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OCR for page 298
Pesticide Resistance: Strategies and Tactics for Management.
1986. National Academy Press, Washington, D.C.
Detection and Monitonng of
Resistant Fonns: An Overview
K. J. BRENT
Detection and monitoring are major components of pesticide re-
sistance management, for several reasons. The different steps that
should be taken in any detection and monitoring program, as well
as examples of successful programs, are described. It is important
to monitor for sensitivity and to establish a resistance management
strategy early in the life of a new product. The need to distinguish
clearly between detecting less-sensitive forms and concluding that
practical resistance problems have arisen is also stressed. The most
effective programs can be developed and carried out only with the
collaboration of private and public organizations.
INTRODUCTION
What precisely is meant by "the detection and monitoring of resistance"?
This basic question must be considered at the outset of any discussion on
this topic, because much vagueness and misunderstanding exist about the
terms involved and their meanings.
"Detection" indicates simply the obtaining of initial evidence for the
presence of resistant forms in one or more field populations of the target
organism. Consideration of the degree of resistance, the proportion of resis-
tant variants in a population, or the effect on practical field performance of
the pesticide is not involved.
"Monitoring" needs more consideration. To many people it denotes a
routine, continuous, and random "watch dog" program, analogous to the
official monitoring for levels of pesticide residues in foodstuffs. Such year-
in, year-out surveillance aims to detect and then follow the spread of any
298
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DETECTION AND MONITORING OF RESISTANT FORMS
299
markedly abnormal forms should they arise, or with sufficiently sensitive
and quantitative methods, to reveal any gradual erosion of response, as has
occurred with certain plant pathogens. Campaigns of this kind can be pro-
tracted and unrewarding, although sometimes they may be justified for certain
very important pesticide uses when the risk of resistance is already known
to be considerable. More specific, shorter term investigations are also (less
aptly) referred to as monitoring. These are done either to gain initial or
"baseline" sensitivity data before the widespread commercial use of a new
pesticide or, more commonly, to examine individual cases of suspected
resistance indicated by obvious loss of field efficacy of the product. Thus,
monitoring can be used to indicate either continuous surveillance or ad hoc
testing programs; this double use is acceptable, providing the meaning of
the term is made clear in any particular context.
"Resistance" and "resistant" have many different shades of meaning.
For precision either a particular usage must be specified as the correct one
or resistance must be defined clearly whenever it is used. The first of these
options is unattractive, because new, narrow definitions of commonly used
and fairly general terms are seldom adopted universally or even remembered,
and they force us to define a whole range of other narrow terms. Hence,
"resistance," "tolerance," "insensitivity," and "adaptation" should not,
as some suggest, be given separate, precise meanings. The second option,
however, is both feasible and sensible and should be encouraged. Resistance
can be used in a general way and interchangeably with the other terms to
mean any heritable decrease in sensitivity to a chemical within a pest pop-
ulation. This can be slight, marked, or complete and may be homogeneous,
patchy, or rare within a population. It can cause complete loss of action of
an agrochemical or may have little practical significance. Thus, resistance
and similar terms must, like monitoring, be defined carefully within each
particular context.
In reports on monitoring, the absolute use of resistance (as in "the pop-
ulation was resistant") causes more problems of misinterpretation than rel-
ative use ("population A was more resistant than B"), and a quantitative
definition of how resistance was categorized and measured should always
be given. A "resistance index" or "resistance factor" (the ratio of the doses,
commonly EDso, required to act against resistant and sensitive forms, re-
spectively) is often used, but the basis of its calculation needs careful con-
sideration. The choice of sensitive reference strains (sometimes merely a
single one is used) and any shift in their response with time can affect greatly
the value of the index and inferences made, at least with regard to fungicide
resistance. If a reference strain has been kept away from all chemical treat-
ments for years in a laboratory culture, it may be abnormally sensitive.
The Fungicide Resistance Action Committee (FRAC) has recommended
that the term "laboratory resistance" should be used to indicate strains of
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DETECTION, MONITORING, AND RISK ASSESSMENT
fungi with significantly reduced sensitivity as demonstrated by laboratory
studies, whereas "field resistance" should be used to indicate a causal re-
lationship between the presence of pathogenic strains with reduced sensitivity
and a significant loss in disease control. The intention is to avoid false alarms
such as have occurred when certain authors, having found some specimens
to be more resistant than others in laboratory cultures or field samples, implied
without evidence that these variants were causing or were about to cause
problems in practical pest control. The use of the above terms as suggested
by FRAC, however, can also be misleading: resistant forms found in the
field in low numbers or with a low degree of resistance or fitness are certainly
field and not laboratory resistant, yet such forms may not be affecting practical
control. Whatever terms are selected there is no substitute for defining clearly
the implications and limits of their use in all publications.
THE AIMS OF DETECTION AND MONITORING
There are at least seven distinct motives for resistance, detection, and
monitoring, and whichever of them predominates will affect the scope and
design of the surveys that are done. The aims, which are discussed in turn
below, are as follows:
· Check for the presence and frequency of occurrence of the basic genetic
potential for resistance (expressed resistance genes) in target organism pop-
ulations.
· Gain early warning that the frequency of resistance is rising and/or that
practical resistance problems are starting to develop.
· Determine the effectiveness of management strategies introduced to
avoid or delay resistance problems.
· Diagnose whether rumored or observed fluctuations or losses in the field
efficacy of an agrochemical are associated with resistance rather than with
other factors.
· If resistance has been confirmed, determine subsequent changes in its
incidence, distribution, and severity.
· Give practical guidance on pesticide selection in local areas.
~ Gain scientific knowledge of the behavior of resistant forms in the field
relation to genetic, epidemiological, and management factors.
Potential for Resistance
To obtain an initial indication of possible sources of future loss of effec-
tiveness, we would need to be able to isolate and characterize rare mutants
at, say, 1 in 10~° frequency. This is not feasible, however, without vast
expense and effort. Resistant forms can be detected only after reaching much
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DETECTION AND MONITORING OF RESISTANT FORMS
301
higher frequencies of 1 in 100 or perhaps 1 in 1,000 units (individual disease
lesions, spores, pests, weeds), depending on the number of samples taken
and the degree of statistical significance required. For example, if 1 in 100
units is resistant, 298 samples must be examined to achieve 95 percent
probability of detection of 1 resistant unit; 2,994 samples must be checked
if the frequency is 1 in 1,000. If a particular pesticide application normally
allows 10 percent survivors (i.e., pest control is 90 percent effective), such
detectable frequencies will occur only one or two applications prior to serious
and obvious loss of practical control. With some pests and diseases this may
be too late to allow any avoidance action to be introduced in the area con-
cerned.
The relatively late first indication of the occurrence of resistance forms,
however, can still give a valuable alert for certain purposes or situations.
For example, it can indicate to other regions or countries that the potential
for resistance exists. Or there may be time to introduce or modify avoidance
strategies in cases where the rate of reproduction of target organisms is low
(one or two generations per year), where lack of fitness in resistant mutants
leads to an interrupted or fluctuating buildup (as with resistance of Botrytis
cinerea to dicarboximide fungicides), or where a range of variants with
different degrees of resistance arise and resistance tends to build up in a
stepwise manner (as in the resistance of powdery mildews to 2-amino-
pyrimidine and triazole fungicides). In such situations loss of efficacy is still
a gradual process, even after relatively high frequency levels are first de-
tected.
Shifts in Frequency or Severity of Resistance
After initial detection systematic monitoring can reveal subsequent changes
(if any) in the frequency and degree of resistance and in its geographic
distribution. For this reason repeated surveys have been done by public-
sector organizations such as the Food and Agriculture Organization of the
United Nations (FAG), the World Health Organization (WHO), and national
agricultural and health research authorities. Surveys are also increasingly
done by agrochemical companies, sometimes in cooperation with Resistance
Action Committees. Examples are considered in the later section on achieve-
ments in resistance monitoring. Shifts in resistance can be very rapid. Sen-
sitive populations have been known to be replaced completely by resistant
ones over large areas within a year of first detection, particularly when the
variants are highly resistant and retain normal or near normal fecundity and
the ability to invade a host crop or animal. Shifts may be much more gradual,
however, as mentioned above. It is essential to obtain information at each
sampling site on the efficacy of field performance of the chemical following
the latest and earlier applications, on the numbers and types of chemical
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DETECTION, MONITORING, AND RISK ASSESSMENT
treatments applied, and on management factors (e.g., cultivar grown, method
of cultivation), in order to permit assessment of the practical impact of
resistant forms at different stages of their buildup and to aid identification
of factors that encourage or suppress resistance.
Checking Resistance Management Strategies
It is sometimes said that monitoring for resistance is a waste of time and
money, because if positive results are obtained it is then too late to take
effective action. This point of view may be valid under circumstances where
the first variants detected are sufficiently resistant to cause loss of control
and sufficiently fecund and competitive to accumulate rapidly and persist
and where selection pressures are sufficiently heavy and widespread to induce
large-scale shifts. Such has been the case with certain combinations of fun-
gicides and plant pathogens, for example, the use of dimethirimol against
cucumber powdery mildew (Sphaerotheca juliginea) in Holland (Brent, 1982)
or of benomyl against sugar beet leaf spot (Cercospora beticola) in Greece
(Georgopoulos, 1982b). Insecticide resistance commonly arises in this way
(Keiding, this volume). There is now, however, an increasing and very
welcome trend toward establishing, in the light of risk assessments, some
kind of strategy of resistance management at the very outset of the commercial
life of a new chemical. Monitoring then is done not to warn of the need to
initiate action but with the much better aim of checking whether an established
strategy is working adequately or needs to be modified or intensified. This
type of approach is indicated in Table 1.
investigation of Suspected Resistance Problems
When observed losses of field efficacy are reported, they may be so
dramatic that testing a few samples under controlled conditions against high
doses of the chemical is sufficient to confirm resistance as the cause. The
situation is sometimes less clear-cut: farmers may be using higher and higher
rates of a chemical to achieve the same degree of control, or the period of
persistence of protection may be gradually shortening. In such situations
studies that are more extensive in area and time can reveal a great deal about
the cause of these problems, and if there are correlations of reduced sensitivity
of the target organism with loss of field performance, then the need for a
change in the strategy of chemical use is indicated.
Subsequent Changes in Resistance
Later surveys, following a demonstration that resistant populations exist,
can indicate whether shifts toward resistance are spreading or contracting in
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DETECTION AND MONITORING OF RESISTANT FORMS
TABLE 1 Phases of Monitoring and Resistance Management for a New
Pesticide
303
Timing
1-2 years before start
of sales
Resistance Monitoring
Activities
-
Establish sampling and
testing methods
Survey for initial
sensitivity data
(include treated trial
plots)
Other Management
Activities
During years of use
As soon as signs of
resistance are seen
visually or through
monitoring
Monitor randomly in
treated areas for
resistance, only if
justified by risk
assessment or special
importance
Monitor to determine
extent and practical
. . ,~ ~
slgnlrlcance or
resistance
Subsequently Check rate of spread or
decline of resistance
Assess risk
Decide strategy of use
Work the decided use
strategy
Watch practical
performance closely
If resistance problem is
confirmed, review
strategies and
modify
Study cross-resistance,
hltness of variants
and other factors
affecting impact of
resistance
Watch performance,
review strategies
SOURCE: Brent (unpublished).
geographic distribution, whether they are increasing or decreasing in fre-
quency or severity, or whether an equilibrium is reached. Attempts should
be made to correlate any such changes in resistance with either initial or
modified strategies of chemical use or crop management.
Guidance in Pesticide Selection
Immediate practical guidance to individual growers, based on resistance
monitoring on the farm, may be feasible in some situations. The only
example known to the author is in the control of Sigatoka disease of
bananas (caused by Mycosphaerella spp.) in Central America, where the
United Fruit Company and du Pont have recommended that growers use
a simple agar-plate test every month and postpone the use of benomyl if
they find that the proportion of resistant ascospores exceeds 5 percent (du
Pont, 19821.
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DETECTION, MONITORING, AND RISK ASSESSMENT
Scientific Knowledge
The use of monitoring to aid our understanding of the nature of the resis-
tance phenomenon is important because of our present limited state of knowl-
edge of the population dynamics of resistant forms in relation to biological,
agronomic, and environmental factors. For example, are different races of
target organisms or cultivars of host plants more prone to resistance problems
than others? There is evidence of this in the resistance of barley powdery
mildew to fungicides (Wolfe et al., 1984~. How far are theoretical models
borne out in practice? Surprisingly few attempts have been made to validate
the various proposed mathematical models of the progress of resistance in
insects, plant pathogens, and weeds. How do factors such as dose applied,
spray coverage, and timing affect the rate and severity of resistance devel-
opment? The few studies that have been made for fungicides (Skylakakis,
1984; Hunter et al., 1984) have depended greatly on the development of
precise and reproducible detection and monitoring procedures.
TIMING AND PLANNING OF SURVEYS
A new pesticide should work well initially on the target organisms against
which it is recommended. If not, it would have failed in the large number
of field trials that generally are done before marketing. Surveys should be
started early, however, by testing field samples of each major target pest for
degrees of sensitivity under controlled conditions before the chemical is used
extensively (Table 11. Such testing provides valuable initial sensitivity (or
baseline) data against which the results of any subsequent tests or surveys
can be compared. These data could indicate the initial incidence of forms
with resistance genes if their frequency and the number of samples tested
were sufficiently high. Normally, however, testing will reveal the range of
initial sensitivities of different populations of the pest; it also will provide
an early opportunity to gain experience with and to check the precision of
test methods that may be required at short notice if problems arise later.
Some degree of variation in the results of initial sensitivity tests will occur,
and it is necessary by replication or repetition of tests to separate experimental
variation from real differences in response between populations. As part of
the baseline exercise, it is very useful to check the sensitivity of surviving
target populations shortly after successful use of the chemical in field trials:
the less-sensitive elements of heterogeneous populations tend to predominate
after treatment. Although these might persist and create problems later, often
they lack fitness or are unstable and decline as the effects of the chemical
wear off (Shepherd et al., 1975~.
Once initial data are obtained a decision must be made as to whether
further surveys are needed. Unless there is a special reason such as the
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DETECTION AND MONITORING OF RESISTANT FORMS
305
critical importance of the particular target-chemical combination, an indi-
cation of high risk from a risk-assessment exercise, considerable variation
between samples in the initial survey, or evidence from other regions for
resistance phenomena the effort and expense of further sampling will not
be justified until signs of practical loss or erosion of efficacy are seen. A
close watch should always be maintained, however, on the efficacy of treat-
ment in practical use ("performance monitoring"), in comparison with initial
field trial results and with the performance of other kinds of chemicals. If
either an obvious major loss of effect or a gradual decline of performance
are observed, all possible alternative causes of the difficulty (e.g., poor
application, misidentification of target organism, increased pest or disease
pressure) should be investigated, in addition to resistance. If possible, re-
sistance sampling should be done at sites of poor and good control and at
sites where the particular chemical has and has not been used. Positive
correlations of degree of resistance with practical performance and with
amount of use at the sampling sites must be sought. Sometimes highly
resistant strains of fungi or insects have been detected readily at sites where
the effectiveness of the product has been retained (Carter et al., 1982; Den-
holm et al., 1984~.
If tests indicate an appreciable shift in sensitivity from the baseline position,
then further monitoring, preferably at the same sites, may well be justified
to reveal whether resistance is spreading, worsening, declining, fluctuating,
or showing little change and how far it is associated with losses of control.
METHODS OF SAMPLING AND TESTING
In an extensive survey many sites (e.g., farms, fields, or glasshouses)
containing the target organism throughout a region or country are examined,
and one or a few representative samples of the population are taken at each
site. At the extreme, area populations of insects or spores can be trapped by
using suction traps for aerial populations of insects or by mounting test plants
on a car top and driving through a cropping area to sample the powdery
mildew spore population (Fletcher and Wolfe, 1981~. In an intensive survey
one or a few sites are visited, and many smaller samples perhaps comprising
single disease lesions or even spores, single insects, or single weed seeds-
are collected on several occasions. Often, it is best that an extensive survey
be done first, followed by a more detailed study if necessary. These two
approaches are complementary, however, and it may be advantageous to use
both concurrently or to adopt an intermediate method.
Information gathered at each sampling site should include the types, tim-
ing, and effectiveness of past chemical treatments and the amounts of target
pests, disease, or weeds present. Differences in these factors should be
compared with differences in sensitivity.
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DETECTION, MONITORING, AND RISK ASSESSMENT
Sample size should relate to the circumstances. If searching for first signs
of resistance in a largely sensitive population, a large bull; sample is more
likely to find the "needle in a haystack." To determine the proportion of
resistant forms in a population or the differences in degree of resistance, a
number of small, specific samples should be tested.
Samples should be as fresh as possible, and repeated culture in the
absence or presence of chemical should be avoided or minimized. One
way to achieve this, which is particularly useful for obligate parasitic fungi,
is to place treated test plants in pots in the field crop, allow them to collect
inoculum, and then remove them for incubation in a controlled-environment
facility or glasshouse to determine response. Conversely, it is valuable to
retest samples after repeated subculture in vivo or in vitro to check for genetic
stability of response.
For increased accuracy and to check degree of resistance, it is generally
best to use a range of concentrations during initial testing rather than a single,
arbitrary, discriminating dose. The response can be scored in various ways.
The EDso value is often used; it is a good "general purpose" value that is
widely understood and can be measured relatively accurately, compared with
an ED's value. For large-scale surveys, however, and particularly where
responses of sensitive and resistant forms are well separated (as with some
fungicide and herbicide resistance and most insecticide and rodent resistance),
the use of a single discriminating dose permits quick and adequate testing.
When resistance is clear-cut, different methods tend to reveal similar trends;
only in marginal cases does the method of testing or scoring affect the picture.
It is advantageous where possible, however, for one agreed method to be
used by different workers nationally or internationally. The WHO standard
tests for insecticide resistance in a range of insects of public health importance
(WHO, 1970, 1980) have been used internationally since the first test, on
anopheline mosquitoes, was introduced about 27 years ago. Test kits, based
on diagnostic test dosages for susceptible, fully resistant, and sometimes
intermediate populations, are available at cost for about a dozen pest species,
including rodents. FAO-recommended methods to measure pest resistance
in crop and livestock production and in crop storage have also been adopted
widely: Busvine (1980) has drawn together details of tests against 20 im-
portant pests, published at intervals since 1969 in the FAO Plant Protection
Bulletin; more recent issues of the bulletin contain new or updated procedures.
Recommended methods for testing fungicide resistance in crop pathogens
have also been published by FAO (1982), and general reviews of procedures
are given by Georgopoulos (1982a) and Ogawa et al. (19831.
During testing it is important to investigate differences in pathogenicity,
growth rate, reproductive rate, and other properties that contribute to the
fitness of an organism. Often the more highly resistant forms are less fit or
competitive than normal forms in the absence of chemical treatment, and
knowledge of this can help to explain and predict their behavior.
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DETECTION AND MONITORING OF RESISTANT FORMS
307
Biochemical methods for detecting and monitoring resistant forms have
been developed for insecticides and are increasingly used in surveys (Miyata,
1983; Devonshire and Moores, 19841. In some situations they can detect
resistance at lower frequencies than do bioassays. They can also be more
convenient and permit the degree of resistance to be measured quantitatively
without the need to test several samples at different doses. Inhibition of
photosystem II, as revealed by loss of chlorophyll fluorescence of herbicide-
treated leaves, leaf discs, or isolated chloroplasts irradiated with short wave-
length light, has proved a convenient method for monitoring atrazine-resistant
weeds (Gasquez and Barralis, 1978, 1979~. Another rapid method for testing
response to photosynthesis inhibitors is the sinking-leaf disc technique. The
buoyancy of discs floated on surfactant solutions appears to depend on the
O2/CO2 ratio in the air spaces, which is decreased by the action of herbicides
(Hensley, 1981~. Biochemical monitoring is not yet used for fungicide re-
sistance because mechanisms of resistance for field isolates are not well
characterized and appear to involve changes at biosynthetic or genetic sites
that are not easily detected. More research on this aspect seems justified.
Specific diagnostic agents, such as cDNA probes or monoclonal antibodies,
may offer new possibilities for future biochemical tests for all types of target
organisms (Hardy, this volume). As pointed out by Truelove and Hensley
(1982), however, biochemical methods should be used with caution, since
resistance that depends on alternative mechanisms to the method under test
could be missed; in this respect, bioassay tests on whole organisms remain
the most reliable indicators of resistance.
ACHIEVEMENTS IN RESISTANCE MONITORING
Only a few examples of the many monitoring projects done in different
countries and on different target organisms can possibly be considered here.
Since the first case of insecticide resistance was reported by Melander in
1914 (Melander, 1914), response to insecticides has been monitored exten-
sively in many countries (Georghiou and Mellon, 19831. Global programs
have been organized by WHO to survey insecticide resistance in anopheline
mosquitoes (WHO, 1976, 1980) and by FAO to survey insecticide resistance
in pests of stored grain (Champ and Dyte, 1976) and acaricide resistance in
ticks (FAO, 1979~. These very large projects have provided valuable infor-
mation on the geographic distribution and intensity of resistance, on its
relationships to the successful use of chemicals, and to failures in control.
The coordination and interpretation of results have benefited greatly from
the general use of recommended methods of testing and reporting mentioned
earlier.
Many national surveys have been conducted. An outstanding example is
the study of resistance in house flies on farms in Denmark, discussed in this
volume by Keiding, which has been sustained since 1948 and has shown
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DETECTION, MONITORING, AND RISK ASSESSMENT
clearly the large-scale shifts in response to successive introductions of dif-
ferent types of insecticide (organochlorines, organophosphorus compounds,
and pyrethroids). Other notable programs have included studies of rice leaf-
hoppers and planthoppers in Japan (Hama, 1980), cotton leaf worm in Egypt
(El-Guindy et al., 1975), and the aphid Myzuspersicae in the United Kingdom
(Sawicki et al., 1978~. In the last study biochemical (esterase-4) tests as well
as bioassays were used; both approaches gave rapid and satisfactory results
and to some extent were complementary in distinguishing different types of
resistance.
International surveys comparable with those undertaken with pests have
not been done for fungi. Although some recommended methods have been
published by FAG, in practice a variety of test methods have been used by
different workers. National or regional programs have included surveys of
resistance of cucumber powdery mildew to dimethirimol in glasshouses in
Holland (Bent et al., 1971) and later to other systemic fungicides (Schepers,
1984), the response of barley powdery mildew to ethir~mo} in the United
Kingdom (Shepherd et al., 1975; Heaney et al., 1984) and to triazole fun-
gicides (Fletcher and Wolfe, 1981; Heaney et al, 1984; Wolfe et al., 1984),
of metalaxy} resistance in Phytophthora infestans on potatoes in Holland
(Davidse et al., 1981) and in the United Kingdom (Carter et al., 1982),
benomyl resistance in sugar beet leaf spot in Greece (Georgopoulos, 1982b),
and dicarboximide resistance in Botrytis on grape vines in West Germany
(Lorenz et al., 19811. Each of these studies, as well as others not mentioned
here, to some extent tells an individual story. Two main patterns can perhaps
be distinguished: a rapid, widespread, and persistent upsurge of resistance
and loss of disease control (as with dimethirimol and cucumber powdery
mildew, metalaxy} and P. infestans in Holland, benomy! and sugar beet leaf
spot) and a slower, fluctuating increase in resistance, with either partial or
undetected loss of disease control (as in the cases of ethirimo! or triazoles
and barley powdery mildew, metalaxyl and P. infestans in the United King-
dom, and dicarboximides and Botrytis). The intensity and exclusivity of
fungicide use and the degrees of resistance and fitness of the resistant forms
are important factors in determining these patterns. In the former cases mon-
itoring tended to follow reports of loss of control and results were obtained
too late to permit any management strategy other than withdrawal of the
product, but in the latter, where monitoring preceded any major breakdown
in performance, avoidance strategies either were already operating or were
introduced following the results of monitoring.
Since the early 1970s the incidence of triazine-resistant biotypes of various
weeds in different crops has been monitored extensively in different parts of
the United States, mainly by collecting seeds and growing progeny for glass-
house tests. The initial indications of resistance, obtained after 10 years of
widespread use of these herbicides, came from farmer observations of obvious
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DETECTION AND MONITORING OF RESISTANT FORMS
309
lack of control; the monitoring has served primarily to confirm resistance
and to follow the problem in time and space (Bandeen et al., 19821. Atrazine
resistance has also been observed in monitoring studies in several countries
of continental Europe (Gressel et al., 19821. The rate of development of
resistance appears to have varied between different parts of the United States
and has been relatively slow in the United Kingdom (Putwain et al., 1982~.
Forms resistant to other herbicides, for example, phenoxy compounds and
bipyridyls, have been detected in different countries, but their incidence has
been sporadic, their resistance less marked, and little monitoring has been
done.
COOPERATION AND COMMUNICATION
Detection of and monitoring for resistance call for close cooperation be-
tween scientists as individuals and as representatives of industrial and public-
sector organizations. Although coordination does take place, such as in the
work of the Fungicide Resistance Action Committee (FRAC) and Insecticide
Resistance Action Committee (IRAC), much of the research is still too frag-
mented and haphazard. Industry has felt it has been excluded from some
collaborative schemes and planning meetings organized by the public sector,
but, equally, the RAC system does not fully involve the public sector, since
it is primarily an intercompany concern. There is much that scientists in
industry and the public sector can do to increase contact, review progress
and priorities, and plan collaborative research. Such collaboration would be
best focused on particular resistance problems and should be in work groups
rather than in conferences, with one person or organization as the focal point
for each topic. At this time of retrenchment of national research expenditures
in many countries, the selection of priorities in resistance monitoring which
despite its importance is an expensive and essentially defensive area of re-
search-is especially important.
The results of monitoring programs should be reported in the open scientific
literature, not retained in confidential reports or computer files. The storage
of information from many sources in a data bank from which it can be
retrieved and disseminated readily is valuable, however; the data bank for
insecticide resistance at the University of California (Riverside) is a good
example (Georghiou, 1981~.
Education in resistance monitoring is improving. Conferences are helpful,
but the international courses on fungicide resistance organized by Professor
Dekker and colleagues and held at Wageningen and more recently in Ma-
laysia- have proved particularly useful, since they included laboratory ses-
sions and a tactical exercise in addition to lectures and group discussions.
Perhaps similar courses could be organized on insecticide and herbicide
resistance.
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DETECTION, MONITORING, AND RISK ASSESSMENT
CONCLUSION
Detection and monitoring form an integral part of pesticide resistance
management. To avoid misunderstanding and waste of effort, very careful
definition, planning, and interpretation of these activities are required. Mon-
itoring denotes different operations, ranging from global surveillance pro-
grams to much smaller investigations of cases of suspected resistance.
Distinction must be made between detecting resistant forms and establishing
that resistance has reached levels of severity and frequency sufficient to cause
practical loss of pesticide performance. Cr~tena for defining resistance and
sensitivity have differed greatly, especially when several different degrees
of resistance occurred, and must always be made clear.
Test methods should be developed and initial sensitivity data sought before
new compounds are brought into widespread use; avoidance strategies should
also be established prior to widespread use, since monitoring cannot be relied
on to give sufficient early warning of the need for such strategies.
Subsequent monitoring should be done if risks are considered high, if the
particular pest-control system is especially important, or when visible signs
of resistance problems arise. Selection of test procedures will depend on the
nature of the pest and of the pesticide treatment, but the adoption of inter-
nationally recommended methods aids the comparison and coordination of
results. Biochemical methods have already proved useful and have a prom-
ising future. Further collaboration between and within the industrial and
public sectors in planning and conducting monitoring programs must be
fostered.
ACKNOWLEDGMENTS
The author is grateful to a number of persons for providing information,
and especially to Drs. A. Devonshire, G. P. Georghiou, H. LeBaron, and
L. R. Wardlow.
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Representative terms from entire chapter:
fungicide resistance
