Combating Insecticide Resistance

August 22, 2002 - 08:55

Entomology professor Ron Oetting advises on how to get the most out of your Marathon applications--and how to avoid resistance.

We have been plagued with resistance problems in the ornamental
industry for several years. My first exposure to resistance was when the
leafminer, Liriomyza trifolii, became resistant to the broad spectrum of
insecticides used in greenhouse production in the 1970s. This was a new thing
to our industry, and we could not figure out why we were getting good results
with one pesticide in some areas and not in others. First, there were only a
couple that could not get the expected control with this insecticide, then a
few more, and finally most could not get the expected results. Then it was
everything else; nothing worked anymore. It was like a plague that spread
across the state. By 1980, leafminer resistance was recognized as a major
problem nationwide, and monies were channeled into the industry to help solve the
problem. We worked with various insecticides and finally worked out solutions.
It became very obvious that this was a problem we were going to have to deal
with in the future with various pests.

Since that time, we have had major pest problems because of
the failure of pesticides to perform as well as they had in the past. Western
flower thrips became a major problem in the early 1980s and silverleaf
whiteflies in the late 1980s. These were major control failures, but several
other pests have been problems as well, including mites, aphids, worms,
mealybugs and others. Resistance is not as big a puzzle as we once thought; we
understand what causes it. However, we still do not have a clear picture of why
some pesticides are not as subject to the development of resistance or how to
deal with preventing resistance from occurring. There are several theories, and
applying these help in creating a management strategy to try and delay the
development of resistance to specific compounds. An accepted practice is to
rotate pesticides so pests are not constantly exposed to one specific mode of
action (the site and method of activity of a pesticide against a pest.)

What Is Resistance?

Resistance occurs when the susceptibility of a particular
organism to a chemical changes such that it takes a larger dosage of that
compound to cause the same level of activity. This is usually the result of
natural selection, where a population is exposed to a certain level of a
toxicant, and a few individuals survive. Some of the offspring of these
surviving individuals inherit the ability to survive that dosage of the
toxicant, and this continues to a point that it takes extremely high dosages to
be effective, if they are effective at all. Thus the theory evolved of changing
modes of action: that a new mode of action will be effective against those
individuals that survive the previous one. Even if a different pesticide that
has the same mode of action is used, there could be cross-resistance, and it
would not be any more effective than the original pesticide used.
Cross-resistance is when resistance develops against one compound, and you can
get resistance against other compounds that have the same mode of action, even
before the pest was exposed to the other compounds. We are experiencing
resistance with many pesticide/pest interactions as well as many other
synthetic chemicals used in other areas, such as the management of disease with
antibiotics in humans.

Current Resistance

Because of the before-mentioned resistance problems, a concern
over resistance developing in whitefly against imidacloprid, or Marathon, has
arisen. The silverleaf whitefly problem surfaced in the late 1980s. The theory
was that this pest, thought to be the sweet potato or cotton whitefly (Bemisia
tabaci), was a major problem in field production, and somehow it had changed
its host and site selection to greenhouse crops and thrived on ornamental as
well as field crops. The new whitefly was determined to be a different biotype
and was given a new species name, silverleaf Á whitefly (Bemisia
argentifolii). The important thing is that it was resistant to the insecticides
commonly used to control whiteflies, and it was a major pest on greenhouse and
field crops. Several insecticides were tested, including combinations of
insecticides, with mixed success. This resulted in a tremendous number of
applications of insecticides used to control whiteflies to get one crop to
market. In 1996, Marathon was introduced to the greenhouse market with
extremely good success against the silverleaf whitefly. As a result, Marathon
has been the most commonly used pesticide against this pest. Any time one
compound is relied upon for the management of a single pest, there is great
concern over resistance developing. This concern is justified because when you
cannot rotate away from one mode of action, the probability of resistance
developing is greatly enhanced.

Marathon was introduced as a new class of insecticide, the
chloronicotinyls. Since it is a new class of insecticide, one would not expect
to have cross-resistance because it has a new mode of action. Silverleaf
whiteflies have developed resistance against several insecticides, including
some pyrethroids, organophosphates and carbamates. Imidacloprid (Marathon) is
an acetylcholine receptor agonists/antagonists, which means that it binds to a
nicotinic acetylcholine receptor, disrupting nerve transmission. This is the
same receptor that organophosphates and carbamates are active against, but the
mode of action is different. Carbamates and OPs are acetylcholine esterase
inhibitors, which cause the inhibition of the enzyme acetylcholinesterase,
interrupting the transmission of nerve impulses. These classes are nerve
poisons, which impact the nerve impulses, but imidacloprid has a different site
of activity.

Is there resistance to imidacloprid (Marathon)? I am not
aware of any confirmed resistance to Marathon in the field in the United
States. I do feel that there are cases of difference in susceptibility to
imidacloprid in populations of whiteflies in the United States. This is not
resistance, but it could be an indication of populations where resistance could
develop if not properly managed. Is Marathon immune to the development of
resistance? I doubt it. If the compound is not properly managed, then
resistance will occur. Á We have had a difference in susceptibility of
whiteflies to Marathon ever since it came on the market. We have observed
problems controlling the greenhouse whitefly (Trialeurodes vaporariorum) in
cooler parts of Georgia ever since Marathon came on the market. I feel that to
control greenhouse whiteflies with Marathon, it must be used before the
whitefly population has a chance to become established. The silverleaf whitefly
is more susceptible to Marathon, and we get very good control of this pest in
our area. However, there are populations within the silverleaf species that are
more susceptible than others.

In some areas, imidacloprid is used for many different crops
to manage whiteflies and other pests. In these areas, whiteflies could
potentially be exposed to imidacloprid continuously throughout the year, so we
have to be especially careful to monitor whitefly populations for reduction in
susceptibility to imidacloprid. These will probably be the first areas where
resistance can develop and areas where it is especially important to rotate the
insecticides used to manage whitefly populations. The ideal situation would be
to control the amount of one chemical used in all of the crops in a region and
maintain a limited exposure to that particular chemical.

There are some countries that have programs recommending the
use of a particular chemical only during a certain time. One of these programs
is called the ?windows? method, where pesticide X can only be used
between certain dates or a window of application. This could only be
accomplished with cooperation among neighbors and would surely delay the
development of resistance to insecticides. Chemical companies are trying to do
the same thing by limiting the number of applications on a particular crop or
season. The problem with this approach is that it does not control what a
neighbor is doing.

Avoiding Resistance

What can we do to avoid the development of resistance to
Marathon? One key thing that each person can do is to rotate modes of action
such that a pest is not exposed to a particular one continuously. There are
three formulations of Marathon currently on the market for greenhouse
production. It is important that these be considered as one chemical and not be
used repeatedly, especially if a grower feels that they have not gotten the
results that they expect from Marathon. If you are not satisfied with control
obtained with Marathon, it is important not to come back with a repeat
application with any of the three formulations. Á Marathon II, applied
as a foliar spray, can be used in repeat applications ? if no
media-applied Marathon has been used on that crop. This could increase the
chance of a low-susceptibility population developing resistance to Marathon.
Media-applied Marathon can be used once every 16 weeks.

Good application technique is key to resistance management.
Make sure the proper recommended dosage is used, application equipment is in good
working condition and good coverage on the plants treated is achieved. If good
coverage is not obtained, there could be pests not reached within the canopy
that later come in contact with the reduced dosage of the chemical residue and
start developing immunity to the chemical. Marathon does have systemic activity
when applied to the root zone, which is an added advantage to this compound
because it can be applied to the potting medium and taken up by the plant to
give control of pests throughout the canopy. Timing is still important because
the age of the leaves and root system development will influence the uptake of
Marathon. Marathon is taken up by the roots and translocated to the actively
growing foliage. This is not a problem on newly established plants with a good
root system, but on older plants the chemical will not be translocated to the
old leaves as efficiently. The result could be control failure on these older
leaves, which is not a sign of resistance but the lack of chemical in these leaves.
A foliar spray can also be taken up in the leaves to move within the leaf, but
you would not get the systemic activity typical of application to the root
zone. Systemics are by nature water-soluble, and excess watering could also be
a problem, especially for a few days after chemical application. The roots must
have a chance to take the chemical up into the vascular system before it is
leached out of the pot. The type of medium in the potting mix can also affect
the speed of uptake.

There are many things that can affect the development of
resistance of any pesticide, and many things must be considered before blaming
a control failure on resistance. In most situations it is something else that
is the cause. The most common cause is in the application and handling of the
pesticide. If you suspect control failure as the result of the application of
any chemical, it is a good idea to consider all things that could have
contributed to the failure. When you repeat an application to get better
control, do not immediately return with the same chemical and do not increase
the dosage above the labeled rate ? select an alternative. Rotate modes
of action whenever possible, and follow the programs recommended by your
extension service.

About The Author

Ron Oetting is professor of entomology at The University of Georgia. He can be reached by phone at (770) 412-4714 or E-mail at roettin@griffin.peachnet.edu.

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