Photos courtesy of Raymond A. Cloyd

Pesticides (insecticides, miticides, fungicides and bactericides) are used to manage arthropod (insect and mite) pests and diseases (fungi and bacteria) in greenhouse production systems. In general, pesticides are easy and convenient to apply, and effective (in terms of control or management). A number of factors affiliated with greenhouses favor pest population growth, including environment (e.g. temperature, relative humidity and photoperiod), cultural practices (e.g., watering and fertility) and continuous food source (many plants in the greenhouse spaced close together). Consequently, greenhouse producers apply pesticides to prevent or minimize plant damage. However, relying continually on pesticides promotes the development of resistance in insect/mite pest and plant pathogen populations.

This is the fourth article in a series of six scheduled for publication in Greenhouse Management associated with pesticide resistance and resistance management of arthropod pests and diseases. Three articles were published in 2019. In this article, we discuss the importance of developing pesticide rotation programs to mitigate insect and mite pest, and plant pathogen populations developing resistance.

Arthropod pests

Pesticide rotation is the temporal (time) alternation of pesticides with different modes of action. If the same mode of action is used repeatedly, this will result in increased selection pressure on an arthropod pest population, which enhances the rate (speed) of resistance development. Consequently, reducing selection pressure by using pesticides with discrete modes of action can increase the longevity and preserve the effectiveness of pesticides.

What is mode of action? Mode of action is how a pesticide, such as an insecticide or miticide, negatively affects the metabolic or physiological processes of an insect or mite pest. The Insecticide Resistance Action Committee (IRAC) has grouped insecticides and miticides based on mode of action using number and letter designations (irac-online.org), which will help in developing sound rotation programs against insect and mite pest populations in greenhouse production systems. Also, be sure to rotate common names (active ingredient), not trade or brand names, since some products have different trade names but contain the same active ingredient.

Exclusive reliance on one or more pesticides, in this case, insecticides or miticides with similar modes of action to suppress a particular pest population, will result in selection pressure placed on that pest population, which will increase the rate of resistance development. Therefore, always rotate pesticides with different modes of action. For example, use one pesticide for the duration of one generation of an insect or mite pest population (approximately two to three weeks), then rotate or switch to another pesticide with a completely different mode of action. Do not rotate pesticides with different modes of action within a single pest generation. As part of a resistance management strategy, many pesticides contain label information that is designed to assist greenhouse producers in developing sound rotation programs. The label indicates that one or two applications can be made before a pesticide with a different mode of action must be used.

Do not use pesticides with site-specific modes of action exclusively. Pesticides with site-specific modes of action target a single mechanism or biological pathway in insects and mites. Consequently, these pesticide types are more susceptible to pests developing resistance. However, pesticides with multi-site modes of action are less susceptible to pest populations developing resistance. Pesticides with multi-site modes of action include: insecticidal soap (potassium salts of fatty acids), horticultural (petroleum, mineral, or neem-based) oils, selective feeding blockers and entomopathogenic fungi (Beauveria bassiana and Isaria fumosorosea).

Furthermore, be sure to rotate insect growth regulators with different modes of action because certain insect pests (e.g. aphids and whiteflies) have developed resistance to a number of insect growth regulators. There are three modes of action associated with insect growth regulators: chitin synthesis inhibitors (buprofezin, diflubenzuron and novaluron), juvenile hormone mimics (kinoprene and pyriproxyfen) and ecdysone receptor agonists/antagonists (azadirachtin and methoxyfenozide).

However, it is important to understand that the rotation of pesticides assumes that the frequency of individuals resistant to one pesticide will decrease during the application of another pesticide — with a different mode of action. In addition, rotation programs will need to be adjusted during the growing season. For example, as temperatures increase in late spring through early fall, the development time or the duration of the life cycle (egg to adult) of insect and mite pests will be reduced. High temperatures (>75° F or 24° C) will shorten the development time, thus leading to overlapping generations. The simultaneous presence of different life stages (eggs, larvae/nymphs, pupae and adults) warrants more frequent pesticide applications and rotations. Therefore, rotation programs will need to be modified by decreasing the frequency of switching pesticides with different modes of action. However, during the winter, the cool temperatures (<65° F or 18° C) and shorter day lengths (<12 hours) will extend the development time, thus reducing the frequency of rotating pesticides.

Plant pathogens

Rotation is one way to reduce the chances of fungicide or bactericide resistance developing in plant pathogen populations. The alternative is using pre-mixes or tank-mixing. However, pre-mixes and tank-mixing only work when both active ingredients are effective against a particular plant pathogen. A combination of fungicides in the Fungicide Resistance Action Committee (FRAC) groups 7 and 11 can be effective against some diseases, such as powdery mildew, Fusarium and rust. In other cases, like the combination of etridiazole and thiophanate methyl (Banrot), where both active ingredients are not effective on “anything,” this is an additive mixture with a broad-spectrum of control.

Tests on fruit and vegetable crops have indicated that rotation or tank-mixing are equally effective in delaying resistance development. Many greenhouse growers seem to prefer using pre-mixes (formulations from the manufacturer that contain multiple active ingredients) because there are so many effective pre-mixes commercially available.

One drawback of tank-mixing is the expense of incorporating two fungicides into a tank for resistance management, which is more controversial than simply rotating fungicides with different modes of action. Using pre-mixes may be preferred because the manufacturer has already determined chemical compatibility.

It is also important to utilize biopesticides (beneficial fungi and bacteria) in a resistance management program. Many biological control agents have more than one mode of action. For example, Trichoderma harzianum T-22 demonstrates the following: systemic acquired resistance (SAR) where the biological control agent triggers the plant’s defensive system, which helps to ward off diseases; promote changes in the soil biological profile that negatively affect plant pathogens; direct parasitism of pathogenic fungi and inhabiting spaces on plant roots that crowds out the plant pathogen. These different modes of action make resistance development much less likely than using a single mode of action fungicide with specific targeted effects.

Bacteria present an interesting challenge in ornamental production since there are few conventional modes of action that are effective on bacteria. Interestingly, in many cases, some biopesticides provide equivalent control as conventional bactericides. These include Bacillus spp. and several plant extracts. If products (such as those with Bacillus spp.) synthesize many chemicals during production, they may not have to be alive, which would influence their effectiveness. In addition, they may be tolerant of copper and can actually be tank-mixed.

Many greenhouse growers seem to prefer using pre-mixes (formulations from the manufacturer that contain multiple active ingredients) because there are so many effective pre-mixes commercially available.

Would you want to use two biological control agents in a tank-mixture? I do not think so, since they are living organisms and there is the possibility they will compete with or even attack other biological control agents. Always follow manufacturer labels carefully concerning mixing or rotating biopesticides (especially biological control agents).

As with Raymond’s discussion above regarding frequent rotations using different modes of action due to multiple life stages present, this is also good strategy for plant pathogens. In this case, the reason is not different live stages, but the very short life cycles associated with bacteria and many fungi (hours to days).

Additionally, the fact that “individuals” are in general all similar genetically (due to asexual reproduction=clones) means that once resistance develops in a small portion of the population, resistance can spread throughout a population very rapidly. Furthermore, resistance to a fungicide or bactericide can result in lack of vigor in the plant pathogen. In other words, plant pathogens pay a price for developing resistance. Consequently, they will not survive as well as the original population and eventually, if the fungicide or bactericide is no longer used, the original plant pathogen population can revert back to being susceptible. The end result is that plant pathogens are no longer resistant to the active ingredient, thus allowing the active ingredient of the fungicide or bactericide to be used again.

In conclusion, resistance management is a strategy designed to preserve the effectiveness of currently available pesticides. Therefore, it is important to develop rotation programs that include an assortment of pesticides with different modes of action when managing insect and mite pests and plant pathogens. In addition, biopesticides should be included in rotation programs, especially if they demonstrate effectiveness by themselves.

Raymond A. Cloyd is a professor and extension specialist in horticultural entomology/plant protection at Kansas State University. You can reach him at rcloyd@ksu.edu.

A. R. Chase is president of Chase Agricultural Consulting. You can reach her at archase@chaseresearch.net.