With HD cameras and the best views of athletic field playing surfaces, sports turf managers’ fields are inspected closely by many. These high visual quality expectations have increased our reliance on effective pesticides to manage playing surfaces. It is easy to continually use the same product or class of products when they have provided exceptional control of the pest in the past; however, this can lead to some very serious problems. Pesticide resistance is becoming much more common and prevalent in a number of our common turf pests including weeds, diseases, and insects.
How do pest populations become resistant?
All pesticides have a mode-of-action (MoA) that describes the biochemical process or physical cells the product will target in controlling the susceptible pest. Most pesticides are grouped based on the specific MoA with similar products in the same pesticide class. There are generally some individuals within a pest population that are naturally resistant to a pesticide class. It is nearly impossible to recognize these individuals when initial applications of an effective pesticide are applied according to label recommendations. However, if the same pesticide or a pesticide class is continuously applied for a given pest problem, selection pressure for the resistant population will continue to grow. The individuals that naturally carry the resistance will reproduce increasing the resistant population over time.
This process may take a number of years to become evident or it can happen very rapidly for pesticides that have highly specific MoA and pests that can produce multiple generations fairly quickly. Some highly specific pesticides have developed resistance within 1 to 2 years of consistent use for a pest.
When a pest population is dominated by resistant individuals, this is termed practical resistance. Once a pest population reaches practical resistance, the pesticide will no longer control that pest for a long period of time. Additionally, many pesticide resistant pests will exhibit cross resistance, which means the population is resistant to other pesticides within that class. There are also numerous turf pests that have developed multiple resistances. Multiple resistances occur when a pest population exhibits high levels of resistance to pesticides in two or more pesticide classes with different MoA.
The specificity of the pesticide MoA can alter the significance of resistance and reduce the time required for practical resistance to occur. There are two general terms used to describe pesticide resistance. Qualitative resistance occurs with pesticides that have highly specific MoA, potentially targeting a single gene or amino acid group within the susceptible pest. Think of qualitative resistance as a simple “yes” or “no” answer. An individual within the population will be resistant (Yes), and no amount of that pesticide will control the pest any longer. If you were to apply two times the rate of a product or shorten the interval between applications, there will be no injury on those resistant individuals.
Quantitative resistance occurs with pesticides that target multiple locations within a target pest, but still have some specificity to a biochemical process. Any pests that carry quantitative resistance may be controlled with increased pesticide rate or shortened application intervals allowable on the pesticide label.
Strategies to avoid pesticide resistance or manage potentially resistant populations:
Do not use any one product or mode of action exclusively
This type of action may quickly result in the development of a resistant population. Many of our current pesticides are highly effective and control an overwhelming majority of the target pest population. The lack of susceptible individuals remaining after application can provide greater selection pressure for the resistant population to expand quickly. You can quickly review the “Group” number on many pesticides to determine if they have different MoA or if they are the same. Pesticides with the same group number have very similar MoA and multiple applications should be avoided to the same target pest.
Be sure to follow the manufacturer’s label recommended application rate
Avoid using rates that are below the manufactures labeled rate to control the problematic pest. The rates on the label are developed from years of studies by the chemical companies and their partners to determine the most effective rates to manage the pests on the label. Applying lower rates can expose the pest population to the chemical but may not control the pests sufficiently enough, which could provide a greater number of individuals for reproduction and growth of the pest.
Use preventive applications for common pests based on prior experiences and environment
Once you have managed fields in your area for a number of years, you likely have a strong grasp on the common pests you are working with and when they will appear on your fields. Using that prior knowledge of environmental conditions or seasons when the pests become problematic, you can schedule preventive applications for those common pests. Managing the pest in this manner before the populations explode in size requiring curative applications can extend the livelihood of the pesticides you currently use.
Consider tank-mixing single-site mode of action pesticides with multi-site products
Tank-mixing various MoA into single products at purchase or mixing compatible pesticides on your own is a great way to manage populations that may have some resistance building up. The more specific MoA products will effectively control all the susceptible individuals in the population, but the alternative product can provide some detrimental effects to the resistant individuals to keep them in check as well.
Incorporate an integrated pest management (IPM) plan using cultural practices and other resources to inhibit disease formation
Pesticides are always going to provide short-term benefits when managing common turf pests. Even before any concerns of pesticide resistance may arise, the application of a pesticide should effectively control a target pest. However, the pest will normally come back at some point when the environment is favorable for it again. Longer-term control requires changing the management practices and environment to make the athletic field less desirable for the pest and more desirable for the turf growing on it. Some things are out of your control as a turf manager, but your cultivation and management practices can alter the environment to be less favorable for pests.
Joseph Young, PhD, is an assistant professor in the department of Plant and Soil Sciences at Texas Tech University, and the technical editor for SportsTurf.