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s
Tactics for
Prevention and Management
THE FREQUENCY OF RESISTANCE in a pest population is in large part a
result of selection pressure from pesticide use. Strategies to manage
resistance aim to reduce this pressure to the minimum, using tactics
designed to increase the useful life of a pesticide and to decrease the interval
of time required for a pest to become susceptible to a given pesticide again
(Chapter 31. Strategy is used here in the sense of an overall plan or methods
exercised to combat pests, whereas tactic is used to mean a more detailed,
specific device for accomplishing an end within an overall strategy. This
chapter will focus on promising strategies and tactics.
Judicious use of pesticides reduces the selection pressure on pest pop-
ulations for developing resistance. Use of pesticides only as needed not
only avoids or delays resistance but tends to protect nontarget beneficial
species. These practices are an essential part of Integrated Pest Manage-
ment (IPM), which implies the optimum long-term use of all pest-control
resources available. Excessive use or abuse of pesticides for short-term
gains (e.g., minor yield increase) may be the worst possible practice long-
term because it may lead to the permanent loss of valuable, efficient, and
often irreplaceable pesticides. Such practices represent a serious issue
affecting all segments of society. Catastrophic events, such as the failure
of an entire pesticide class against a target species, have in the past, and
may again in the future, force dramatic changes in our crop production
and pest-control practices.
Genetic, biological, ecological, and operational factors influence devel-
opment of resistance. Operational factors, including pesticide chemicals and
how they are used, obviously can be controlled (Georghiou and Taylor, 1977;
313
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314
TACTICS FOR PREVENTION AND MANAGEMENT
Georghiou, this volume). The biological factors are considered beyond our
control, but current studies in biotechnology and behavior have shown that
components of genetic, reproductive, behavioral, and ecological factors may
be manipulated and have potential for use in management (Leeper, this
volume).
While the basic principles of resistance management apply to all major
classes of pests (insects, pathogens, rodents, and weeds), there are some
important differences among these classes that influence the applicability
of management strategies and tactics. Tactics are site and species specific.
For example, many insects and plant pathogens have considerable mo-
bility, whereas rodents and weeds generally have less. The usefulness of
maintaining refuges can vary substantially among pest classes. Weed seeds,
egg sacs of some nematodes, and the resting structures of some plant
pathogenic fungi may remain dormant in soils for many years, thus pre-
serving susceptible germ plasm. This does not occur for other classes of
pests. Rates of reproduction, population pressure, and movement of sus-
ceptible individuals from refuges into a treated area are often very high
with plant pathogens, moderate to high with insects, and comparatively
low for weeds and rodents (Greaves, this volume). The residual nature or
persistence of pesticides varies greatly, which will affect the success of
various tactics to manage resistance. Generally, the greater the persistence,
the greater the probability of resistance. The number of target species
being controlled with a given pesticide varies with the class of pest.
Biological control agents are critical for many insect pests but have not
yet become as important in control of pests in other classes. Other dif-
ferences exist, but their strategic significance is poorly understood.
Some of the most important issues that impinge on the development and
selection of management tactics are: differences among classes of pests and
pesticides; dynamics of resistance (differences between high- and low-risk
pesticides, and variations in the rate of resistance development within species
and geographic areas); complexes of pests on crops or locations requiring
multiple pesticides for control; and lack of supporting data and validation in
the field. Pesticides considered to be at high risk for resistance generally
have a single site of toxic action and, in fungicides, are usually systemic,
while low-risk compounds have multiple sites of action. Our current insec-
ticides and most of our new systemic fungicides tend to have single sites
and would, therefore, fall within the high-risk category. On the other hand,
few plants have evolved resistance to herbicides, which also tend to have
single sites of action. Although experience with inorganic insecticides (i.e.,
lead arsenate) shows that resistance can also develop to multisite compounds,
such resistance is rare.
The rate at which pesticide resistance develops is extremely variable among
species as well as among different field populations of the same species.
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315
Rate of reproduction, pest movement, relative fitness of resistant members
of a population, mechanisms of resistance, etc., all contribute to the dy-
namics of resistance and determine the severity of its effect on economic
efficacy and the viability of continued use of a given compound. Therefore,
the applicability of specific management tactics must be established on the
basis of specific cases and locations.
Although resistance poses a most serious threat to a pesticide's economic
life and has resulted in total loss of previously valuable chemicals from some
major pest-control programs, no pesticide has been lost from the marketplace
solely because of resistance. Resistance is not absolute throughout a pest's
range, and susceptible populations of some pests continue to exist. Further-
more, in an area where resistance has occurred, a pesticide's continued use
may be required to control other pests that are still susceptible. This may
confound management attempts, but documented cases of resistance do not
necessarily warrant removal of a pesticide.
On the other hand, industry has a responsibility to adjust marketing plans
(and perhaps propose label changes) to reflect a product's efficacy or inef-
ficacy, leaving the marketplace to determine its actual value and life. In
addition, public-sector research, extension, and regulatory programs have a
key role to play in ensuring that growers are completely informed of resistance
situations that are identified, so that rational decisions can be made among
pest-control alternatives.
Several major deficiencies in scientific understanding currently frustrate
efforts to develop and implement tactics to manage resistance. Resistant
strains of pests selected in the laboratory may differ from field strains in
some ways, including fitness and number of alleles conferring resistance.
Therefore, tactics should be validated for a wide range of pests under field
as well as laboratory conditions. Monitoring technologies must be developed
to evaluate the strategies, validate the tactics, accurately determine critical
resistance frequencies for pests under different conditions, and guide the
implementation of optimum tactics (Chapter 41.
TACTICS FOR RESISTANCE MANAGEMENT
Several concepts discussed below have been proposed as tactics for man-
aging specific cases of resistance. Most of these tactics have been used, often
inadvertently or without confirming data, in pest-control practices. Owing
to lack of rigorous field and laboratory evaluations, our inability to establish
and detect critical frequencies of resistance, and the limitations of space, no
attempt is made here to detail the strengths and weaknesses of the tactics.
Sweeping generalizations about the applicability or feasibility of specific
tactics are not justified. These caveats must be kept in mind in interpreting
the data presented in Table 1. The ratings are usually only valid within the
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TACTICS FOR PREVENTION AND MANAGEMENT
TABLE 1 Tactics for Management of Resistance to Pesticides and Their
Suitability for Classes of Pests
Tactics
Insecticides Fungicides Herbicides Rodenticides
+ + (T) + + (T) + + (T) + + (T)
+ + (T)
Variation in dose or rate
Frequency of applications
Local rather than
areawide applications
Treatments only to
economic threshold
Less persistent pesticides
Life stages of pest
Pesticide mixtures
Alternations, rotations, or
sequences of pesticide
applications
Pesticide formulation
technology
Synergists
Exploiting unstable
resistance
Pesticide selectivity
New toxophores with
alternate sites of action
Protection and use of
natural enemies with
pesticide tolerance
Reintroduction of
susceptible pests
Code for Suitability Ratings:
+++ +
+++ +
+ +
+ + (T)
+ + (T)
+ + +
+ (T)
++ +
+ +
+ + (T)
+ + (T)
+ + + (T)
+ + +
+ +(T) + +(T) + +
+ + (T) + (T)
+++ +++
+
o
O O
o
o
o
+ + +
o
+ + + (T)
+ +
+++ +++
+ (T) + + +
+ (T) o
+
O(-) O(-)
+++ +++
++ + O O
+ (T) o
O O(-)
+ + + Very useful, generally supported by laboratory data and/or field experience.
+ + Moderately useful.
+ Minor use, in exceptional cases only, or supported by few data.
O Not applicable or assumed to be of no value.
(T) Supported by theoretical assumptions only. No data or experience.
( - ) May actually be detrimental to managing resistant populations.
The suitability ratings presented in this table are very tenuous, may be theoretical or supported only
by a few examples, and should not be assumed to be generally valid for each pesticide, pest, or
tactic within each class.
limitations of a few examples, often weakly supported, for each tactic within
each pest group.
Variation in Dose or Rate
With this tactic, resistance may be delayed or minimized by preserving a
sufficient population of susceptible individuals or alleles by using low rates
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317
of a given pesticide so as not to select against heterozygotes where resistance
is recessive. On the other hand, the use of high doses has also been proposed,
but as a means of eliminating or reducing the frequency of heterozygotes
where resistance is dominant. While laboratory studies have supported the
latter approach with insecticides, there is limited evidence to confirm its
success under field conditions with the possible exception of some pests of
stored grain. Using fungicides at dosage rates giving less than 100 percent
control may minimize the threat of resistance, if low levels of disease can
be tolerated or if a high level of resistance may occur (e.g., benomyl or
metalaxyl). If resistance is linked to decreased fitness, however, or if low
levels of resistance are likely to occur (e.g., dicarboximides), high dose rates
might be recommended to control all individuals in the populations. Also,
because of the explosive reproductive capacity of some pathogens or the high
premium paid for a totally disease- and insect-free crop (e.g., apple scab
and coaling moth), some disease situations require virtually total control.
There is no proof that herbicide-use rate has any effect on the development
of resistance in weeds, although circumstantial evidence indicates that high
rates may favor resistance. Because of the short generation time of rodents,
any treatment that leaves significant numbers of survivors fosters selection
for resistance. Both low concentrations in baits or inadequate applications
fit this category. Unfortunately, specific field data are lacking.
Frequency of Application
Fewer or less frequent applications, which reduce the selection pressure
over time, should reduce the rate and probability of resistance development.
This tactic is assumed to be valid for management of resistance to insecticides,
but it is unconfirmed. Circumstantial-evidence indicates that in areas where
a fungicide is used only once or twice a season, the threat of resistance
development is reduced compared to full season programs. For example, in
northern Europe, resistance quickly developed when metalaxyl was used full
season to control late blight. Based on limited experience, it may be possible
to continue cautious use of such fungicides even after resistance has devel-
oped. A specific herbicide is most commonly used only once per crop season.
Postemergent herbicides or those having brief soil activity could be applied
several times, especially in perennial crops, but this would tend to increase
selection pressure for resistance. Paraquat-resistant weeds have occurred in
a few areas following frequent applications of this herbicide. If applications
of rodenticides are made monthly (as by a Pest Control Operator EPCOl),
the selection pressure would be persistent and could speed selection. Treat-
ments once or twice a year (as with urban rat control programs) would be
nearly as efficient in selecting for resistance, however, because removing
susceptible individuals from each generation as it reaches reproductive age
speeds selection.
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TACTICS FOR PREVENTION AND MANAGEMENT
Local Rather Than Areawide Applications
Control of a pest with a particular pesticide in a single field or site, rather
than over a large area, can leave refuges in surrounding areas to thwart
resistance development; this is believed to be a useful tactic, especially with
insecticides. Susceptible individuals move into previously treated areas, thus
diluting the frequency of resistance. The success of this tactic may vary with
insect species, refuges, and other factors. In some cases, an areawide ap-
plication of the right insecticide can severely reduce a particular generation
of specific insects when properly timed, thereby reducing or eliminating the
need for further applications. Plants, even weeds with seeds that are easily
spread, are not sufficiently mobile to allow this tactic to be very successful
with herbicides; seeds probably serve more often as a way of introducing
alleles conferring resistance than of moving in large enough numbers of
susceptible alleles to swamp those conferring resistance. Some plant pa-
thologists feel that this tactic is not appropriate for airborne pathogens with
potential for resistance under high population pressure. For example, resis-
tance has often occurred when metalaxyl was used to stop heavy infestations
of late blight (potatoes) or blue mold (tobacco). When metalaxyl has been
used over a wide area as a preventive treatment before the disease started,
however, resistance has not developed in these pathogens, at least in North
America. On the other hand, some experts suggest that we should "confuse"
the pathogen by localized use of two or more fungicides having different
mechanisms of action, together with multiple cultivars that have a number
of alleles conferring resistance to the pathogen (although the latter tactic
assumes a single fungicide is used in the area). To the extent that resistant
rodents are considered less fit competitors (the British view), localized control
would result in islands of resistance that would not readily spread. Areawide
control, however, is likely to result in areawide resistance (as in Denmark)
(Greaves, this volume).
Treatments Based on Economic Threshold
This tactic delays pesticide applications until the economic threshold is
reached and may allow a certain level of crop damage to occur. This is a
means of reducing the selection pressure for resistance. The success of this
tactic in managing insecticide resistance varies with the insect pest and con-
ditions. The establishment of valid economic thresholds and the use of pes-
ticides only when the threshold is exceeded is a major principle of IPM. The
economic threshold often varies because it depends on commodity prices.
The benefit of this tactic in managing resistance to fungicides is generally
unconfirmed. It may be applicable with less virulent or localized plant path-
ogens, when total disease control is not necessary, or when the disease occurs
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319
only occasionally. It is probably not useful for the more virulent and systemic
diseases. Under current management practices, this tactic is only of marginal
benefit in limiting resistance in weeds and rodents. Introduction of the newer
postemergence herbicides, however, provides the potential to exploit this
tactic to control weeds, based on the number of weeds that compete for
resources with the cultivated plant.
Use of Less Persistent Pesticides
The selection and use of pesticides or formulations having a lower bio-
logical persistence can be a useful tactic for managing resistance. Insecticides
with short residual lives tend to slow the development of resistance due to
reduced exposure, but success may depend on the nature of the insect and
insecticide. Persistence of a fungicide will always prolong the period of
selection pressure and thus favor the build-up of resistance. It is important
to point out that a less persistent fungicide applied more frequently will have
the same effect on resistance (e.g., a 14-day treatment schedule of one
fungicide versus a 7-day schedule of another with half the persistence).
Relatively long persistence and excellent control of most weeds are believed
to be mainly responsible for the numerous occurrences of triazine-resistant
weeds. This tactic is not considered to be applicable to rodenticides.
Life Stages of Pest
This tactic is based on using a pesticide against the life stage of the target
pest that is not so likely to develop resistance. For example, in some lepi-
dopterous species the adults and/or very early larval stages (instars) are
apparently less able to metabolize insecticides than are late instars. In theory,
the rate of developing resistance would be lessened by targeting insecticides
against the adults or early instars, thereby reducing the selection pressure on
later instars that have a higher resistance risk due to their greater enzymatic
activity for pesticide metabolism. Applying a fungicide during the sexual
stage theoretically should increase the chance of selecting for a higher level
of resistance in the fungus. On the other hand, when fungicides have been
applied during the asexual stages (e.g., late blight and apple scab), resistance
has developed very rapidly. This tactic is not applicable to herbicides or
rodenticides.
Mixtures
Simultaneous use of two or more pesticides having differing mechanisms
of action or target sites (Chapter 2) has been and will continue to be a very
important tactic to avoid and manage resistance. Certain limitations and
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TACTICS FOR PREVENTION AND MANAGEMENT
conditions must apply for this tactic to be successful in managing resistance
in insects and other pests. The use of mixtures must start early before resis-
tance occurs to one of the components (unless negatively correlated toxicity
or enhanced susceptibility is present), each component must have similar
decay rates (preferably short stability), and they must have different modes
and sites of action or different resistance mechanisms (with fungicides, sim-
ilar translocation). Nevertheless, resistance to two or more different insec-
ticides can develop by the same process as with a single pesticide it just
takes longer. Mixing chemicals sometimes leads to potentiation, rather than
merely additive effects, thus delaying or preventing resistance even further.
In case of established resistance, potentiation may become the only means
of controlling the pest (V. Dittrich, Ciba-Geigy Corporation, Baste, personal
communication). Mixtures are assumed to be an important tactic in avoiding
or delaying the development of resistance to single-target-site fungicides by
plant pathogens. Limited laboratory data show that mixed populations of
resistant and susceptible Phytophthora infestans shifted to the resistant pop-
ulations more slowly when mixtures were used. On the other hand, some
reports indicated that resistance to a specific site-inhibitor fungicide can
continue to increase when one is used in combination with a multisite fun-
gicide, due in part to the pathogen population's not being controlled by the
multisite inhibitor (e.g., lack of translocation). The use of mixtures has been
a major tactic in preventing both the development and spread of weed re-
sistance to herbicides. Resistant weeds have not usually occurred where
herbicide mixtures are used, but triazine-resistant weeds have often developed
after 5 to 10 years where this class of herbicide has been applied alone and
frequently. Once resistant weeds have developed in an area, they usually
take over completely if the single-problem herbicide is used exclusively. The
use of mixtures has not been a usual tactic for rodenticides. To mix an acute
poison with an anticoagulant is illogical. Mixing of warfarin and vitamin D
(calciferol) in England seems not to have enhanced efficacy significantly.
Alternations, Rotations, or Sequences of Pesticide Applications
The use of pesticides of differing classes or modes and sites of action in
rotation, alternation, or sequence to control the same pests has been much
studied and accepted to avoid resistance. It assumes that the number of
generations or length of time between uses of any one material is sufficient
to allow resistance to decline below a critical frequency (Georghiou, 1980;
Georghiou et al., 19831. Whether this tactic is superior to pesticide mixtures
and the optimum sequence, frequency, and rate of each component will likely
vary according to the pest, pesticide, and other factors. It is based on the
relative instability of particular resistance mechanisms and is especially viable
when it is known that cross-resistance does not occur. Annual rotation or
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321
alternation is probably not a good strategy for many high-risk fungicides
because resistance can develop within one growing season, but sequences
of applications of different fungicides is often quite useful. Rotation of lower-
risk compounds (e.g., ergosterol biosynthesis inhibitors) may be an accept-
able way to prolong the life of a fungicide. As with some of the other tactics,
voluntary compliance or enforceability often prevents the general use or
success of this tactic in management of resistance to fungicides. Although
no direct evidence documents the effectiveness of annual rotations for man-
agement of resistance to herbicides, abundant circumstantial data support the
use of annual rotations or alternations of herbicides. This has not been used
as a resistance-avoiding tactic, but has been used inadvertently due to the
very common practice of rotating crops, mainly for other reasons, which
usually requires different herbicides to avoid crop phytotoxicity and to max-
imize control of the different weeds. Variable sequences of different her-
bicides during a crop season are often used to control or manage resistant
weeds once they have developed. Mixing or alternating anticoagulants is
ineffective because of cross-resistance in rodents. However, the use of an
acute rodenticide alternately (or periodically) in a control program with an-
ticoagulants is thought to be the best way to prevent resistance from being
selected (Greaves, Jackson, this volume).
Pesticide Formulation Technology
Although additional research is needed to substantiate this tactic, formu-
lation technology can be used in several ways to combat pesticide resistance.
It can reduce the dose or rate of pesticide applications. Synergists, adjuvants,
penetrants, and materials that improve bait attractancy can be incorporated
into pesticide formulations. If resistance is due to differential penetration of
an insecticide, the adjuvants or penetrants used in the formulation could be
useful to delay or reduce resistance. Changing the attractant in an insect bait
could modify the effectiveness or potential resistance to a less effective
attractant. Controlled release or longer residual type formulations might en-
hance the rate of resistance development due to longer selection pressure,
but this has not been sufficiently tested and would depend on other factors,
such as the life span of the target pest species and the effect of low levels
of the insecticide on insect reproduction. No data are available, but the same
factors would likely apply to fungicides and herbicides, except for bait at-
tractants. Poorly formulated rodenticide baits could enhance the selection for
anticoagulant resistance because these compounds require multiple feedings
to be effective. Baits with low palatability will be insufficiently consumed,
thus leaving significant numbers of survivors and fostering the selection for
resistance. Other factors discussed above also would likely apply (Jackson,
this volume).
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Synergists
The use of pesticide synergists as a tactic for resistance management has
been of special interest, but further study is required to evaluate the practical
use of this tactic. It is generally based on the use of a second chemical that
counteracts or inhibits the mechanism responsible for resistance to the pes-
ticide. Insecticide synergists inhibit specific detoxification enzymes and thus
can reduce or eliminate the selective advantage of individuals possessing
such enzymes. Synergists as inhibitors of oxidases (e.g., piperonyl butoxide),
dehydrochlorinase (e.g., chlorfenethol), esterase (e.g., DEF), and other more
recent enzyme inhibitors have found some use in field applications. Their
utility to inhibit the evolution of resistance would depend on the absence of
an efficient, alternative mechanism of resistance in the target population.
The relatively high cost of the synergist, formulation problems, the potential
synergism of mammalian toxicity, and the high level of biochemical adap-
tation in some major insects (e.g., house fly), have militated against their
use. Increased rate of metabolism by the target pathogen is not a common
mechanism of fungicide resistance, but it does occur in a few cases. Fur-
thermore, it has been shown that synergism may counteract development of
resistance (e.g., a fungicide that inhibits respiration has increased the uptake
of fenarimol by a fenarimol-resistant strain, thereby making the resistant
strain again sensitive). The use of synergists may not be applicable with
herbicides. Some synergistic interactions between herbicides (e.g., atrazine
and tridiphane) have been reported due to reduced metabolism of atrazine
triggered by an enzyme inhibition from tridiphane. Herbicide resistance,
however, has not been due to enhanced metabolism of the herbicide by the
resistant weeds. A combination of antibiotic (to reduce production of vitamin
K by gut bacteria) with anticoagulant (Prolin@) appeared to give no field
advantage to the formulation and would not be expected to impact on resis-
tance development with rodenticides. Other synergist-type compounds have
not been suggested.
Exploiting Unstable Resistance
Pesticide resistance often carries with it, especially during its original
development, some deficiencies in fitness, vigor, behavior, or reproductive
potential. These characteristics often make the resistant biotype of the target
pest more susceptible to other control measures. Unstable resistance can be
exploited by using other insecticides or control programs to control resistant
insects preferentially or selectively until resistance diminishes. Resistant plant
pathogens may be unstable at time of initial mutation or development and
should be more easily controlled then. It is important to determine if resistance
is stable, fit, and genetically based. By use of fungicides in which resistance
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323
is associated with a lack of fitness, the resistant mutants would not survive
when the selection pressure is removed. Weed biotypes resistant to herbicides
are usually less fit or competitive than the susceptible population and may
be more easily controlled with alternate herbicides. In England resistant rats
reportedly have higher vitamin K demand than normal rats and thus do not
survive well (Greaves, this volume), although resistance in monitored English
populations seems to have reached an equilibrium point rather than decreasing
toward extinction.
Pesticide Selectivity
Selective insecticides often eliminate the pest species while preserving or
causing less injury to the predators and beneficial insects. This is an IPM
approach and will help to delay or prevent resistance development by pro-
viding additional mortality factors for resistant pests. A selective pesticide
is often a specific single-target-site chemical with a higher resistance risk,
but this danger might be alleviated somewhat by using less specific pesticides
applied more selectively, for example in baits, as systemic insecticides in
furrow, or as seed treatments. This approach is the most useful in management
of resistance in insects and mites. The use of compounds with multisite action
has not been a tactic to manage resistance in weeds or rodents.
New Toxophores with Alternate Sites of Action
The discovery and development of new pesticides has often been viewed
as a major approach to management of resistance to earlier pesticides. Re-
placing older pesticides with new ones because of pest resistance has never
been the primary objective of this predominately industrial activity, however.
It is obvious that future priorities in pesticide development should give more
attention to new or alternate target sites that will have lower risk of resistance
development. While we need to encourage new discoveries, we must do
everything possible to preserve all of our present pesticides. This strategy is
a vital and relatively long-term solution to the control of pests resistant to
current pesticides, but it can never be a permanent solution. Pests are likely
to evolve various means to survive any new pesticides and other control
measures. It is also becoming more difficult and expensive to make new and
novel chemical discoveries. We are fortunate to have available many types
of herbicides with different modes of action, but we can still benefit from
breakthroughs in new chemistry to control resistant or problem weeds in
certain crops. Development of new types of rodenticides has contributed to
resistance management in recent years. New materials include bromethalin,
vitamin D3, and alphachlorhydrin.
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Protection and Use of Natural Enemies with Pesticide Tolerance
The intentional protection of natural enemies of pests or the introduction
of such predators, especially those with natural or induced tolerance to the
specific pesticide, has become a tactic of much interest in resistance man-
agement. The development and release of predators with some level of re-
sistance has shown promising results in managing insecticide resistance. Such
predator resistance is usually intentionally developed by laboratory exposure
during several generations. Genetic engineering offers even more potential
in this area. It should be pointed out that the use of several of the previously
described tactics will tend to interfere with or counteract this tactic. The use
of resistant beneficials has not been used to manage fungicide resistance
successfully. Laboratory data indicate that it could be a useful tactic where
populations of soil antagonist strains of microorganisms (e.g., Trichoderma
and Gliocladium) are used in an IPM approach. The use of resistant bene-
ficials is not applicable to herbicides and is often not compatible with ro-
denticide use. With most rodents, their predators are slow breeding, are
unable to match the rapid build-up of mouse and rat numbers, and are
ineffective in structured urban environments.
Reintroduction of Susceptible Pests
Increasing or encouraging the immigration of susceptible pest genotypes
can be effective in dealing with a small insect population with a high pro-
portion of resistant individuals. This tactic shifts the population away from
a critical frequency of resistance. The reintroduced susceptibles must be
numerous enough to swamp the endemic, resistant population, thereby re-
ducing the likelihood of mating between resistant individuals (Suckling,
1984~. This tactic is often most applicable where pest control is not intensive.
It is not likely to be an appropriate tactic for managing resistant fungi, weeds,
or rodents.
RECOMMENDATIONS
1. Efforts should be expanded to develop IPM systems and steps taken
to encourage their use as an essential feature of all programs to manage
resistance.
2. Increased research and development emphasis should be directed to-
ward laboratory and field evaluation of strategies and tactics for preventing
or slowing resistance development, including efforts to:
a. Develop models, to be tested in laboratory and field experiments,
to assist in formulating hypotheses on managing resistance.
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b. Develop and validate sampling and bioassay techniques for mon-
itoring low levels of resistance.
c. Identify chemicals with negatively correlated cross-resistance and
develop rotations or mixtures based on this information.
d. Determine stability of resistance in pest populations to specific
pesticides.
e. Evaluate alternations and rotations of pesticides shown by research
findings or field experience to have high potential as tactics to
manage resistance. Design rotation schedules that will maintain
acceptable levels of susceptibility.
f. Investigate inlaboratory end geld basic genetic, toxicological,
and ecological factors that influence the rate of resistance devel-
opment.
g. Use traditional and biotechnological genetic methods to produce
pesticide-resistant biological control agents and herbicide-resistant
crop plants.
h. Investigate pest migration and the factors that influence it to de-
termine the potential for assessing the spread of resistant forms to
new areas and the reinvasion of resistant populations by susceptible
pests from refuges.
3. Population biologists, toxicologists, and modelers should be involved
in designing and executing research and validation efforts.
4. The private sector, extension personnel, and regulatory agencies should
encourage the use of promising tactics to manage resistance, while attempting
to confirm or validate their usefulness (Davies, 1984~.
5. As part of overall IPM strategy to manage resistance, increase efforts
to understand and use components of those genetic, reproductive, behavioral,
and ecological factors that may minimize the need for pesticide use and
reduce resistance development.
6. The traditional method for dealing with resistance has been to switch
to a new pesticide. This does not address the problem of resistance, but at
best simply delays its recognition and may exacerbate it through cross-re-
sistance. For these reasons and because further discovery and development
of new and better pesticides is uncertain, greater efforts must be made to
conserve existing materials as finite resources.
7. Do not depend totally or too much on any one pesticide or means to
control any pest, especially with high-risk pesticides against major pests.
8. When resistance occurs, move promptly to take necessary actions and
apply the best tactics to manage the resistance with all tools and technology
we have available.
9. Encourage the use of crop rotations so that different herbicides will
be used in successive seasons on different crops.
10. Industry should continue to search for and develop new toxophores
OCR for page 326
326
TACTICS FOR PREVENTION AND MANAGEMENT
and, in some cases, new synergists, with emphasis on new mechanisms or
approaches (e.g., behavioral-type insecticides, multisite fungicides, etc.),
rather than to kill the pest by direct, immediate, and single-site action.
REFERENCES
Davies, R. A. H. 1984. Insecticide resistance: An industry viewpoint. Pp. 593-600 in 1984 Proc.
Br. Crop Prot. Conf. Pests and Dis.
Georghiou, G. P. 1980. Insecticide resistance and prospects for its management. Residue Rev.
76:131-145.
Georghiou, G. P., A. Lagunes, and J. D. Baker. 1983. Effect of insecticide rotations on evolution
of resistance. Pp. 183-189 in IUPAC Pesticide Chemistry, Human Welfare and the Environment,
J. Miyamoto et al., eds. Oxford: Pergamon.
Georghiou, G. P., and C. E. Taylor. 1977. Operational influences in the evolution of insecticide
resistance. J. Econ. Entomol. 70:653-658.
Suckling, D. M. 1984. Insecticide resistance in the light brown apple moth: A case for resistance
management. Pp. 248-252 in Proc. 37th N.Z. Weed and Pest Control Conf.
WORKSHOP PARTICIPANTS
Tactics for Prevention and Management
HOMER M. LEBARON (Leader), Ciba-Geigy Corporation
DANIEL ASHTON, Bowling Green State University
AHMED NAss~R BALLA, Agricultural Research Corp., The Sudan
FAUSTO C~sNERos, The International Potato Center, Peru
R. A. H. DAv~Es, ICI Plant Protection Division, Great Britain
DoNA~D E. Davis, Auburn University
JOHAN DEKKER, Agricultural University, Wageningen, The Netherlands
TIMOTHY J. DENNEHY, Cornell University
VOLKER DITTRICH, Ciba-Geigy, Ltd., Switzerland
GEORGE P. GEORGHIOU, University of California, Riverside
EDWARD H. G~Ass, New York State Agricultural Experiment Station, Cornell
University
KENNETH S. HAGEN, University of California, Albany
WAYNE HARNISH, FMC Corp.
WILLIAM B. JACKSON, Bowling Green State University
JOHN R. LEEPER, E. I. du Pont de Nemours and Company
JAMES V. PAROCHETTI, U.S. Department of Agriculture
FRED W. SEIFE, University of Illinois
HOWARD WEARING, DSIR, New Zealand
Representative terms from entire chapter:
selection pressure
