
targets due to wind drift, runoff, or leaching, they are considered to be pollutants. The
potential for pesticides to move depends on the chemical properties and formulation of
the pesticide, soil properties, the rate and method of application, pesticide persistence,
frequency and timing of rainfall, irrigation, and depth to ground water.^35
Pesticide Toxicity
Toxicity is the inherent ability of a pesticide to cause injury or death, indicating how
poisonous the chemical is. Acute toxicity is the ability of a substance to cause harm as
the result of a single dose or exposure to a chemical. Chronic toxicity is the ability of a
substance to cause injury as the result of repeated doses or exposures over time. Any
chemical substance is toxic if it is ingested or absorbed in excessive amounts. Table salt,
for example, if consumed in excess, can be toxic. The degree of danger or hazard when
using a pesticide is determined by multiplying toxicity times exposure.
The Pesticide Problem | 7