Whitepaper: Microbial Pest Control—Is the Best Yet to Come?

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Naturally occurring pest control has intrigued scientists for thousands of years. The earliest known reference to diseases of insects is from the writings of Aristotle over 2,300 years ago. Throughout history, many scientists have expanded our knowledge of insect diseases and laid the foundation for the use of these diseases in pest management.

Modern scientists have expanded the knowledge about microorganisms that will control not only insects, mites, spiders, and nematodes but also bacterial, viral, and fungal diseases. These microbial pest controls include bacteria, fungi, viruses, nematodes, protozoa, and other microorganisms that impact populations or control pests.

While scientific advancements continue the exploration, is the best yet to come? The journey already has been long, yet fruitful; but new technologies create a high level of promise for the future.

The development and understanding of pathology and physiology of both insects and plants are fundamental to successful use of microbial pest control. This evolution faces many challenges in current crop production practices. Obstacles range from insect and weed resistance to product formulation, production scalability, shelf-life, application timing, and, of course, environmental conditions. Despite the lengthy scientific investigations into microbial pest controls, successful role models do exist.

Bt: Bacteria use as a Microbial Pest Control

Early development of the concept of microbial control was dominated by research on diseases of two important beneficial insects of the time, silkworm and honeybee. While working to control a problem, scientists realized a valuable use for the cause of the problem—a soil-dwelling bacteria known as Bacillus thuringiensis (Bt).

In 1901 a Japanese biologist, Shigetane Ishiwatari, was studying wilt disease in silkworms. His is the earliest record of the isolation of this Bt bacteria from an infected insect.

Between 1901 and 1938 researchers explored the mode of action that resulted in Bt’s insecticidal action. Bt produces proteinaceous crystals that are potent insect toxins. The conditions of how and when these toxins are produced were identified and the result was the realization of the insecticidal potential of Bt. Further basic research revealed that different strains of Bt produced toxins that were active against different groups of insects and each toxin had a very narrow host range. Some Bt strains produce insect toxins and others produce toxins effective against nematodes.

While manufacturers struggled with early formulations for commercial products, excellent performance was achieved with improved application and timing. The performance of microbial control products has continued to improve due to advancements in fermentation and harvesting procedures, identification of new and more active strains, and rigorous quality control. These have contributed to Bt products becoming the most developed of all microbial control products. In fact, Bt is the most commonly used biological pesticide worldwide. However, an apparently unlimited number of strains and extractions are yet to be discovered and developed.

Ongoing Discoveries

Currently 20 of the 50 Bt strains listed as active ingredients in the United States Environmental Protection Agency (EPA) product database are in registered pesticide products. These 20 strains represent five subspecies of Bt and a total of 346 active product registrations. After 100 years of research, new strains of Bt with novel pest activity are being identified regularly. There is no end in sight. Advanced technology currently allows the identification of new strains faster and more efficiently; the search for new strains through collections of wild populations is taking place at an accelerated rate.

In addition to the discovery of new strains, further characterization of known strains will continue to be identified. Better formulations and more efficient production methods will likely elevate many microbial control agents to commercial acceptability. Improvements in our understanding of the ecology of target insects and their pathogens, together with the optimization and use of genetic tools, will continue to contribute to the development and optimization of microbial control measures.

Further development will need to be supported and promoted by various stakeholder sectors to facilitate the market reach of these environmentally friendly solutions. The rapidly growing public interest in sustainable production practices is creating a highly receptive marketplace around the world.

AgriThority® is experienced at testing hundreds of different species of microbial control agents. Through our knowledge and expertise, we partner with technology developers throughout the entire development process. Our business, market and product development services, with regulatory support, covers the U.S. and many other countries around the world. Our expert network and services help move innovative and biological products across borders. This includes logistics for compliance with specific import and release permit requirements, as well as shipping and customs clearance management, protocol design, and implementation; all of which is supported by business and market development consultation. We are proud to play a role in transforming production practices toward sustainable and eco-friendly food systems, not only for the planet but also for all individuals who are part of it.

Read on to learn more about the evolution of microbial pest control agents.

Authors

Evolution of Microbial Pest Control—The Best is Yet to Come

By Richard Shaw, Eleonora Da Riva, and Dr. Paula Prieto

Safe and effective pest management has long been a goal of crop protection. Civilizations have struggled to protect themselves and their food supplies from pest attacks for thousands of years. The earliest known reference to diseases of insects is from the writings of Aristotle over 2,300 years ago. Throughout history, many scientists have expanded their collective knowledge of insect diseases and laid the foundation for the use of these diseases as a pest management strategy.

The first attempt to use applied microbial control with bacteria was in the early 1900s when a bacterium, Coccobacillus acridiorum was applied for control of a grasshopper, Schistocerca americana in several Latin American countries with mixed results.

Over time, microbial pest control developed to include known microorganisms that will control insects, bacterial, viral, and fungal diseases, as well as mites, spiders, and nematodes. Microbial pest control is a subcategory of the practice of biological pest control, which is the use of naturally sourced pest control products. Microbial pest control organisms include bacteria, fungi, viruses, nematodes, protozoa, and any other microorganism that can be used to control pests. Biological control methods extend beyond microorganisms to include a variety of pest management strategies, such as the use of predatory insects, plant extracts, naturally occurring minerals, and synthetically produced versions of naturally occurring chemical compounds. Each category of biological control has its own fascinating history. However, this article will concentrate specifically on microbial pest control, exploring its evolution, current applications, and potential for future innovations in sustainable agriculture.

Bt: A Success Story Using Bacteria as a Microbial Pest Control

Early development of the concept of microbial control was dominated by research on diseases of two important beneficial insects of the time, silkworm and honeybee. While working to control a problem, scientists realized a valuable use for the cause of the problem—a soil-dwelling bacteria.

The earliest record of the isolation of Bacillus thuringiensis (Bt) from an infected insect was in 1901 by a Japanese biologist Shigetane Ishiwatari studying wilt disease in silkworms. He named it Bacillus sotto and it was later renamed Bacillus thuringiensis by a biologist, Ernst Berliner, who isolated it from a diseased Mediterranean flour moth in Germany. Around 1927, another German worker, Mattes, isolated the Bt strain discovered by Berliner and conducted field studies against the European corn borer.

Between 1901 and 1938 researchers explored the mode of action that identified the Bt insecticidal action. It was observed that Bt produces proteinaceous crystals that are potent insect toxins. The conditions of how and when these toxins are produced were identified and the result was the realization of the insecticidal potential of Bt. Further basic research revealed that different strains of Bt produced toxins that were active against different groups of insects and each toxin had a very narrow host range. Some Bt strains produce insect toxins while others produce toxins effective against nematodes. Bt strains toxic to other invertebrates are not yet developed as economic control measures.

The corn borer trials by Mattes showed promise and, by 1938 led to the development of the first commercial Bt insecticide in France. The first registration of Bt in the U.S. was in 1961 after many years of testing formulations that didn’t work. Although by the late 1960s when formulations were much improved, manufacturers still struggled with dry and dusty powders that were susceptible to lumping when mixing. But excellent performance was possible when the application and timing were right. Other advancements in the performance of microbial control products include improved fermentation and harvesting processes, the identification of new, more active strains, and rigorous quality control measures. While Bt products are the most developed among microbial control solutions, there still appears to be an almost limitless potential for discovering new strains and extractions.

Currently fifty Bt strains are listed as active ingredients in the United States Environmental Protection Agency (EPA) product database. Many of these are registered for use in genetically modified (GMO) crops or are not currently being used in a registered product. Twenty strains are used in registered pesticide products. These twenty strains represent five subspecies of Bt and a total of 346 active product registrations. After 100 years of research, new strains of Bt with novel pest activity are being identified regularly. There is no end in sight. Advanced technology currently allows the identification of new strains faster and more efficiently, and the search for new strains through collections of wild populations is accelerated.

In addition to the discovery of new strains, further characterization of known strains will continue. Better formulations and more efficient production methods will likely elevate many microbial control agents to commercial acceptability. Improvements in our understanding of the ecology of target insects and their pathogens, together with the optimization and use of genetic tools, will continue to contribute to the continued development and optimization of microbial control measures. Development will need to be supported and promoted by various sectors to facilitate the market reach of these products, where demand for more environmentally friendly solutions has increased, along with growing interest in organic and sustainable production practices.

Fundamental to the development of microbial control of insects has been the expansion of scientific understanding of pathology and physiology of insects and plants. One of the most noted pioneers and historians of insect pathology and biocontrol was Dr. Edward A. Steinhaus, credited by much of the scientific community as the father of insect pathology. In 1956, Steinhaus wrote “Microbial Control—The Emergence of an Idea” in Hilgardia, published by the California Agricultural Experiment Station. At the time of his death in 1969, he was working on a book on the history of insect pathology. His manuscripts were posthumously published in 1975 by the Ohio State University Press. The work is titled “Disease in a Minor Chord” and is one of the most complete accounts of the beginnings of the science of biological control, predominantly microbial control, of plant pests through the middle of the 20th century. Steinhaus’s publications were relied on for much of this summary.

The Role of Fungi in Biological Control of Insects Pests

Throughout the latter part of the 19th century and the first half of the 20th century, many efforts to control important insect pests biologically were with fungi. These included attempts in Ukraine from 1879-1884, to control sugarbeet curculio (beet weevil), Cleonus punctiventris, with a fungus, Metarhizium anisoplia, and from 1888-1902, programs targeting chinch bugs, Blissus leucopterus, in the US Midwest with the fungus Beauveria bassiana.

Fungi are the most visible of insect diseases and in some cases the most virulent, causing massive die-off of insect populations. The positive and negative results with entomophagous fungi have affected the approach to all microbial control efforts. In recent years, an increased understanding of critical population densities for fungal outbreaks has made the manipulation of natural populations of fungal pathogens a critical part of some integrated pest management programs.

In 1835 an Italian bacteriologist, Augustino Bassi, was able to artificially transmit the fungus Beauveria bassiana to silkworms and other species. The fungus is common in soil and parasitizes many different species of insects. Currently, seven different strains of B. bassiana are registered by US EPA in numerous insecticide products.

A decade or more after the work of Bassi, the famous French microbiologist, Louis Pasteur, known primarily for his accomplishments in germ theory of disease, development of vaccines for rabies, cholera, and Anthrax, and the process of Pasteurization, also studied diseases affecting the silkworm industry in France. He is credited in Steinhaus’s writings with “initiating the real scientific development of insect pathology”. It is also observed that Pasteur’s work in insect pathology contributed to his historical advancements in the understanding and prevention of human disease.

The work of Bassi, Pasteur, and numerous other scientists across the globe intensified to define control methods for different pests by using microbial pathogens. Many insect/pathogen relationships were identified from these various works. More importantly, each effort revealed original information on host and pathogen interactions with the environment that led to the eventual commercial success of this approach to the management of insects and other pests of crops.

Interest in microbial control of insects grew rapidly in the United States as evidenced by the writings of J. L. Leconte. He was a famous and influential MD by training and entomologist by his life’s work. In 1875 he proposed an overhaul of economic entomology in the United States and included microbial control as an alternative that should be developed. This is believed to be the first formal, published proposal for the use of microbial pest control.

The Role of Fungi in Biological Weed Control

Plant pathogens were also introduced in 1970 as bioherbicides. Biological control of weeds involves the intentional use of living organisms, known as mycoherbicides, to reduce the vigor, reproductive capacity, density, or impact of weeds, positioning them as a potential alternative to chemical herbicides. Several selected microorganisms have been extensively studied, and many have been developed or are under development for commercial use.

Early mycoherbicides primarily relied on highly virulent fungal pathogens that infected the aerial parts of weeds, resulting in visible disease symptoms. Fungal genera such as Colletotrichum, Phoma, and Alternaria have received significant attention as promising microbial herbicide candidates. However, despite progress, little is known about host-pathogen interactions in natural plant communities, and even less is understood about weed-pathogen dynamics specifically.

Most of the existing research on classical biological weed control with plant pathogens is on the -Puccinia chondrillina. This relationship involves an apomictic plant population and an overly specific rust pathogen, which highlights the complexity of achieving broader applicability with bioherbicides.

Successful incorporation of bioherbicides into conventional agriculture will only be achieved if they are able to effectively and consistently suppress multiple weeds of economic importance on a very large scale. Continued research and innovation are essential to unlocking the full potential of bioherbicides as a reliable and environmentally friendly solution for weed control. These also target specific weed species without affecting crops or beneficial plants, thus reducing the chance of resistance development and helping to manage weed populations more effectively over time.

Nematodes and Viruses to Control Insects and Weeds

In the decades following, other researchers discovered promising nematodes that attacked Japanese beetle. Use of live nematodes for insect control is exempt from registration by the U.S. EPA. According to an extension publication by University of Florida, six nematode species of the Genus Steinernema and five species of the nematode Genus Heterorhabditis are available for use on a wide range of insects attacking a wide range of crops. Target pests include numerous species of grubs, weevils, cutworms, armyworms, chinch bugs, thrips, mole crickets, and other soil infesting insect larvae.

Baculoviruses of two groups; nuclear polyhedrosis viruses (NPV) and granuloviruses (GV) were identified as pathogens of many lepidopterous pests of forests and agronomic crops. As of 2002, US EPA had registered occlusion bodies of five NPVs and two GVs as active ingredients for control of numerous lepidopteran pests. NPVs have been used in large-scale control programs in forestry for control of gypsy moth, pine sawfly, and other pests.

In select cases, viruses that affect undesirable plants have also been considered as bioherbicide candidates. Tobacco Mild Green Mosaic Tobamovirus for control of tropical soda apple (Solanum viarum) was investigated in Florida; Araujia Mosaic Virus for control of moth plant (Araujia hortorum) in New Zealand; and a virus resembling Tobacco Rattle Virus has also been proposed as a control agent for Impatien glandulifera, an invasive species of concern in central and western Europe.

The biological activities of viruses are very distinct from pathogenicity caused by bacteria or fungi and may present additional opportunities for biological weed control in some situations.

Microbials in Plant Disease Control

Additionally, the use of beneficial microorganisms as biological control agents against plant diseases is well documented. The most common agents include Trichoderma and bacteria from the genera Pseudomonas and Bacillus. These microorganisms employ various modes of action to suppress plant pathogens. These include competition for nutrients and space, the production of secondary metabolites and the induction of plant systemic resistance. Furthermore, they can target a wide range of pathogens, including fungi, bacteria, and viruses. Products containing these microorganisms are already available on the market.

Several bacteria have also been investigated as potential biological weed control agents with Pseudomonas fluorescens and Xanthomonas campestris drawing significant interest. Many strains of P. fluorescens, some of which are beneficial to plants whereas others are inhibitory. Notably, certain strains have been observed to selectively suppress the growth and germination of problematic grassy weeds like downy brome, showcasing the potential of microbial agents not just in disease control, but also in integrated weed management strategies.

A Growing Product Line

Microbial active ingredients registered in the EPA product database include 148 different microbes used in hundreds of registered pesticides. Chart 1 presents data from US EPA on the number of microbial active ingredients registered by EPA through October 2024. The rapid and extensive acceptance of microbial-originating plant incorporated protectants (PIP) through genetically modified organisms (GMO) has not slowed the continued development of new microbials for use as foliar, soil, and seed treatments. Sources available estimate the current 2024 global microbial pesticide market is between $1.37 Billion US and $2.68 Billion US. With projected annual growth forecast of five to six percent per year, this growth is approximately the rate of the entire global market for pesticides.

Appendix 1

Key Events and Developments