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Biopesticides: Categories and Use Strategies for IPM and IRM

Biopesticides contain active ingredients of natural or biological origin that include plant extracts, microorganisms, microbial metabolites, organic molecules, minerals, or other such natural materials that have pesticidal properties.  Pests such as herbivorous arthropods, pathogens, parasitic nematodes, mollusks, rodents, and weeds cause significant crop damage when they are not managed.  Pest suppression is a critical part of crop production to maintain plant health, prevent yield losses, and optimize returns.  As agriculture advanced from subsistence farming to a global enterprise, crop protection also evolved over millennia.  When farming was less organized, nature maintained a balance and provided solutions initially.  Then natural solutions were actively implemented until industrialization led to the use of synthetic inputs in the 20th century.  While synthetic fertilizers and pesticides contributed to a tremendous improvement in the yield potential, the indiscriminate use of some of them and the resulting damage to the environment and human health steered food production in the recent past towards organic farming with the use of nature-based solutions.

Although biopesticides have been around for a few decades, the growth of organic farming gave an impetus to the biopesticide industry during the past few years resulting in the development of new active ingredients and improved formulations.  Now, biopesticides are considered an important part of integrated pest management (IPM) strategies in both organic and conventional systems.  With a considerable industry investment in research and development, the quality and efficacy of biopesticides have also significantly improved.  This has also contributed to optimizing the cost of some formulations.  However, there is still a need to fill the knowledge gaps in biopesticides and their use.  Depending on the active ingredient, the mode of action for biopesticides, their target pests, their storage and handling, and the use strategies are quite diverse, and a thorough understanding of these aspects is critical for their successful use.  As emphasized in the new IPM model (Dara, 2019), while biopesticide use is an integral part of crop protection, understanding the pest biology, using biopesticides appropriate for the target life stage of the pest, applying them at the right time and rate using the right technology, avoiding incompatibility issues, building and sharing effective use strategies, and continuously investing in research and outreach are essential elements of biopesticide use.  Biopesticides also play an important role in insecticide resistance management (IRM) to address resistance issues associated with synthetic pesticides.  This article provides an overview of various biopesticide categories and general strategies for their successful use for IPM and IRM.

Biopesticides can be used for managing arthropod pests, bacterial or fungal pathogens, plant-parasitic nematodes, weeds, and snails and slugs.  Some formulations or active ingredients have multiple roles and can be effective against more than one category of pests.  While some active ingredients are very specific to a particular pest or related species, others have a broad-spectrum activity.  Based on the source, biopesticides can be placed in four broad categories: i) botanicals, ii) microbials, iii) toxins, and iv) minerals and other natural materials.

Botanical extracts: Plants are a rich source of numerous phytochemicals or secondary metabolites that have a wide range of properties including pesticidal activity.  Acids, alkaloids, flavonoids, glycosides, saponins, and terpenoids in plant extracts or oils obtained from seeds and other plant parts are some of the compounds present in various biopesticides (Pino et al., 2013).  Azadirachtin, BLAD (polypeptide from sweet lupine seeds), citric acid, essential oils, pyrethrins, soybean oil, and extract of the giant knotweed are used for their acaricidal, insecticidal, fungicidal, nematicidal, or herbicidal properties.

Microbials: Some of the microbial pesticides have live microorganisms (such as entomopathogens, Bacillus spp., Streptomyces spp., and Trichoderma spp.) while others (such as Burkholderia rinojensis and Chromobacterium subtsugae)have heat-killed microorganisms and fermentation solids as the active ingredients.  Entomopathogenic microorganisms [Bacillus thuringiensis (bacterium), Beauveria bassiana and Cordyceps fumosorosea (fungi), Heterorhabditis spp. and Steinernema spp. (nematodes), and granuloviruses and nucleopolyhedroviruses] primarily kill their hosts through infection; microbe-based fungicides antagonize plant pathogens through competitive displacement and production of toxic metabolites; nematophagous fungi parasitize plant-parasitic nematodes; and plant pathogenic bacteria, fungi, and viruses infect and suppress weeds.  Bacteriophages, which are viruses that parasitize bacteria, are used against the plant pathogenic species of ClavibacterErwiniaPseudomonasXanthomonasXylella, and other genera.

Toxins and other organic molecules: There are multiple examples of toxic organic molecules derived from various organisms.  Avermectins from the bacterium Streptomyces avermitilis and spinosad from the bacterium Saccharopolyspora spinosa, strobulurin from the mushroom Strobuluris tenacellus, and cerevisane from the yeast Saccharomyces cerevisae are some of the microbial toxins that are effective against insects, plant-parasitic nematodes, or snails and slugs.  A venom peptide from the Blue Mountains funnel-web spider, Hadronyche versuta, from Australia is a recently developed insecticide active ingredient with its unique mode of action class.  Chitosan, a polysaccharide from the exoskeleton of shellfish, is a fungicide.

Minerals and other natural materials: Diatomaceous earth, mineral oil, and minerals such as sulfur are used for controlling multiple categories of pests.  Potassium salts of fatty acids of plant or animal origin, known as insecticidal soap, have insecticidal and fungicidal properties.  Organic acids such as acetic acid and citric acid are derived from plants and have fungicidal and herbicidal properties.  Since these are different from other botanical extracts, they are placed in this category.

Except for the microbial pesticides that have live microorganisms, most biopesticides have chemical molecules of microbial, fungal, botanical, or mineral origin and work through various modes of action similar to synthetic pesticides.  Several synthetic pesticides are developed from natural molecules.  Abamectin, pyrethroids, neonicotinoids, spinetoram, and storbulurins are synthetic analogs based on avermectins, pyrethrins, nicotine, spinosad, and strobulurin, respectively, and were developed for improved stability, safety, or ease of commercial-scale production.

Integrated pest management and resistance management: Biopesticides are very diverse in their origin and mode of action and have been successfully used in several cropping systems for managing a variety of pests.  They have complex interactions with plants, soil microbiota, pests, and environmental conditions.  It is critical to have a good understanding of the source of biopesticides and how they act on their target pests.  Certain biopesticides may have special storage and handling requirements or tank-mixing restrictions.  It is essential to refer to the manufacturer’s guidelines or label instructions to avoid incompatible tank-mix combinations, understand proper application sequences, and to store, transport, and apply under unfavorable conditions.  While it is very important to use biopesticides as a part of the IPM program and tools for IRM, caution is warranted to avoid repeated use of the same or a similar type of biopesticide.  Pests can develop resistance to biopesticides just as they do to synthetic pesticides (Dara, 2020).

Strategies for using biopesticides: From the seed or transplant treatment to soil or foliar application, biopesticides can be used throughout crop production.  Certain combinations can have an additive or a synergistic effect on pest suppression.  At the same time, certain inputs or practices can negatively impact biopesticide efficacy.  For example, alkaline tank-mix components breakdown the protein coat of entomopathogenic viruses and Bacillus thuringiensis.  Botanical oils can be incompatible with cold water.  Some fungicides such as captan and thiram are incompatible with entomopathogenic fungi like Beauveria bassiana while several others are compatible (Dara et al., 2014).

Investing in biopesticides: Environmental safety and resistance development are two major concerns for excessive use of synthetic pesticides and incorporating biopesticides into IPM will help address both issues.  Substituting biopesticides for synthetic pesticides will reduce the total amount of the latter during a production season and their potential negative impact on the environment and human health.  Several biopesticides are not harmful to pollinators and in some production systems, pollinators are used to deliver biopesticides to the crops they pollinate.  Adding biopesticides to the standard crop protection program will also increase pest control efficacy.  Additionally, by not continuously using synthetic pesticides, the risk of resistance will be reduced and thus their efficacy will continue to be maintained.  Although some biopesticides can be more expensive than synthetic pesticides, investing in them will be a good strategy for both the short-term benefit of effective pest suppression and the long-term benefit of a healthy and resilient ecosystem.  Since pests do not have boundaries, area-wide implementation of good agricultural practices with a balanced use of synthetic and natural inputs is necessary for maintaining the productivity of the cropping systems.

Productive collaborations among the pesticide industry, researchers, extension educators, and the grower community are critical for successfully using biopesticides for sustainable food production.  While research helps to develop effective formulations and their use strategies, outreach helps with the implementation of those strategies. — By Surendra K. Dara, UC Cooperative Extension Advisor, Entomology & Biologicals

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