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RESISTANCE TO PESTICIDES
Although pesticides are designed to kill pest populations, they are seldom 100% effective
-- a few individuals usually survive and reproduce. These survivors may have a
behavioral trait that helps them avoid the pesticide, a biochemical trait that allows them
to detoxify the
pesticide,
or some other
genetic
characteristic that
reduces their
susceptiblity to the
pesticide. If these
survivors mate and
pass on this
"resistance" to their
offspring, then
subsequent
generations will
contain fewer
susceptible
individuals.
Eventually, the
entire population
may become
"resistant". There
are two major
variables that
determine the rate at which a resistant trait is likely to spread throughout the population:
1. Its mechanism of inheritance (dominant, recessive, or co-dominant; sex linked,
pleiotropic, or polygenic), and
2. The severity of selective pressure (what percentage of susceptible individuals
survive each generation and whether mortality occurs before or after the
susceptible individuals reproduce).
In general, resistance will spread through a population most rapidly when it is inherited
as a single, dominant allele and selective pressure is high (meaning very few susceptible
individuals escape and reproduce).
Class Resistance and Cross Resistance
When an insect population develops resistance to one pesticide, it may also prove to be
resistant to similar compounds that have the same mode of action. This phenomenon,
known as class resistance, occurs frequently in pest populations that develop resistance
to organophosphate, carbamate, or pyrethroid insecticides. In some cases, a population
may develop a form of resistance that protects it from compounds in more than one
chemical class. This cross resistance may produce a population that can no longer be
controlled with chemical insecticides.
Resistance Management
Frequent and/or indiscriminant use of chemical insecticides typically leads to the
development of resistance and to a need for more powerful poisons. But wise use of
pesticides can delay (or maybe even prevent) the evolution of resistant genotypes. There
are basically three strategies that can be used to "manage" the resistance problem:
1. Management by Saturation involves excessive, heavy, or frequent use of a
pesticide that is designed to leave absolutely no survivors. This strategy
overwhelms a population's resistance, suppresses any detoxification mechanism,
and precludes reproduction by survivors. It is most effective when the resistant
gene is dominant and the target population is small, isolated, or living in a limited
habitat (e.g. greenhouse).
2. Management by Moderation uses only the minimum control necessary to
reduce a population below its economic threshold. Growers use low rates, shortresidual compounds, infrequent applications, and/or incomplete coverage to
ensure that significant numbers of susceptible individuals survive and reproduce.
This strategy tries to ensure that susceptible genes are never eliminated from the
population. It works best when the susceptible trait is dominant over the resistant
trait.
3. Management by Multiple Attack involves the use of several control tactics
that work in different ways. By rotating insecticides with different modes-ofaction or by alternating chemical with non-chemical control tactics, a pest
population is exposed to selective pressures that change from generation to
generation. Natural selection for a resistant genotype is less likely to occur when
selective pressures are highly variable.
CHARACTERISTICS OF RESISTANCE MANAGEMENT STRATEGIES
Saturation
 High rates
 Frequent
applications
 Long residual
 Apply before
reproductive stage
 Eliminate refugia
Moderation
 Low rates
 Infrequent
applications
 Short residuals
 Apply after
reproductive stage
 Preserve refugia
Multiple Attack
 Rotate treatments
 Use mixtures
 Alternate with nonchemical controls