Biopesticides and the environment

Definition: Biological agents that are used to control insect and weed pests

Biopesticides have significant advantages over commercial pesticides in that they appear to be environmentally safer, given that they do not accumulate in the food chain and they have only slight effects on ecological balances.

Pests are any unwanted animals, plants, or microorganisms. When the environment has no natural resistance to a pest and when no natural antagonists are present, pests can run rampant. For example, the fungus Endothia parasitica, which entered New York State in 1904, caused the nearly complete destruction of the American chestnut tree because no natural control was present.

Biopesticides represent the biological, rather than the chemical, control of pests. Many plants and animals are protected from pests by passive means. For example, plant rotation is a traditional method of insect and disease protection in which the host plant is removed for a period long enough to reduce pathogen and pest populations.

Biopesticides have several significant advantages over commercial pesticides. They appear to be ecologically safer than commercial pesticides because they do not accumulate in the food chain. Some biopesticides also provide persistent control, because pests require more than a single mutation to adapt to them and because they can become an integral part of a pest’s life cycle. In addition, biopesticides have only slight effects on ecological balances because they do not affect nontarget species. Finally, biopesticides are compatible with other control agents. The major drawbacks to using biopesticides are that, in comparison with chemical pesticides, biopesticides work less efficiently and take more time to kill their targets. There were 390 biopesticide active ingredients registered with the US Environmental Protection Agency in 2020.

Viruses, bacteria, fungi, protozoa, mites, and flowers have all been used as biopesticides. Viruses have been developed against insect pests such as Lepidoptera, Hymenoptera, and Dipterans. These viruses cause hyperparasitism. Spongy moths and tent caterpillars, for example, periodically suffer from epidemic virus infestations.

Many saprophytic microorganisms that occur on plant roots and leaves can protect plants against microbial pests. Bacillus cereus has been used as an inoculum on soybean seeds to prevent infection by the fungal pathogen Cercospora. Some microorganisms used as biopesticides produce antibiotics, but the major mechanism for protection is probably competitive exclusion of a pest from sites on which the pest must grow. For example, Agrobacterium radiobacter antagonizes Agrobacterium tumefaciens, which causes crown gall disease. Two bacteria—Bacillus and Streptomyces—added as biopesticides to soil help control the damping off disease of cucumbers, peas, and lettuces caused by Rhizoctonia solani. Bacillus subtilis added to plant tissue also controls stem rot and wilt rot caused by the fungus Fusarium. Mycobacteria produce cellulose-degrading enzymes, and their addition to young seedlings helps control fungal infection by Pythium, Rhizoctonia, and Fusarium. Bacillus and Pseudomonas are bacteria that produce enzymes that dissolve fungal cell walls.

The best examples of microbial insecticides are Bacillus thuringiensis (B.t.) toxins, which were first used in 1901. They have had widespread commercial production and use since the 1960s and have been successfully tested on 140 insect species, including mosquitoes. B.t. produces insecticidal endotoxins during sporulation and also produces exotoxins contained in crystalline parasporal protein bodies. These protein crystals are insoluble in water but readily dissolve in an insect’s gut. Once dissolved, the proteolytic enzymes paralyze the gut. Bacillus spores that have also been consumed germinate and kill the insect. Bacillus popilliae is a related bacterium that produces an insecticidal spore that has been used to control Japanese beetles, a pest of corn.

Saprophytic fungi can compete with pathogenic fungi. Among the fungi used as biopesticides are Gliocladium virens, Trichoderma hamatum, Trichoderma harzianum, Trichoderma viride, and Talaromyces flavus. For example, Trichoderma competes with the pathogens Verticillium and Fusarium. Peniophora gigantea antagonizes the pine pathogen Heterobasidion annosum through three mechanisms: It prevents the pathogen from colonizing stumps and traveling down into the root zone, it prevents the pathogen from traveling between infected and uninfected trees along interconnected roots, and it prevents the pathogen from growing up to stump surfaces and sporulating.

Nematodes are pests that interfere with commercial button mushroom (Agaricus bisporus) production. Several types of nematode-trapping fungi can be used as biopesticides to trap, kill, and digest the nematode pests. The fungi produce structures such as constricting and nonconstricting rings, sticky appendages, and spores, which attach to the nematodes. The most common nematode-trapping fungi are Arthrobotrys oligospora, Arthrobotrys conoides, Dactylaria candida, and Meria coniospora.

Protozoa have occasionally been used as biopesticide agents, but their use has suffered because of such difficulties as slow growth and complex culture conditions associated with their commercial production. Predaceous mites are used as a biopesticide to protect cotton from other insect pests such as the boll weevil.

Dalmatian and Persian insect powders contain pyrethrins, which are toxic insecticidal compounds produced in Chrysanthemum flowers. Synthetic versions of these naturally occurring compounds are found in products used to control head lice. Molecular genetics has also been used to insert the gene for the B.t. toxin into cotton and corn. B.t. cotton and B.t. corn both express the gene in their roots, which provides them with protection from root worms. Ecologists and environmentalists have expressed concern that constantly exposing pests to the toxin will cause insect resistance to develop rapidly and thus reduce the effectiveness of traditionally applied B.t.

Bibliography

Churchill, B. W. Biological Control of Weeds with Plant Pathogens. Edited by R. Charudattan and H. Walker. New York: Wiley, 1982. Print.

Deacon, J. W. Microbial Control of Plant Pests and Diseases. Research Triangle Park: Instrumentation Systems & Automation, 1983. Print.

Metz, Matthew, ed. Bacillus Thuringiensis: A Cornerstone of Modern Agriculture. Binghamton: Haworth, 2003. Print.

Ohkawa, H., H. Miyagawa, and P. W. Lee, eds. Pesticide Chemistry: Crop Protection, Public Health, Environmental Safety. New York: Wiley, 2007. Print.

"What Are Biopesticides?" United States Environmental Protection Agency, www.epa.gov/ingredients-used-pesticide-products/what-are-biopesticides. Accessed 22 Feb. 2023.