Editor's note: This is the sixth, and final, article published in Greenhouse Management affiliated with pesticide resistance and resistance management of arthropod pests and diseases. The first three articles were published in 2019, (part 1, part two, part three) and the fourth and fifth articles were published in the 2020 March and June issues of Greenhouse Management.

Pesticides have been safely and effectively used for decades in greenhouse production systems.

In general, pesticides are easy and convenient to apply, and useful in terms of control, suppression, or management of arthropod pest and plant pathogen populations. Greenhouse producers regularly apply pesticides to alleviate or minimize plant damage from insect or disease pressure.

However, the continual reliance on pesticides can promote the development of resistance in arthropod pest and plant pathogen populations.

In this final article, we discuss how integrating biopesticides into rotation programs and pesticide mixtures can help in resistance management

Where biopesticides fit in an integrated approach

First of all, if you’re new to the subject of Integrated Pest Management (IPM), perhaps you are asking yourself: what are biopesticides?

Biopesticides are a classification of pesticides that are derived from natural materials, such as, plants, bacteria, animals, and certain minerals. Biopesticides are categorized into three major classes: microbial pesticides, plant-incorporated protectants, and biochemical pesticides.

Below is a detailed description of each of the three major biopesticide classes:

  1. Microbial pesticides: consist of a microorganism as the active ingredient (e.g. fungus, bacterium, virus, or protozoa), which is highly selective in activity against specific target insect pests.
  2. Plant-incorporated protectants: substances that plants produce based on genetic material that is incorporated into plants.
  3. Biochemical pesticides: naturally occurring substances that control insect pests by non-toxic mechanisms.

Microbial insecticides commonly used in commercial greenhouse production systems are based on bacteria and fungi. The difference between the two is that bacteria must be consumed by the targeted insect to cause death, whereas fungi can directly penetrate through the insect cuticle and initiate an infection.

The microbial insecticides currently registered for use in greenhouse production systems include:

Fungi

Beauveria bassiana Strain GHA (BotaniGard)

Isaria fumosorosea Apopka Strain (Ancora)

Metarhizium brunneum (Met52)

Bacteria

Bacillus thuringiensis subsp. kurstaki (Dipel)

B. thuringiensis subsp. israelensis (Gnatrol)

Saccharopolyspora spinosa or spinosad (Conserve)

Chromobacterium subtsugae Strain PRAA4-1T (Grandevo)

Why should biopesticides be considered for use in rotation programs and pesticide mixtures?

Well, there are two different modes of action associated with pesticides (insecticides and miticides): site-specific (narrow spectrum) and non-specific (broad-spectrum).

Arthropod pest populations can develop resistance faster to pesticides that have a site-specific (narrow spectrum) mode of action due their ability to metabolize or detoxify the active ingredient. However, biopesticides have non-specific (broad-spectrum) modes of action. Therefore, it is more difficult for arthropod pests to develop resistance to a biopesticide with a non-specific mode of action due to the diversity of target sites affiliated with inducing insect mortality. It is recommended to integrate site-specific and non-specific modes of action in rotation programs to alleviate selection pressure when using site-specific modes of action alone, which will preserve the effectiveness and longevity of currently available pesticides.

Pesticide mixtures that include biopesticides may help alleviate the potential for resistance.

For instance, using pesticide mixtures that include conventional pesticides with site-specific (narrow spectrum) modes of action with biopesticides (e.g. entomopathogenic fungus) that have non-specific (broad-spectrum) modes of action may mitigate the development of resistance. Furthermore, there are cases when mixing biopesticides with another active ingredient results in synergism (toxicity of a given pesticide is enhanced by the addition of a less or non-toxic pesticide).

For example, mixing Metarhizium brunneum (Met52) with a product containing azadirachtin (e.g. Azatin, Ornazin, and Molt-X) can result in higher mortality of certain insect pests, such as the western flower thrips (Frankliniella occidentalis), than when the two active ingredients are applied separately. Azadirachtin is an insect growth regulator, more specifically, an ecdysone antagonist, that inhibits or slows down (delays) the molting process. By delaying the molting process, the hyphae of an entomopathogenic fungus are able to penetrate the cuticle and initiate an infection before the insect pest sheds the old cuticle (ecdysis). This allows more time for fungal hyphae to initiate an infection, which can enhance efficacy and death of an insect pest.

Plant pathogens

Resistance management of fungicides using different mode of action (MOA) groups is one of the common issues encountered by greenhouse producers. Research has shown that rotations and tank-mixing are effective in mitigating resistance development in plant pathogen populations.

There are characteristics of biopesticides that make them ideal candidates for use in rotation programs including: providing better control, and delaying resistance from developing in plant pathogen populations.

In our opinion, the ideal number of fungicides to use in a rotation program are two to three, because using more than three can make it difficult to know which active ingredients worked, which ones failed, or which ones damaged your crop. If you are producing crops organically, the number of choices is substantially limited.

However, you should still know how to incorporate biopesticides into rotation programs. Using a biopesticide that is alive provides distinct advantages. The Trichoderma spp. in the product RootShield Plus has at least four modes of action:

  • Exclusion: occupies space on plant roots and absorbs nutrients normally available to fungal pathogens.
  • Shielding: attacks and consumes fungal pathogens.
  • Antagonism: releases anti-pathogen substances and creates a zone inhospitable for fungal pathogen development.
  • Induced Host Resistance: stimulates accumulation of defensive compounds that ward-off fungal pathogens.

It is also important to consider that we are learning conventional products are not single action products. In fact, many stimulate a diversity of host resistance mechanisms.

With that in mind, here are some factors to consider before adding a biopesticide to conventional rotation programs:

1. What is the disease target?

If you have Pythium root rot and use a product that only works on Rhizoctonia; you have wasted your money and will probably have to “dump” the crop.

2. What are the best conventional and biopesticide products for a specific disease?

If you use a product that is marginally effective, this will reduce the efficacy of the entire rotation program, even if you use highly effective conventional products. Remember, there is no single product that is effective on all plant pathogens.

3. Is the biopesticide alive?

If you use a living fungus or bacterium and rotate with a fungicide that kills fungal plant pathogens then control will be diminished. However, if the biopesticide (even if biological) does not need to be alive to work, then you do not have to be concerned about killing any biological organisms with the biopesticide. Be sure to check with the supplier to determine if the biological organisms are sensitive to bactericides or fungicides used in your rotation program.

Years ago, we were informed by a well-known biopesticide manufacturer that the Bacillus (bacterium) in the product, Marrone BioSolutions’ Cease, was not sensitive to copper and could be tank-mixed with copper without influencing efficacy. In addition, the chemicals the Bacillus in Cease synthesizes during production will work with or without a living bacterium. Therefore, the tolerance of copper and efficacy of the chemicals produced are responsible for the efficacy of Cease.

4. Is the biopesticide sensitive to specific environmental parameters?

The product BotryStop, which contains the active ingredient, Ulocladium oudemansii, must be stored under refrigeration. The active ingredient is killed if the product is stored under conditions that result in exposure to extreme temperature swings. Check the website of the manufacture for information on proper storage conditions and sensitivity to other pesticides.

Some biopesticides; especially those containing biological organisms, must be used before pest pressure is high. Sometimes, applying a conventional product first in a rotation program followed by a biopesticide can result in optimal control (e.g. fusarium wilt and cyclamen). In addition, a biopesticide and conventional fungicide mixture may provide better control than when used in a rotation program.

In any event, considering the difficulty in controlling plant diseases and pests in the greenhouse, why not use all effective IPM tools available to you?

Raymond A. Cloyd is a professor and extension specialist in horticultural entomology/plant protection at Kansas State University. Reach him at rcloyd@ksu.edu. A. R. Chase is president of Chase Agricultural Consulting. Reach her at archase@chaseresearch.net.