by Erin Hodgson
Every year, farmers seek ways to become more efficient in their operation. Tank-mixing pesticides, or combining multiple products in a sprayer tank, is a common practice in the Midwest.
But just because we can do something doesn’t mean we should be doing it. An example of this is making a Jello salad with carrots. Does tank-mixing pesticides make sense — or cents — in field crops?
Benefits exist
At first glance, reducing the number of times equipment has to run through the field is convenient. Combining products in a foliar application means fewer trips through a field. The short-term benefit is reduced plant injury from equipment, especially later in the growing season.
The long-term gain to this strategy is reducing soil compaction, which can affect soil porosity, infiltration and aeration. There also is an immediate cost savings when spending less on fuel and people power.
When is the last time you saw just one species of weed or insect pest in the field? Ideally, a tank-mix application could target multiple pests that are active at the same time.
In reality, multiple pests overlap spatially and temporally in every field each year. A common practice that makes sense is using multiple herbicide active ingredients to target grasses and broadleaf weeds that are present in the spring. Unfortunately, multiple modes of action applied together are sometimes needed because genetic resistance develops in a pest population due to frequent exposures.
Here’s the downside
There are several environmental and financial reasons not to tank-mix in field crops. While tank-mixing multiple modes or sites of action of herbicides or fungicides is common to target multiple weed or disease pests, it is challenging to jointly target insect, disease and/or weed pests at their most susceptible timings with a single application.
Every pesticide application is an exposure to target and nontarget pests, as well as an opportunity for that pest to develop resistance. Some pests can quickly adapt to exposures by tolerating or breaking down the active ingredient, especially from poorly timed applications. It is happening and intensifying for weeds, plant pathogens and insects in field crops across the world.
Minimizing applications will prolong the efficacy of available and affordable pesticides. The keystone of a sustainable pest-management plan is to be aware of pest activity and yield loss potential. This concept is especially true for insects.
Understanding the true cost of an application (dollars per acre) will help determine if an application is justified based on future market values (dollars per bushel). The goal of a foliar insecticide application is to protect yield and, hopefully, the effort results in a profitable choice.
Insecticides offer no value to the plant except to knock down pest activity. Thus, there has to be a certain number of pests present to meet an economic threshold. The injury potential varies by pest, as not all are created equal.
Prophylactic tank-mixed sprays often miss the mark for insect suppression because they are applied too soon. A typical example in the Midwest would be applying a foliar fungicide plus insecticide tank mix at corn silking to reduce adult corn rootworm abundance. Most insecticides used in corn and soybean kill by contact or ingestion. Do not rely on residual activity to fend off feeding injury. The odds of breaking even on an insecticide applied too soon are small compared with a well-timed treatment.
In addition to the presence or absence of a pest, other important considerations for tank-mixing pesticides are product compatibility and mixing order. Just because we can put multiple products together doesn’t mean the result will be successful.
Be aware of synergistic crop damage, antagonism and physical issues with mixing chemistries. Finding sludge or sediment or seeing phytotoxicity is a negative consequence for application mishaps.
Plus, the equipment cleanup for incompatible products can be time intensive. Mixing a jar test before a full tank of a new tank mixture is best practice to assure physical compatibility. However, it cannot quickly ensure that no chemical synergism or antagonism will occur.
Drawbacks often outweigh benefits
There will always be some insects in field crops, but the damage is not always measurable in bushels per acre. For persistent field crop insect pests, understanding when and where to scout is essential for making management decisions. Historical records also will help provide context for pest fluctuations over time.
Tank-mixing can be an effective tool, but it should be considered carefully, as the detriments often outweigh the benefits.
Hodgson is an Iowa State University Extension entomologist.