The Truth Behind Tank Mixes

August 22, 2003 - 07:34

Tank mixes were common years ago, but are they worth it now?

"Can I use tank mixes?" This is a question I have
heard from growers for more than 25 years, and I still do not have an answer
that is backed up by facts and figures. Tank mixes were a common practice
during my early experience with greenhouse growers, but mixes are not as common
today. When I first started working with pest management in greenhouse production,
it was common for chrysanthemum growers to use weekly sprays of tank mixes.
These tank mixes usually included a pesticide for worms, one for mites and at
least one for diseases. It was also common to apply a systemic pesticide,
usually aldicarb, as a general preventative against several pests. Integrated
pest management was being researched and practiced, especially for field and
orchard crops, but not much emphasis had been placed on ornamental crops and
research in this area was in its infancy.

When Liriomyza leafminer became a management problem in the
mid 70s, there was a change in traditional management strategy. Pesticide
resistance became a new topic of concern, and recommendations for pest
management took on a new picture. There was a rush to find a management tool
for leafminers, and pest biology became an important part of this strategy.
Prior to this time, the only concern was pure efficacy of pesticide against the
target pest. Researchers across the country diligently searched new and old
chemistry for a solution to the problem of leafminer resistance. When a
solution was found, it was used extensively to reduce leafminer populations; as
a result, resistance was developed against some of these compounds. The prime
example was the rapid development of leafminer resistance against permethrin in
about 1980.

Resistance management really became an issue when western
flower thrips became resistant to available pesticides and spread across the
United States and around the world. Thrips were already hard to control in the
mid 70s, but there were a few insecticides that were effective. Methomyl was
the standby, and acephate was new on the market at that time — both were
effective. By the early 80s, western flower thrips and the accompanying tospovirus
(tomato spotted wilt virus) had become a dominant concern in pest management
for greenhouse growers. This was a result of western flower thrips' resistance
to available insecticides and the lack of any means of controlling the
tospovirus vectored by the thrips. A management approach was to develop a
rotation of insecticides to control western flower thrips. The theory was that
this rotation would slow down the development of resistance when new
insecticides were found. At that time, the recommendation of rotating chemicals
was the standard, and tank mixes were discouraged in extension recommendations.

What is the role of tank mixes, or the lack thereof, in
resistance management today? This is a good question with answers supported by
various theories, but the most common theory is that mixing compounds will
result in resistance developing faster than if each pesticide is used
separately in a rotation. The purpose of this article is to address some of the
reasoning behind the use of tank mixes and rotations in pest management.

Tank Mix Problems

The first major problem with tank
mixes is the increased potential for phytotoxicity. I have seen numerous
examples of increased phytotoxicity as a result of mixing two or more chemicals
together. Two serious examples come to mind. One grower lost his newly rooted
poinsettias because of a foliar spray of a tank mix of three chemicals all
targeting silverleaf whitefly, and a second situation resulted in complete loss
of a poinsettia crop because of the mixing of an insecticide and an
off-the-shelf spreader sticker formulated for homeowners. In these situations,
all chemicals were sprayed at label rates. None of the selected pesticides were
illegal. Data does not exist for all possible mix combinations, let alone for all
plants. Growers who use tank mixes must take the responsibility of checking out
a tank mix on a few plants to make sure that phytotoxicity will not occur.

Another problem sometimes encountered with tank mixes is the
incompatibility of components in the mix. This incompatibility results because
the chemistries of the two compounds are not compatible, and they react with
each other, often reducing the activity of mix against the target pests. It
could also cause the new tank mix to be more phytotoxic to the host plant as
discussed above. Often, if the chemicals are incompatible, there is a chemical
reaction between the two compounds, which can be seen in the mixed solution.
The common test of physical compatibility is to mix the correct proportions of
each pesticide component with water in a quart jar and shake the mixture.
Observe the mixture to see if they mix together uniformly or if there is
separation of layers, precipitation to the bottom of the jar or other abnormal
mixing characteristics. The problem of incompatibility is not limited to mixing
two or more pesticides but could also be the result of mixing a pesticide with
fertilizer or other horticultural chemicals.

Resistance Management Programs

The most common tank mix is the use of a pesticide with an
additive to enhance efficacy of the pesticide. Additives such as buffers are
added to adjust the pH, without which the pesticide will break down too fast or
not be as active as it should be; surfactants are often used to increase
coverage on plants, especially waxy plants; pyrethrin has been used as a
flushing agent to irritate a pest and make it move, allowing better contact and
control; attractants have been used to attract pests to feed on the pesticide
component of the mix; and sugar has been mentioned as an additive for increased
thrips activity. Many of these additives are not only enhancing the activity of
the primary pesticide, but some of them also have pesticidal activity
themselves. The pesticide label often contains guidelines concerning the use of
additives in the tank mix.

The big question is whether tank mixes are bad for
resistance management programs? Resistance is the result of a pest becoming
more tolerant of a pesticide, which was once effective against it. This begins
when a few individuals are more tolerant to a chemical than the remainder of
the population. These individuals survive and pass this genetic trait of
tolerance on to the next generation. As a result, more individuals in the next
generation are less susceptible to being killed by the pesticide. If this
continues for several generations, the tolerance can be much greater than what
was present in their ancestors. Most new pesticides now have resistance
management instructions on their labeling, and most recommend limiting the
number of applications on a crop and/or using a rotation of a different mode of
action. The purpose of restricting the frequency of use and limiting exposure
to that chemistry is to decrease the chance of having a population that has
developed resistance against that chemistry. How does the use of tank mixes fit
into resistance management programs? The key concern in tank mixes and
resistance management is whether the mix is targeting one pest or different
pests.

One-Pest Mixes

The use of tank mixes, which targets only one pest, goes
against the general philosophy of many researchers and is not good resistance
management. My general perspective is that we do not use tank mixes targeting
one pest unless it is absolutely necessary. One philosophy is that by mixing
two effective insecticides from two different classes, you should be able to
kill all of the individuals of the species, and there will not be any
individuals to carry on resistant genes. This is good in theory, but I don't
think it is true that we will kill all individuals in the greenhouse. It is
more probable that a few individuals that do survive will demonstrate some
tolerance to both pesticides and will pass that on to the next generation.

You probably would not be thinking about mixing compounds if
you were not already having problems controlling this pest with one or both of
the components of the mix. We also know that if you can remove the population
of a particular pest from exposure to an insecticide, in some cases, they will
actually recover and become more susceptible to that insecticide again.
Therefore, the rotation of compounds is a better management practice against a
particular pest species. There are many different types of pesticides and
different pests that enter this picture, and each must be considered separately
to make a sound decision on tank mixing two or more pesticides to manage a
single pest species.

There are exceptions where mixes are recommended, but these usually
follow the loss of efficacy of the primary component. For example, when we
encountered problems with abamectin for the management of western flower
thrips, we used a tank mix of abamectin and horticultural oil, and it improved
efficacy. The use of oil is acceptable in tank mixes by many researchers.
Another example was when acephate and a pyrethroid were used to manage
silverleaf whiteflies when no other control was available. If possible, it is
still a good practice to rotate these tank mixes with other alternative control
practices.

A question is often raised about mixing two insecticides
together to kill different developmental stages of a pest. A common example is
an insecticide spray for adult fungus gnats and an insecticide drench for
immatures in the medium or a miticide to kill active mites and an ovacide to
stop reproduction. This is a little more valid than the situation discussed
above, but I still feel that we need to be careful with these mixes. We need to
fully evaluate the situation. Do we really need to attack both developmental
stages? If the crop is going to be in the house for a few weeks, then an
effective material will reduce the population, and you can still rotate
pesticides appropriately. If you are ready to ship the crop this week, then
there may be more justification for looking at controlling all stages.

Mixes for Different Pests

What about mixing together pesticides that target different
pests? A mix that targets different pests might include an insecticide for an
insect, a miticide for a mite or a fungicide for a disease. There are varying
opinions on the validity of this mix, but the truth is that growers are going
to use them. The biggest problem is the already-mentioned phytotoxicity.

A commonly asked question is what is the difference in a
tank mix and spraying the pesticides separately within a very short time of
each other? The answer is probably nothing as far as resistance management goes
but everything when it comes to phytotoxicity.

Mixes targeting different pests make better sense in
resistance management, but care should be taken that the components do not
overlap in their activity. For example, using abamectin for mites and spinosad
for thrips does not make sense. Both are used for thrips control, so they overlap
in that use and should be rotated in a thrips management program. In addition,
abamectin is used at a lower rate for mites than for thrips. Applying an
insecticide at a sublethel dose is a sure fire way to increase the probability
for resistance. If you are going to mix different pesticides for different
pests, you need to really study the characteristics of the components. You
definitely do not want to mix a fungicide with a fungal bioinsecticide that has
insecticidal activity, such as Beauvaria bassiana. Care should be taken to test
any tank mix before using it on the entire range. Spray a few plants, and
observe for plant damage and pest control to see if they are safe and
compatible.

Tank mixes are much easier for growers to use than trying to
spray all pesticides at different times. The use of mixes will save time, labor
and applicator exposure to pesticides. However, if the end result is that you
loose the effectiveness of those pesticides, this could be a temporary saving.
More information is needed to assist growers with compatibility and other
concerns of tank mixes. Pesticide labels do contain more information about
mixing and resistance management than in the past, but growers still need to
take care to test pesticides and mixes on a few plants before using over a
large area.

About The Author

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

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