The management of insecticide resistance is a somewhat
controversial subject, and not because we are ignorant of the problem. In fact,
scientists have a good handle on the mechanisms of pesticide resistance
development. It is controversial because scientists do not have a good handle
on the solutions to insecticide resistance. At several recent conferences,
there were varying opinions on the subject, with many extension scientists
begging for answers they could take to growers. The growers, like my uncle in
suburban Seattle, just want the pests dead and gone — although my uncle
wants them to suffer and, with their last dying breath, tell their bug buddies
never to return.
Professional opinions on the best way to manage insecticide
resistance range from those that suggest we need to stop using pesticides
altogether to those that recommend tank-mixing the most efficacious products.
Of course, these two examples are at the extremes and neither is likely to be
an appropriate solution, especially in the world of ornamentals where nothing
is simple. Few studies have actually tested the theories of insecticide resistance in practical applications. So in most cases, people can only make educated guesses at which strategies should work best for each product and each pest.
Through the 1990s, more detailed investigations of the
mechanisms involved in pesticide resistance have shown that insects have an
innate ability to develop resistance to most pesticide chemical classes, and
some insects have the ability to develop resistance to many classes
simultaneously. Everyone is aware of the classic example of cross-resistance
between carbamates and organophosphates. However, with the development of new
insecticide chemistries, new forms of cross-resistance (resistance to more than
one pesticide with the same or similar mode of action) and multiple-resistance
(resistance to two or more pesticides with different modes of action) have been
exhibiting themselves, such as multiple-resistance to both a chitin synthesis
inhibitor and a carbamate or an imidacloprid resistant whitefly that is more
tolerant to bifenthrin. Many of the highly resistant insects we see today are a
result of overuse or off-label use of pesticides.
So why is insecticide resistance management important,
especially since it seems so difficult? Primarily because the number of
pesticides registered is dwindling, and the cost and time required to develop
and register new ones is high. In addition, resistance management is important
to manufacturers so that their products have a long life in the industry.
Therefore, they are imposing user restrictions on the label or recommending a
specific IPM program on the label to delay or reduce the chance of resistance
buildup.
Classifying Chemicals
Understanding insecticide resistance management requires
some knowledge of pesticide chemistry and mode of action. The actual chemical
structure, or molecular structure, of the active ingredient in the pesticide
determines the chemical class. Unfortunately, the chemical class is not on the
pesticide label, and in many cases it is not on the MSDS. Researchers have been
telling growers to rotate by chemical class for a long time, but until
recently, there’s not been an easy way to identify a pesticide’s
chemical class. This information is available from many sources, including
Olympic Horticultural Products’ Chemical Class Chart, various Web sites
and the GPN Resource and Buyer’s Guide. The EPA has been listening and
now has an extensive chemical class list for pesticides used in the industry
(see appendices, http://www.epa.gov/opppmsd1/PR_Notices/).
The mode of action is the mechanism by which the pesticide
kills the pest. The reason for suggesting rotation based on chemical classes is
that the chemical class usually denotes the mechanism of action of the
pesticide, i.e., the way the chemical works (See Table 1, page 34) It is
important to note, however, that it is possible for different chemical classes to
have the same or similar modes of action, such as with organophosphates and
carbamates. Back-to-back rotation of these chemical classes would not be
recommended. For a more technical description of insecticide mode of action,
see the following Web site: http://ipmworld.umn.edu/chapters/bloomq.htm.
Figure 1, above, is a simple representation of the nervous
system in animals. Nerve cells are connected by synapses. The nerve impulse
moves along the axon (sodium NA+ and chlorine CL¯ channels) until it
reaches the synapse and Acetylcholine (ACh) is produced to carry the impulse to
the next nerve cell. Once the Acetylcholine has done its job, it is metabolized
by Acetylcholine esterase (AChE) to empty the synapse for the next nerve
impulse. You will note that the majority of the modes of action listed in Table
1, page 34, affect the nervous system in some way.
Mechanisms of Resistance
Resistant pest populations that can no longer be controlled
by a pesticide usually develop over several generations. Resistance develops
fastest in insects that have a high rate of reproduction and are under heavy
pesticide pressure. The more you spray, the more you increase the proportion of
resistant individuals in the population. This fact has been demonstrated many
times. Insecticide resistance is genetically based, and a resistant population
may have developed one or more of the following resistance mechanisms:
• They may have a different behavior.
• They may have changed the outer cuticular layer so
the pesticide can’t penetrate.
• They may produce an increased amount of pesticide
insensitive ACh.
• They may produce an increased amount of pesticide
insensitive AChE.
• They may have increased the number of types and
quantities of detoxifying enzymes.
The insect cuticle and blood are full of many different
types of enzymes called mixed function oxidases (MFOs) and cytochrome p450s.
The latest research shows that some resistant insects have the ability to
produce increasing amounts of and different types of detoxifying enzymes. In addition to the metabolic enzymes, resistant insects have the ability to produce
pesticide-tolerant or increasing quantities of ACh and AChE, so much so that
the pesticide is rendered ineffective. Unfortunately, some of the highly
resistant insects (leafminer, western flower thrips, etc.) use several of these
mechanisms at the same time. This is known as multiple resistance.
It is important to understand the basic biology of the pests
in question. How long does it take for the pest to complete one generation? With
aphids, it’s very short, so rotation should occur in approximately two
weeks. However, whiteflies take about 25 days to develop into an adult. Their
generation time is much longer.
Resistance Factors
Before we can discuss pesticide resistance, we should
investigate the other potential causes for control failure. Control failure may
not be a result of resistance. It may be due to some other factor in the pest
management scheme.
Possible reasons for control failure include:
• failure to initiate other parts of an overall
integrated pest management program;
• failure to intensively monitor for pest species;
• misidentification of pest species;
• wrong choice of pesticide;
• incorrect rates/off-label use/use of an old or
degraded pesticide;
• poor choice of tank mixes, adjuvants, pH, water
quality, etc.;
• misuse of equipment, inadequate agitation, improper
calibration, inadequate maintenance, etc.;
• inadequate coverage, improper placement of the
pesticide; and/or
• very high pest populations, tolerant or resistant
populations.
Managing pests should begin with the basics of integrated control. For instance, all efforts should be made to exclude or inhibit the
development of pest populations using cultural or environmental controls.
Biological control should be considered where possible, and an intensive
monitoring or scouting program should be in place so that one can treat or spot
treat when necessary. Another good reason to intensively monitor for pests is
that a very heavy pest infestation is very difficult, if not impossible, to
control before damage occurs. Ten aphids on a terminal today can become 250 in
a week. If spider mite webbing is visible, the population is probably enormous.
Very high pest populations should be avoided at all costs. Concentrating on
integrated methods of control will reduce the reliance on chemical control. When chemical control is necessary, proper application of the chemicals is the next best method of avoiding or delaying pesticide resistance.
Some of the most critical components of successful pesticide
applications are coverage, timing and placement. Not spending enough time
covering dense foliage will, in all likelihood, allow some pests to survive.
Sub-lethal doses due to inadequate coverage may increase the chance of
pesticide resistance. The timing of pesticide application must coincide with a
stage of the pest that is vulnerable to the application. For instance,
whiteflies are most susceptible when they are in the early nymphal stages. They
are most tolerant when they are in the redeye stage. Therefore, monitoring to
establish the pest stage will help determine which pesticide to use and when to
use it. In addition, whitefly nymphs are located on the lower surface of the
leaves. So, the application should be directed to the undersides of the leaves
for effective coverage. Knowing where to direct the pesticide is just as
important as proper coverage.
Using the New Mode
Rotation of pesticides with different modes of action is a
very important concept. Research has shown that pests can develop high levels
of resistance, like western flower thrips, which can detoxify more than one
chemical or mode of action per generation. Quite simply, this means that
treating insect populations with tank mixes of insecticides that have several
modes of action can result in a pest population with multiple resistance
mechanisms and produce what seems to be a superbug. Therefore, it is important
to use the pesticide according to the label and to rotate by both chemical
class and mode of action. Rotation should occur every one or two pest
generations so that the new chemical will kill surviving individuals tolerant
to the previous pesticide.
So what is the best approach to manage pests without
enhancing pesticide resistance? We cannot in good conscience suggest that
everyone just stop using pesticides. We also cannot suggest enhancing the
problem by adding a greater amount of pesticide or a greater number of
pesticides to the tank. Minimizing chemical control by incorporating other pest
management tactics is a more sensible solution. I have observed that successful
growers monitor their crop very closely and treat hot spots. On the other hand,
growers who are treating on a scheduled basis and tank mixing more than one mode
of action at one time are usually less successful. If you want to examine a
recent example of how things can go wrong, see “The Return of the
Leafminer,” in the June 2001 issue of GPN. The products that successful
growers use are highly effective because the pests they treat are sensitive to
the pesticides.
Tank mixing insecticides with different modes of action is
risky and, in my opinion, should be avoided. It exposes the pest population to
several modes of action at the same time, enhancing the potential for
resistance development. There are a few successful tank mixes that are
well-known such as the mix of an OP with a pyrethroid (e.g., acephate +
permethrin), or avermectin with insecticidal soap. In my opinion, it would be
risky to rely on the OP-pyrethroid mix religiously without rotation. However,
resistance to soaps and oils is unlikely, so the inclusion of soaps and oils to
a rotation or a tank mix is an option to keep open.
Many have asked me about tank mixing to kill both the adult
(e.g., a pyrethroid) and immature (e.g., an IGR) at the same time. Again, this
is where I believe some growers have gotten into trouble with resistant pests.
All stages of the pest are being exposed to two modes of action at the same
time. It is possible, with an effective monitoring program, to know which stage
the population is in and treat only that stage. It is also possible to treat
the untreated stage with another pesticide at a different time, thereby
delaying the exposure of both stages to both modes of action.
If a resistance problem occurs, then it’s time to take
a step back and review the overall integrated pest management program. The
tendency is to seek a new chemical solution rather than looking at the entire
program. There is evidence that reversion can occur. That is, if the use of
pesticides against a resistant insect is curbed, in time, that chemical can be
used again. This needs more study in ornamentals.
