Pests are a given on a golf course. While it may be tempting to think of certain golfers as pests – volumes can be written on this subject – the real ones are the lower-order organisms that cause a disruption in the aesthetic appeal and functional aspects of golf play.

A number of materials have been used over the years to control pests. The first fungicide was used on grapes in France many years ago, when a mixture of hydrated lime and copper sulfate was sprayed on the vines to discourage pilfering of the fruit by the local townsfolk. After application, the growers noticed a significant decrease in the fungus disease powdery mildew. A healthier and more bountiful crop was harvested, and the rest is history.

When it comes to golf turf, we need to look closely at both successes and failures in pest control. In some cases, the failures are due to the resistance of certain insects, diseases and weeds to the applied pesticide. In other instances, nonpesticide factors are responsible. Sorting through these causes provides insight into overall successful pest management.

Aerification is an important cultural practice that can improve drainage and lessen the need for repeated fungicide applications.


The topic of pesticide resistance can be discussed in many ways. Regardless of the approach, an important first step is to know more about the makeup of the chemistry of the potential applied products. As most superintendents are well aware, pesticides have three names: chemical, common and trade. For example, the three names for a commonly used fungicide are:

  • methyl (E)-2-{2-[6-(2-cyanophenoxy) pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate (chemical),
  • azoxystrobin (common), and
  • Heritage (trade).

In addition, pesticide products are grouped by their mode of action: contact, penetrant or systemic.

Scientists that study these modes of action for fungi, insects and weeds classify them into groups to provide information that can be used to slow resistance to various products. Fortunately, most if not all of this information is present on the product labels; it may be a bit hard to find and require reading glasses to see it, but like the tomato sauce, Prego, “It’s in there” – as are many other helpful guidelines for delaying or preventing resistance.

Modes of action and sites

In terms of resistance, one of the most important distinctions is whether a product is active via single or multisite points of contact with the pest. Modes of action vary widely, attacking various protein sites in metabolic pathways. In general, single-site pesticides bind to specific proteins in cells and have a higher tendency for resistance. Multisite pesticides bind to a greater number and variety of sites and therefore have a lesser tendency for resistance, although both are certainly susceptible.

Crabgrass and goosegrass are prolific seed producers.

Tolerance and resistance

Some turf professionals view the terms “tolerance” and “resistance” as synonyms, but they really aren’t. Tolerance is an inherent capacity of a species to survive and reproduce after a pesticide treatment. No reaction to management, selection pressure or genetic manipulation is involved in making the plant tolerant. It just naturally is.

Other factors can be confused with tolerance, including the results of:

  • excessive or inadequate product rate,
  • lack of required additives,
  • antagonism of one or more active ingredients in a combination application,
  • photodegradation,
  • phytomobility (not getting the chemical to the necessary area, such as when grub-control products get hung up in the thatch rather than reaching the soil),
  • missed areas or gaps in coverage,
  • application timing,
  • sprayer calibration,
  • formulation calculations, and
  • pest identification.

Each of these failure factors can be confused with tolerance, or be responsible for partial control of the pest, with the pest indifferent to the application to some degree.

Resistance, on the other hand, occurs when a whole or a significant part of a pest population survives a previously lethal application of a pesticide. An example of resistance would be when 2,4-D no longer controls plantains or dandelions. When considering tolerance and resistance occurrences, it’s helpful to consider mode of action and application rate.

Genetic changes play a role in the issue of tolerance and resistance as well. Mutations – spontaneous changes that occur on their own – are usually sporadic and rare, but do occur from time to time. Mono-resistance conferred by changes in a single gene, and poly-resistance from several genes, could limit uptake, detoxify applied materials, or develop alternative pathways for continued existence. Overall, resistance becomes noticeable when mutants begin to dominate the playing surface.

Selection pressures

Resistant populations are often selected by management practices that offer insufficiently diverse strategies and encourage the development of uncontrollable pests. Cultural practices such as maintaining an extremely low mowing height, keeping the root zone overly dry or overly moist, inadequate compaction relief, and poor air circulation commonly lead to a reliance on pesticides.

Repeated applications of products with the same modes of action (such as DMIs, QOIs and dinitroanilines) is another selection pressure. To the extent possible, rotating pesticide classes will reduce pressure and the tendency toward resistance.

For example, more consideration should be given to the calendar-based approach of application on historic dates regardless of pest pressure. While certain pests such as grubs and crabgrass are pretty much year-in and year-out pest, applications made by the day of the month – rather than growing degree days and moisture conditions – tends to increase pressure. Strictly reactive pest-control measures can also lead to resistance because they often allow pests to reach large numbers or advanced life cycle stages before applications are made.

Population size is a difficult but important selection pressure because preventing large outbreaks of infestations such as grubs, goosegrass, chinch bugs and brown patch can significantly reduce reliance on pesticide applications.

Reducing leaf wetness lowers disease pressure and overall need for applied fungicides.

The price of resistance

Although it’s an understatement, the costs and negative outcomes associated with pesticide resistance are quite concerning. Depending on how you count, at least four exist, both tangible and intangible.

First, the likely result of resistance to a particular product is that a higher rate is required to achieve the same result as before. A shorter interval of control is also a common outcome. Higher rates and shorter intervals lead to increased overall costs due to the greater number of applications needed to do the job.

Second, a common drawback is the complete loss of previously effective products. Once they are unable to control the target, they’re usually deemed useless.

After this loss, the result is fewer products on the market, which leads to an increase in the potential of resistance to more than one mode of action, regardless of whether a single or combination product was applied.

Finally, cross- or multiple-resistance can occur, in which a lack of capacity develops for all products within the same class of chemistry.

Scouting and early detection are important strategies.

Weed specifics

Although there is overlap or commonality between groups of pests in terms of resistance, each also has its own set of specific considerations. Due to shading and competition from turfgrasses, there are naturally fewer weeds present in a turfed sward, which reduces the overall population and lessens the need for herbicide applications. Fortunately for superintendents, the goal of management is to maintain a healthy, dense stand of turf, unlike field crop agronomy, in which bare soil conditions are often present for parts of the year.

Certain weeds, such as annual bluegrass, goosegrass, smooth crabgrass, spotted spurge and annual sedge, are prolific seed producers, which is another factor in building the local population. As a result, attention should be focused on these pests to prevent buildup and the need for future herbicide applications. When necessary to control these types, rotating the classifications of herbicides designated by the Weed Science Society of America’s Herbicide Resistance Action Committee is helpful in delaying resistance.

Disease specifics

As with weeds, pathogens that produce many spores or are active throughout the growing season pose a greater risk than those that are not as prolific or favored during certain environmental conditions. Dollar spot, microdermium patch, anthracnose, grey leaf spot and several of the Pythium species are more prone to resistance than others. The cultural practices of appropriate turfgrass species/cultivar selection, reduction of leaf wetness, moderate cultivation and low-to-moderate fertilization rates are very helpful to increased efficacy and reduced resistance. Rotation of fungicides between classes designated by the Fungicide Resistance Action Committee, such as QOIs and DMIs, is also a strong recommendation.

Rotation amongst mode of action classifications is helpful in delaying resistance.

Insect specifics

Selection pressures by inappropriate application of insecticides allows pre-adapted, genetically predisposed insects to become dominant. There are also concerns about older and more recently developed insecticides. Older products tended to be quite toxic to beneficial insects and mammals, while the newer products are commonly single-site-active, leading to a greater tendency for resistance. Furthermore, recently developed products may not penetrate the cuticle of the insect body as well as older products, such as DDT and the organophosphates.

Some insects are also able to break down and excrete the active ingredients of recent insecticides, or change the sensitivity of the target site of the product. This is problematic because small changes in the way that toxins interact with the target site can lead to resistance.

As with the other pest groups, entomologists have made recommendations to prevent resistance: proper plant care, selection to reduce the need for applied insecticides, rotating the various modes of action and making sure that the formulation reaches the site of pest activity.

Overall, using the recommended label rate as opposed to half doses is another technique that will lessen the likelihood of control failure and tendency for resistance.

Integrated strategies

Considering the multitude and magnitude of costs associated with resistance, avoiding or at least delaying it is good common sense. Controlling the size of a particular population is a particularly effective step because smaller populations can often be managed with nonchemical strategies.

Reconsideration of the threshold could also delay resistance in certain pests. Scouting before and after a pesticide application will make you aware of the success of a control measure, especially in relation to previous attempts and location on the golf course. When performing post-application scouting, evaluate and document weed escapes, as well as insects that were not controlled and diseases that were not suppressed. This information is useful, especially when evaluated in context of the cultural conditions before, during and after the treatment attempt.