The chemical insecticides developed during
and after World War II initially were effective and cheap. Farmers came to rely on the new chemical methods
of pest control, which rapidly replaced traditional forms of chemical, cultural and biological
control. The 1950s and 1960s were times of an insecticide
boom, but use continued to rise and insecticide application is still the single main pest
control tactic employed today. I’m Dr. DeBusk and in this video, I’m
going to talk about the effect of insecticides, mechanisms of resistance, and tactics for
resistance management. Although pest populations are suppressed by
insecticide use, undesirable effects include the following. 1) Selection for insects that are genetically
resistant to the chemicals. 2) Destruction of non-target organisms, including
pollinators, the natural enemies of the pests, and soil arthropods. 3) Pest resurgence – as a consequence of 1
and 2, a dramatic increase in numbers of the targeted pest can occur. For example, severe outbreaks of cottony-cushion
scale as a result of DDT use in California in the 1940s. If the natural enemies recover much more slowly
than the pest population, the pests can be greater than before the insecticide treatment. 4) Secondary pest outbreak – a combination
of suppression of the original pest and effects 1 and 2 can lead to insect species previously
not considered pests being released from control and becoming major pests. 5) Adverse environmental effects, resulting
in contamination of soils, water systems, and the produce itself with chemicals that
accumulate biologically, especially in vertebrates, as a result of biomagnification through food
chains. 6) Dangers to human health either directly
from the handling and consumption of insecticides or indirectly via exposure to environmental
sources. Despite increased insecticide use, damage
by insects pests has increased. For example, insecticide use in the United
States increased 10-fold from about 1950 to 1985, while the proportion of crops lost to
insects roughly doubled, from 7% to 13%, during the same period. These figures do not mean that insecticides
don’t control insects, because non-resistant insects clearly are killed by chemicals. Rather, many factors account for this imbalance
between pest problems and control measures. Human trade has spread pests to outside their
range of natural enemies. Selection for high-yield crops often inadvertently
resulted in more pest susceptibility. Extensive monocultures are common, with a
reduction in sanitation and other cultural practices like crop rotation. Finally, aggressive commercial marketing of
chemical insecticides has led to their inappropriate use, especially in developing countries. Insecticide resistance is the result of selection
of individuals that are predisposed genetically to survive exposure to an insecticide. Tolerance, the ability of an individual to
survive an insecticide, implies nothing about the basis of survival. Field-evolved resistance is the genetically
based decrease in susceptibility of a population to an insecticide caused by exposure to that
insecticide in the field. Over the past few decades, more than 700 species
of arthropod pests have developed resistance to one or more insecticides, as well as resistance
to toxins that have been genetically engineered into major crop plants. The silverleaf whitefly, the Colorado potato
beetle, and the diamondback moth are resistant to virtually all chemicals available for control. Chemically-based pest control of these and
many other pests may soon become virtually impossible because many show cross- or multiple
resistance. Cross-resistance is where the resistance mechanism
for one insecticide gives tolerance for another with the site mode of action. Multiple resistance is resistance of two or
more pesticides with different sites of action. This figure shows resistance of plants to
herbicides but it is the same in insect pests. Mechanisms of insecticide resistance include:
increased behavioral avoidance, as some insecticides, such as neem and pyrethroids, can repel insects
and physiological changes, such as sequestration (trapping toxic chemicals in specialized tissues),
reduced cuticular penetration, or accelerated excretion. Other mechanisms include metabolic detoxification
by specialized enzymes and increased tolerance as a result of decreased sensitivity at the
insecticide target site. Plant-feeding insects, especially those that
feed on multiple plants, frequently develop resistance faster than their natural enemies. They may be preadapted to evolve insecticide
resistance because they have general detoxifying mechanisms for compounds found in their host
plants. Insects that chew plants or consume non-vascular
cell contents appear to have evolve pesticide resistance more than phloem- and xylem-feeding
species. Management of insecticide resistance requires
a plan for controlled chemical use with the primary goals of avoiding or slowing down
the development of resistant pest population, causing resistant populations to revert to
more susceptible levels, and/or causing resistance in natural enemies. The tactics for resistance management includes
maintaining reservoirs of susceptible pest insects, either in refuges or immigration
from untreated areas, to promote dilution of resistant genes as you can see depicted
in this diagram, varying the dose or frequency of insecticide applications, using less-persistent
chemicals, and/or applying insecticides as a rotation or sequence of different chemicals
or as a mixture. The optimal strategy for reducing resistance
is to use insecticides only when natural enemies can’t control the pests. Resistance monitoring should be an integral
part of management, so you can anticipate problems and assess the effectiveness of management
tactics. In conclusion, recognition of the problems
discussed, cost of insecticides, and a strong consumer reaction to environmentally damaging
agronomic practices and chemical contamination of produce have led to current development
of alternative pest control methods. Chemical controls are increasingly being integrated
with, and sometimes replaced by, other methods.