NAVMED P-5010-8 - Navy Medicine - U.S. Navy

8-13 CHAPTER 8. NAVY ENTOMOLOGY AND PEST CONTROL TECHNOLOGY 8-13
8-13. Resistance to Pesticides
a. Definition of Pesticide Resistance.
Resistance of pests to pesticides is defined as the
ability of a given population to withstand a
poison that was effectively lethal to earlier
generations of the species.
b. Development of Resistance. Most
normal populations of animal species include
individuals that vary in their susceptibility to
pesticides. Consequently, candidate pesticides
will kill some individuals of a species more
readily than others. Individuals in a population
that are less susceptible to a chemical are
considered to be more resistant. Continued
pesticide pressure upon a population will destroy
the most susceptible individuals, permitting the
more resistant individuals to survive and
produce generations of increasingly resistant
offspring. Thus, the species becomes
increasingly difficult to control because of
genetic factors transmissible to subsequent
generations. Development of resistance in a pest
population can be subtle or quite dramatic.
Houseflies were found to develop resistance to
DDT within a year after it was introduced into
areas of Europe. For mosquito control, the use
of the same insecticide as a larvicide and
adulticide is thought to enhance the development
of resistance. Resistance is not confined to
insecticides, nor is it always rapid in
development. Some Norway and roof rats and
house mice have become resistant to
anticoagulant rodenticides in Europe and the
U.S. after 20 years of use. Pesticide resistance
has been reported for more than 225 species of
arthropods. All of the modern day insecticide
groups that include organophosphates,
organochlorines, and carbamates have examples
of the development of resistance. Even crossresistance
between these groups occurs. For
example, chlordane (organochlorine) resistance
may increase propoxur (carbamate) resistance in
the German cockroach, Blatella germanica. This
condition further complicates the situation for
control work and necessitates reliance on specialists
for recommending changes in methods,
materials, and dosage rates. Not all field reports
of resistance are valid. Other factors may be
responsible for unsatisfactory control. Faulty
techniques, chemical agents and equipment,
inexperienced or incompetent operators, increased
breeding rates, migration from outside the controlled
area, and poor sanitation are a few of the
more frequently observed reasons for ineffective
control. It must be continually emphasized that
change to another insecticide should be considered
only when conclusive laboratory proof of resistance
is obtained. The question of whether a
resistant strain will revert to susceptibility when
not exposed to the pesticide for a period of time
has not been completely answered. The consensus
among researchers is that while reversion will
probably occur if there is no further exposure to
the same or related pesticides, the time required
would be dependent upon the degree of resistance
developed. However, it has been experimentally
demonstrated that once a resistant insect species
has reverted back to susceptibility that resistance
may quickly reappear with resumed use of the
original insecticide.
c. Prevention of Resistance. Selection
of an insect population for insecticide resistance
may be averted or delayed by rotating the different
classes of insecticides available for control.
For example, treating a German cockroach
population repeatedly with an organophosphate
insecticide may hasten the development of
resistance to this class of insecticides. The use
of integrated pest management techniques which
include preventive, exclusion, biological, physical
as well as chemical control methodologies
together in a comprehensive pest control
strategy will also help to slow or stop the
development of resistance.
9 Nov 2004
8-13