REVIEW ARTICLE t Microorganisms and their byproducts, nematodes, oils Larry D. Godfrey Elizabeth E. Grafton-Cardwell Harry K. Kaya William E. Chaney t The insect and mite control potential of natural and biological toxins has been recognized for several centuries. Bacteria, viruses, protozoa and fungi are the primary groups of micro- organisms known to reduce insect populations; they often occur natu- rally in fields and function as compo- Natural and botanical products have been nents of biological control. Beneficial used to control insect pests for centuries. nematodes are also being used for Kaolin is a mineral-based particle film that is sprayed onto crops as a barrier to repel pest control, especially against soil in- insects and prevent feeding. sects. The isolation of toxic metabolic compounds from microorganisms con- tinues to be a fruitful research area, The insect-control potential of natu- biological agents that are being used although there are barriers to their ral and biological toxins has been rec- for pest control, especially against soil successful marketing and distribution. ognized for several centuries. As early insects. Several species of beneficial Another, more controversial way to as 2700 B.C., unintended epizootics nematodes are commercially available, (outbreaks of disease affecting many and the animal itself acts as a distribu- deliver these insect-specific toxins to animals of one species at the same time) tion tool for symbiotic bacteria that the target pest is through genetically by natural enemies (microorganisms) actually kill the target pest. As with the modified plants, such as those modi- were reported in beneficial insects such microorganisms, the ability to produce fied to express Bacillus thuringiensis as silkworms and honeybees. The first and formulate these nematodes into in- (Bt) toxins. Oils and particle films also record of microorganisms being inten- secticidal products has enabled their use have important niche uses for pest tionally used to control crop pests was in pest management programs. control. in the 18th century (a fungus against a A common feature of some micro- weevil pest). Bacteria, viruses, protozoa organisms, principally bacteria and and fungi are the primary groups of fungi, is their natural ability to produce pest management moves for- microorganisms known to reduce insect metabolic byproducts that are toxic to ward in the 21st century, alter- populations. These organisms often many organisms. For example, the ASnative control measures are needed occur naturally in fields and function antibiotic penicillin was isolated from a to suppress insects and mites. Federal as a component of biological control. fungus and is used to combat bacterial regulators are closely scrutinizing the Research on these microorganisms as infections in humans. Other metabolic organophosphate (OP) and carbamate “biopesticides” has resulted in the abil- byproducts have toxic activity against insecticides under the Food Quality Pro- ity to isolate, culture and formulate arthropod pests. Instead of relying on tection Act (NRC 2000); in California, some for use in integrated pest manage- the microorganism to produce these surface-water contamination is of par- ment (IPM) programs. These formula- arthropod-active toxins in the field, ticular concern in the Sacramento and tions have improved the shelf life of the microorganism can be cultured in San Joaquin river water basins (USGS the resulting products, their miscibility fermentation facilities and the resulting 2000). The recognized adverse effects of with water or oil, and the ability to metabolites can be harvested, purified, synthetic chemical pesticides (such as spray them with commercial application formulated and used effectively against the OPs, chlorinated hydrocarbons and equipment, as well as provided some major arthropod pests. Two widely used carbamates) on the environment and protection against environmental ex- commercial insecticides, spinosad and human health emphasize the need to tremes that occur after application. abamectin, were developed using this advance and refine current pest- Entomopathogenic (insect-parasitic) approach. Over the centuries, as micro- management strategies (see page 7). nematodes are another example of organisms have evolved in their envi- http://CaliforniaAgriculture.ucop.edu • JANUARY-MARCH 2005 35 In most cases, the use of living organisms — with their inherent growth and survival criteria — introduces considerable complexity into arthropod management schemes. Clark Kelly Jack Photos: ronment, the metabolites they produce following application, but also makes it have provided a competitive advantage; difficult to balance market size with reg- consequently, the isolation of these istration costs. In addition, microorgan- compounds continues to be a fruitful isms generally only have one mode of agricultural research area for the sup- entry into the host. Bacteria, viruses and pression of both arthropod pests and protozoa must be ingested to cause an plant pathogens. infection, whereas fungi cause an infec- One additional way of protecting a tion when the conidium (spore) attaches microbial-derived toxin and efficiently to and penetrates the insect cuticle. delivering it to the target pest is through Bt in widespread use genetically modified plants that express an insecticidal protein, such as Bacillus The notable exception, Bt, was used thuringiensis (Bt). Although there is con- on nearly 800,000 acres in California in siderable controversy worldwide regard- 2001 (DPR 2003). This bioinsecticide is ing the applicability and sustainability of registered on all California field, veg- this technology, it does undeniably rep- etable, orchard and floriculture crops. Bt resent an effective way to deliver a toxic strains (also known as subspecies) are ac- dose to the pest (Shelton et. al 2002). tive against particular groups of insects Other types of natural and biological (Tanada and Kaya 1993). These include toxins that are useful in pest manage- Bt kurstaki and Bt aizawai, which are both ment include plant-derived compounds active against lepidopterous larvae (but In the U.S. South, the cotton bollworm, top, is (such as rotenone, pyrethrum, sabadilla differ in which caterpillar species are controlled with cotton genetically engineered and azadiractin), inorganic products most susceptible); Bt tenebrionis, active to express Bacillus thuringiensis (Bt), but this (sulfur), mineral/refined petroleum oils against certain beetles; and Bt israelensis, pest is of minor importance in San Joaquin Valley cotton. Bottom, a cabbage looper killed and mineral-based particle films. active against mosquitoes and black flies. by a foliar application of Bt, the most widely Many of the alternative control Different Bt products are also char- used “bioinsecticide” in the state. agents that we discuss are not new to acterized by different insecticidal science. However, through innovative proteins, (δ-endotoxins) known as Cry approaches and with a better under- endotoxins A, B, C, and so on, and are and B. sphaericus, with efficacy against standing of how they kill insects and further subdivided into Cry1Aa, Cry Coleoptera (white grubs) and Diptera mites, many have come to the forefront 1Ab, Cry1Ac, and so on. These differ in (mosquitoes), respectively. Despite their in pest management. Others are still their toxicity to specific pests. Because commercial potential, these bacteria cur- plagued with high production costs, these products must be ingested, they rently have limited market share. inconsistent efficacy or special handling require warm weather for active feed- Nucleopolyhedroviruses. Nucleo- requirements, thereby limiting their ing and are most effective on early polyhedroviruses (NPV) can potentially usefulness in agricultural systems and instars. Crop damage may occur for a be used against lepidopterous larvae. as alternatives to broad-spectrum insec- short time, since Bt products are slow- A number of NPVs are registered in the ticides such as the OPs. acting, but the problems of slow kill and United States, with two of particular The natural epizootics of insect short residual activity are offset by their significance to California agriculture. pathogens occur commonly in native lack of toxicity to other natural enemies The NPV from the beet armyworm and managed systems, significantly (such as predators and insect parasites). (Spodoptera exigua) is registered in the assisting pest management. However, Bt products are especially important for United States with a provisional reg- except for Bt, the application of micro- pest control in organic cropping sys- istration for use in California in 2005 organisms for pest control in agricul- tems. However, some Bt products are (the California Department of Pesticide tural systems in California is extremely prohibited by some certifying agencies, Registration is requiring further data limited (Flint 1992). The reasons for this either because they contain inert ingre- with the actual product before full reg- include: (1) the high cost of in vitro or in dients that are prohibited, or because istration is granted), for use in field, vivo production; (2) limited persistence they have been genetically modified us- vegetable and floriculture crops. A simi- and efficacy due to UV light degrada- ing molecular techniques not acceptable lar product contains an NPV from corn tion, high humidity requirements or for organic production and marketing. earworm (Helicoverpa zea) and