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                         TECHNICAL PAPER # 27
                      UNDERSTANDING INSECT PESTS
                           AND THEIR CONTROL
                           Harold R. Willson
                          Technical Reviewers
                           Dr. Ernest C. Bay
                            Kenneth Haines
                             R. Narasimhan
                   1600 Wilson Boulevard, Suite 500
                     Arlington, Virginia 22209 USA
                 Tel: 703/276-1800 . Fax: 703/243-1865
           Understanding Insect Pests and Their Control
                        ISBN: 0-86619-230-1
            [C]1985, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Maria Giannuzzi
as editor, Julie Berman handling typesetting and layout, and
Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Harold R. Willson, is an
extension entomologist and integrated pest management coordinator
with the department of entomology, Ohio State University.  A
former Peace Corps Volunteer in India, Dr. Willson has performed
many overseas consultancies concerned with pest management and
entomology, and is widely published in these fields.   The reviewers
of this paper are also VITA Volunteers.   Dr. Ernest C. Bay is
superintendent and professor of entomology for the Washington
State University Western Washington Research and Extension Center.
He is a specialist in the biological control of mosquitoes
and has consulted for the World Health Organization and VITA in
Nicaragua, the Far East, Africa, and Haiti.   Kenneth Haines worked
in Ghana for several years where he supervised pest, control
as one of his duties as an agricultural extension specialist.
R. Narasimhan is with the Asian Pacific American Chamber of
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
                by VITA Volunteer Harold R. Willson
Insect control is the effort made to protect crops, animals, or
other targets of insect attack from unacceptable injury or loss.
If destructive insect populations are allowed to cause significant
damage or injury to their target host, economic loss or a
decline in human health may result.
Destructive insects threaten the value of most food crops both
before and after harvest.  In addition, insect pests can destroy
structures and cause direct injury to animals including humans.
However, not all insects are destructive.   Some provide direct
economic benefit by producing products such as honey or silk;
others naturally control harmful insect pests.   And of vital
importance is the role of insects in pollinating flowering
A population of a destructive pest and its target host (a crop,
livestock, etc.) are part of a complex system that includes all
other organisms living in a given physical environment or ecosystem.
Elements of a natural system, such as a crop, are in a
state of balance with one organism affecting the other.   Problems
occur when the natural balance between destructive and beneficial
insects is upset.  Corrective action is then required to control
the problem or prevent similar pest problems in the future.
Applied control includes a whole range of practices that become
necessary when natural control factors fail to work adequately.
Important methods of applied control are:
    1.   chemical control;
    2.   biological control;
    3.   cultural control;
    4.   legal control; and
    5.   environmental control.
In discussing each method, note that only general information can
be given since much depends on the type of insect pest, its
habits, and the kind of damage it causes.   Also important are the
nature and condition of the target host to be treated, weather
conditions, application equipment, and the method of application.
For information to meet specific needs, consult an agricultural
organization in your area.
Chemical Control
The use of chemicals--generally referred to as insecticides--has
become the most common approach to dealing with insect problems.
An insecticide may be used as either a preventative or corrective
treatment.  In the case of preventative treatment, it is assumed
that the probability of an insect becoming a problem is high.
Action is taken on the assumption that preventative treatment is
more economical or effective than taking corrective action after
the fact.  However, many insect pests are best controlled after
their numbers are so high that they might become a threat to
their target.
For crops the decision as to when to take corrective action is
best based on an 'economic injury level' previously established
for the host and its insect pest.   This involves the judgement of
the extent to which a particular pest population can be allowed
to grow before an insecticide must be applied to prevent further
crop loss.  The economic injury onset is determined by monitoring
the insect pest population in relation to its natural enemies,
weather, and host condition.
The timing of insecticide application can significantly affect
the potential for crop contamination.   The best times to apply
insecticides, especially if they are sprayed, are in the early
morning or early evening hours when the air is still.   Insecticides
should never be sprayed in wind or when heavy rains--are
expected.  Sometimes, insecticides are applied early in the growing
season; but since this usually coincides with the rainy
season, the runoff merely carries the insecticides away.  If
possible, insecticides should be applied after crops have
emerged, later in the season, or even after the season to prepare
fields for the next season.
People who use an insecticide to control a pest problem must be
warned of the possible harmful effects it may have on the environment,
beneficial insects (e.g., pollinators such as bees), or
the person who applies it.  Safety precautions and directions for
its use should be followed carefully.   Failure to take safety
precautions and to handle insecticides carefully may result in
illness or death or contamination of water and food.
Continuous use of insecticides can destroy beneficial soil microrganisms
and reduce soil fertility.  Some soil microorganisms
kill insect pests.  Overuse and misuse of insecticides can interfere
with the microorganism's ability to kill such pests.   When
this happens, pest problems can actually worsen: the pest may
develop resistance to the insecticide, and since natural controls
have been wiped out, the pest populations can be virtually uncontrollable,
at least for a time.
Remember that many insecticides kill not only target species but
other harmless or beneficial organisms such as honeybees, insect
parasites, or predatory insects.   In many instances, the use of
insecticides to control one insect destroys the controlling
natural enemies of other species, allowing these to become new
If you need to use insecticides, check with local farmers or
extension agency personnel to see what are the possible effects
of using them.  Also, look into alternative control measures that
may meet your needs without harmful effects.
Biological Control
Biological control involves using an insect's own natural processes
or enemies to control it.  Some of these controls are the
result of modern scientific research.
Very few biological controls are as readily available and useful
for controlling insect pests as insecticides.   These are almost
exclusively disease organisms, particularly Bacillus thuringieusis,
which is effective against many Lepidopterous larvae and
Bacillus thuringieusis Israelensis, which is useful for some
mosquito species.  Also, milky spore disease has long been marketed
for Japanese beetle control in the United States.
A few insect predators and parasites, including lacewing larvae
(Chrysopa), ladybird beetles (Coccinellidae) and Trichogramma
species are sometimes marketed but their successful use requires
a professional understanding of the elements in the ecosystem
where they are used.  More often these species are marketed more
for their ease of production then for their effectiveness.  A
notable exception is the mosquito fish Gambusia affinis, which
preys on some pool dwelling mosquito larvae.   Many sites, however,
are not suitable for mosquito control by Gambusia.   Also, care
must be taken that the fish does not become permanently established
where it will eliminate other desired fish by competition.
The introduction of natural enemies to control imported insect
pests has proven to be the most effective biological control.
This usually requires the resources of government agencies, dedicated
research, and years of work.   Where it is successful as
with the classic case of the Vedalia beetle in the control of
cottony cushion scale of citrus, it requires no further application
of the natural enemy.  It simply requires the observance of
good agronomic and ecological practices to ensure the mutual
survival of prey and predator at acceptable pest levels.
Other types of biological control include release of sterilized
males, sterile hybrids, pheromones for attracting or confusing
populations, and other innovative techniques.   These have been
used with varying success.  These practices usually require institutional
The use of chemical repellents is another method of controlling
insect pests.  For example, chemicals that function as repellents
to household pests such as cockroaches can be used alone or in
conjunction with an integrated control program to prevent the
accumulation and reinfestation of cockroaches.   In cases where
immediate relief from, say, biting mosquitoes is needed but other
control measures are impractical, repellents placed directly on
the skin or sprayed on a piece of clothing can be very effective.
Repellent-treated mesh jackets are effective for longer periods.
In some situations, the term biological control also refers to
the presence of native beneficial insects that are natural predators
of problem pests.  It is when natural control fails that
other controls are necessary.   This is the basis of pest management
and integrated control.  It is important to be aware of the
natural enemies of insect pests and to use those insecticides
that are least destructive to them.   As mentioned earlier, the
destruction or disturbance of natural control by some insecticides
can cause other insects previously under natural control to
assume pest status.
Manipulation of beneficial insects to control problem pests generally
requires extensive study and a long-term effort.   However,
awareness of naturally present beneficial insects and the use of
chemicals least destructive to these insects may prevent new pest
Biological control methods have worked well in some small-scale
applications but may or may not work in other situations.  They
should be considered as alternatives that may be used alone or in
combination with other pest control practices.
Cultural Control
Many insect pest problems can be prevented by adopting crop
culture practices that adversely affect the development of certain
pests.  For example, rotation of certain crops can prevent
development of a given pest population that requires the presence
of the host crop over more than one growing season.   Effective
weed control often reduces the probability of some pests infesting
a crop.  The use of cultural insect controls may or may not
provide an economical alternative to chemical control depending
on the situation.
Cultural controls can also be used to alleviate forest pest
problems.  Forest culture practices include removing high-risk
trees, promptly treating pest-infested trees, disposing of logging
residues, and promptly harvesting damaged trees.
In nurseries, cultural control practices include carefully controlling
irrigation, improving soil fertility, and regulating
seedling density.  Proper timing between lifting, fallowing, and
planting can also be useful in reducing pest populations in
nursery beds.
Legal Control
Regulatory action by a governmental agency may be advisable where
insect pests pose a threat to society.   Examples of legal control
include isolating a pest-infested area to prevent insects from
spreading to other areas or requiring farmers to adopt cultural
controls to reduce the impact of a given pest over an area.
Environmental Control
Environmental control involves changing the environment in such a
way as to destroy insect life.   Three environmental control
methods are discussed below.
Physical Control.  Examples of some physical means of excluding
insect pests include properly designed machinery or equipment,
constructing airtight doors, screening windows, controlling and
filtering air, segregating commodities (e.g., grain products)
subject to high pest infestation, rotating commodities in storage
facilities, and developing insect-resistant packaging.
Temperature Control.  Temperatures below 5[degrees]C (40[degrees]F) prevent insect
activity and temperatures much below 0[degree]C (32[degrees]F) for an extended
period usually kill insects.   Also, temperatures above 38[degrees]C
(100[degrees]F) for long periods or 60[degrees]C (140[degrees]F) for short periods are
Sanitation Control.  Sanitation involves good housekeeping practices.
Although sanitation by itself does not usually prevent
insect infestation, it often enhances the effect of insecticides
should these be needed.
The regular removal of dirt, dust, and grease from household
objects helps to prevent infestation.   Rotating heavy pieces of
furniture is also important because household pests usually feed
in areas where cleaning is difficult rather than in the open
where thorough cleaning, light, and the movement of people make
infestation difficult.
Cereals and foods high in protein are attractive to household
pests.  Take care in kitchen cabinets and other storage areas not
to let these materials accumulate in cracks and crevices.  Routinely
removing other food sources such as garbage also prevents
pests from breeding.
Integrated pest management (IPM) is a comprehensive approach that
uses available control methods in an ecologically and economically
balanced program.  Its objective is to optimize pest control
in terms of overall economic, social, and environmental values.
By using a combination of the insect pest control practices
described thus far, an effective IPM program can reduce insecticide
use and thus prevent the damage to the environment caused by
the continuous use of insecticides.   Also, it can provide alternate
controls should any one method fail.
To ensure that an IPM program is economical, an IPM specialist
may recommend the use of low-cost insect control methods for a
field that has a low cash value per acre.   Thus, an IPM program
aims to incorporate cost-effective control practices into a practical
pest management system.
IPM specialists are also aware of the important role beneficial
parasites and predators play in reducing insect pest populations.
In many vegetable production areas, trained IPM specialists try
to reduce insecticide treatments by making full use of beneficial
insects.  They recommend insecticide applications for insect pests
only when needed, thus hoping for the least harm to beneficial
IPM specialists are also trying to reduce the use of hazardous
chemicals and to manage target hosts in a manner less likely to
contribute to pest problems.   In forestry, for example, the
amount of direct chemical control of forest pests has been declining
in recent years.  Biological control agents are often
used to replace conventional chemicals.
Before undertaking an integrated pest management program, one
needs to (1) learn as much as possible about the life cycle 'and
environmental requirements of the insect pest; (2) find out
where or when the pest is most susceptible to control; and
(3) determine the least ecologically harmful way or combination
of ways to control it.
Design of a pest management program for a given pest or combination
of pests, whether affecting a crop, livestock, or other
aspect of human welfare, depends on the availability of adequate
information.  First, the pest must be identified.  Accurate identification
of a pest enables review of the pest's biology, population
activity, nature of damage to host, etc.   Following identification,
a survey of the site should be conducted to assess the
degree of the problem.  Such a survey will provide an idea of the
relative importance of the pest in regard to a particular situation.
Selection of appropriate pest control measures will depend
on the type of insect and available control methods.
The easiest and most common method for controlling destructive
insects is chemical control.   Before a decision is made to use an
insecticide, one should consider: (1) the economic cost/benefit
of the action, and (2) the environmental implications of the
action, including the safety of the applicator.
Specific questions that need to be addressed before applying
insecticides to control crop pests include:
    o   Does the pest threat justify the investment in the
    o   At what stage of the pest's development will the pest
       population cause substantial injury to the crop?
    o   Is the pest population still present to warrant
       chemical treatment? If so, is it vulnerable to such
    o   Do any parasites, predators, or diseases exist that
       might lead to a decline in the pest population?
    o   What effect will prevailing climatic factors have on
       the pest population or the host crop?
    o   Does timing the application of an insecticide have an
       effect on the ability of the insecticide to control the
       pest population?
    o   What effect will an insecticide have on beneficial
       insects in the immediate environment?
    o   What is the distribution of the pest problem and should
       all areas be treated?
    o   Has the application been calibrated recently to deliver
       the desired treatment?
    o   If insecticide use is warranted, which insecticides can
       be obtained and applied within the remaining pre-harvest
    o   What is the effectiveness of available insecticides?
    o   Are the insecticides available in your area registered
       for use? Have you checked with local authorities to
       see if there are specific laws governing the use of
    o   Have adequate precautions been taken to protect workers
       from insecticide poisoning during transport, storage,
       and application of insecticides? Are instructions
       available in local languages?
    o   Could the insecticide suggested for use kill beneficial
       soil microorganisms or beneficial insects?
    o   Have you considered all pest management options?
    o   Is it likely that erosion will carry insecticides into
       downstream water bodies? If so, could such insecticides
       affect fisheries and domestic water use?
    o   Can a species-specific insecticide be used?
    o   Is it possible to switch insecticides to reduce the
       likelihood of target species developing resistance to
       an important insecticide?
    o   Have you contacted local universities and government
       agencies for information on local pest species and
       their control practices to be sure you have considered
       all the alternatives to insecticides?
To answer the above questions, the decision maker needs adequate
information on the status of the crop threatened by a pest
problem.  Such information includes (1) identification of the
pest problem and associated organisms; (2) knowledge of the
biology and population dynamics(*) of the pest population(s);
(*)The aggregate of processes that determine the size and composition
of any population.
(3) familiarity with the host crop's capability to withstand pest
injury; and (4) knowledge of the effectiveness of available
insecticides under prevailing environmental conditions.
The suitability of chemical controls for an insect pest depends
on the availability of insecticides registered for use on the
site in question.  The effectiveness of a given insecticide on a
new pest problem should be evaluated to determine appropriate
treatment, rates, time of application, and impact on the local
Deciding whether or not to use biological or natural controls
requires detailed knowledge of related organisms that may be
parasites or predators of the pest in question.   Use of cultural
methods to control a pest requires a thorough understanding of
the pest under various cultural practices.   Implementation of
biological or cultural control methods often depends on years of
research and evaluation.  If effective chemical methods are
available to control an insect pest, efforts to develop a program
that integrates biological and cultural controls with chemical
controls should be made to minimize dependence on any single
control method.
Once an insect control program has been established, the program's
effectiveness must be monitored regularly, especially if
the program depends heavily on the use of insecticides.   Effective
biological or cultural control methods, once established,
tend to be long lasting.  In contrast, dependence on insecticides
often requires continued re-evaluation and development of new
compounds to maintain adequate control of the pest.   This is due
to the ability of insect populations to develop resistance to
chemicals over time.  The development of resistance by an insect
population to insecticides is most likely in situations where a
single insecticide is used extensively.   Thus, any effort to
minimize excessive use of chemical controls and incorporate biological
or cultural controls will enable more effective use of
chemical controls.
Methods of insect control differ with each combination of pests
and affected site.  Insects pass through various stages and the
stage targeted for treatment may or may not be the damaging stage
in the life cycle of the pest.   The environment of the host site
also has a significant effect on the control method adopted.
Insect pests inhabit a diversity of environments ranging from
aquatic to soil ecosystems, and each environment presents a
different set of ecological factors for consideration.
Information on insect control is available from a number of
agricultural institutions throughout the world having entomological
expertise.  Agricultural chemical industries often provide
extensive information on important pests.   Documentation exists
on over a million insect species around the world, of which only
a few thousand are considered destructive insects of economic
importance.  However, the most important step in dealing with an
insect problem is the collection of accurate observations in the
field which form the basis for future decisions.
Development or selection of a pest management program for a given
pest problem requires the services of personnel with training and
experience in the field of applied entomology.   Such personnel may
be entomologists or specialists in the host commodity field with
substantial training or experience in pest control.
Where new insect control methods must be developed, the services
of research entomologists with training or research experience in
a particular specialty may be required.   Entomologists specialize
in a number of areas depending on the commodity, method of control,
or level of technology being developed.   Insect toxicologists
study the response of insects to toxic substances in the
lab.  Physiologists study various aspects of insects in regard to
their function.  Biological control specialists study the relationship
between pests and natural agents that may be implemented
for control.  Taxonomists are often necessary to accurately identify
pest species.  Entomologists who develop and implement field
methods are often referred to as economic entomologists or pest
management specialists.  Such applied field entomologists often
specialize by commodity fields (e.g., field crops, vegetables,
fruit, forestry, livestock, stored products, human health, etc.).
The institutional source for entomological expertise includes
both public and private agencies.   In the United States, expertise
in entomology is primarily based in a network of state agriculture
experiment stations operated by state universities having
agriculture colleges.  In addition, research and regulatory entomologists
operate out of state and federal departments of agriculture.
Implementation of new technology is performed by extension
entomologists associated with agricultural colleges.   Within
each state, entomology extension specialists work through the
county extention agents to educate farmers and the public concerning
new pest control technology and the safe and proper use
of pesticides.
Private industry--especially the agricultural chemical industry--represents
an important source of pest control expertise.   In
many cases, the level and extent of expertise within private
industry exceeds that available from public resources.   Most chemical
companies have personnel with specific responsibility in
either (1) research, (2) product development, (3) technical support,
or (4) sales and marketing.  The geographical areas served
by industry personnel vary with the scope of the market served.
However, all geographical areas of the world are serviced by this
private network of pest control specialists.   In general, a close
working relationship exists between government and private industry
in the development and implementation of pest control programs.
One of the most important sources of information for farmers is
the local chemical dealer, who often has more contact with local
farmers than do public development personnel or other pest
control specialists.  Provision of education programs on the safe
and proper use of pesticides for such local dealers or merchants
is important if the local user of pesticides is to receive accurate
recommendations.  Education of local chemical dealers or
merchants in the appropriate pest control technology depends on
programs available from both public and private institutions.
Public personnel should realize that the local chemical dealer is
often the primary source of pest control information and target
educational programs accordingly.   Potential customers should bear
in mind that the chemical dealer or merchant has a vested interest
in his products and thus may not be the best source of
information on controlling a pest problem.
Chemical marketing programs should stress that the proper use of
a chemical to achieve effective control depends on the level of
expertise available from local dealers.   Education of local chemical
dealers and applicators often depends on the implementation
of an educational program leading to certification in the proper
use of chemical pesticides.  Such programs often emphasize pest
identification, pest population assessment and control, and pesticide
safety.  If participation in such training programs is a
requirement for the sale, purchase, or use of toxic chemicals,
then participation in the educational program is facilitated and
improper use of pesticides may be prevented.
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