Insect Pest Management for Organic Crops
Microbials for Controlling Insect Pests
Microbial insecticides, also called as biological pathogens or biological control agents, contain microbes like bacteria, fungi, viruses, or nematodes that produce toxins that are harmful to insect pests. Most insect pathogens are relatively specific to certain groups of insects and certain life stages. Unlike chemical insecticides, microbial insecticides can take longer to kill or debilitate the target pest. This may limit their use to crops that can sustain some insect damage. To be effective, most microbial insecticides must be applied to the correct life stage of the pest, and some understanding of the target pest's life cycle is required. Some microbial insecticides must be eaten by the insect to be effective. Good spray coverage is therefore important. Products containing these organisms are regulated by the Environmental Protection Agency (EPA) and use is governed by the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). While the active ingredients of microbials are generally approved for organic production (Organic Materials Review Institute, OMRI listed) because of their natural origin, certain formulated products are prohibited because the inert ingredients or procedures used in making the product are prohibited.
Bacterial insecticides are the most common form of microbial biopesticides. They are typically used as insecticides, although they can be used to control unwanted bacteria, fungi or viruses as well. As an insecticide, they are generally specific to individual species of moths and butterflies, as well as species of beetles, flies, and mosquitoes.
The most widely used microbial pesticides are subspecies and strains of Bacillus thuringiensis (abbreviated as Bt). Each strain of this bacterium produces a different mix of proteins and specifically kills one or a few related species of insect larvae. When ingested by insect larvae, Bt releases endotoxins (proteins) that bind to the intestinal lining of the insect midgut.
Types of Insect Pests Controlled. Insecticidal products comprised of a single Bacillus species may be active against an entire order of insects, or they may be effective against only one or a few species. There are different strains (subspecies) of Bt, each with specific toxicity to particular types of insects. Bacillus thuringiensis var. kurstaki and Bacillus thuringiensis var. aizawai are used against lepidopteran (moths and butterflies) larvae.
Formulations. Formulations are available as liquid concentrates, wettable powders, and ready-to-use dusts and granules. Many formulations of Bt are made using natural processes and have been approved by the OMRI for certified organic production.
Application. Because Bt must be ingested by the insect it must be applied where and when the insect is feeding. Many insects feed on the undersides of leaves and in concealed parts of the plant, so thorough coverage is required. As with most insecticides, young larvae are generally more susceptible than older larvae, so treatments should be timed accordingly. Early detection and application are crucial for good control. If temperatures are too cool or too hot, insects may stop feeding. Multiple applications are often needed for adequate management of the pest.
Resistance. As with any natural or synthetic-organic insecticides, insect populations can develop resistance to Bt; resistance has already occurred with some populations of Colorado potato beetle and diamondback moth. Perhaps the best way to avoid resistance is to avoid continuous Bt applications and to use the product only when necessary. This requires frequent observations or scouting in the field and the use of treatment thresholds whenever possible. Early detection of infestations is especially important when Bt is used as the primary pest control tool.
Potency. Manufacturers often provide potency data for their products; these data are usually reported in terms of International Units per milligram (IU/mg) or billions of International Units per kilogram (BIU/kg).
Recombinant DNA (rDNA) Techniques. The newest form of Bt manufacturing is through recombinant DNA (rDNA) techniques, where specific genes linked to the expression (production) of crystalline protein toxins are inserted into bacterial cells.
Spinosad is a relatively new microbial insecticide that is derived from a soil-dwelling bacterium. It is produced by aerobic fermentation of the soil-dwelling actinomycete species Saccharopolyspora spinosa. Spinosad is a fast-acting, somewhat broad-spectrum material that acts on insects primarily through ingestion or by direct contact with a spray droplet or a newly treated surface. Foliar applications of spinosad are not highly systemic in plants, although some movement into leaf tissue has been demonstrated. The addition of a penetrating surfactant increases absorption by tissues.
Fungi are a diverse group of organisms and can be found in almost every environment on Earth. Most have complex lifecycles, and some are parasitic to various insects. Some species have proven useful as microbial biopesticides. The mode of action is varied and depends on both the insecticidal fungus and the target pest. Unlike bacterial spores or virus particles, fungal conidia can germinate on the insect cuticle and produce specialized structures that allow the fungus to penetrate the cuticle and enter the insect’s body.
Beauveria bassiana is a fungus, which causes a disease known as the white muscadine disease in insects. B. bassiana is applied to the target pest as a spore, which is the reproductive and dispersal structure of the fungus. Once the spores have contact with the insect exoskeleton, they grow hyphae (long, branching vegetative appendages) that secrete enzymes, which in turn dissolve the cuticle (outermost layer of the skeleton).
Trichoderma harzianum provides natural, season-long control of diseases because it grows on the root system in a way that benefits the plant (root colonization). It protects roots from diseases caused by Pythium, Rhizoctonia and Fusarium and permits stronger, healthier root systems.
The most common and effective types are the baculoviruses, a group that includes two types of insect viruses: nuclear polyhedrosis and granulosis viruses. Insect viruses are obligate disease-causing organisms that can only reproduce within a host insect. Most are so specific in their action that they infect and kill only one or a few species of Lepidoptera larvae (caterpillars), making the viral insecticides good candidates for management of crop pests with minimal off-target effects.
Cydia Pomonella Granulosis
The granulovirus of the codling moth Cydia pomonella, or CpGV, is a good example of a commercially successful viral insecticide. CpGV is highly specific to the codling moth. The codling moth, Cydia pomonella (L.), is a major pest causing severe economic damage in apple and pear orchards throughout most of the temperate world.
Nematodes are microscopic roundworms that lack segments or appendages and may be parasitic, free living, or predaceous. Entomopathogenic nematodes have a symbiotic association with a bacterium (Xenorhabdus spp.) that is lethal to many soil-dwelling insects but does not affect animals and plants. Because they are highly mobile and can locate and destroy new victims in just a few days, entomopathogenic nematodes make outstanding candidates for all kinds of biological control.
Species of nematodes vary in their host range and host-finding behavior. Most failures in efficacy of field applications are related to a poor match between the nematode species and target insect pest. Species of nematodes vary in their host range and host-finding behavior. Some nematodes (e.g., Heterorhabditis bacteriophora) are very active in the soil and search a relatively large area for a host insect, whereas the widely available nematode Steinernema carpocapsae is relatively sedentary and tends to sit and wait for a host insect to pass by in close proximity.
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Topics Within This Chapter:
- Introduction to Insect Pest Management for Organic Crops
- Biology of Insects
- Cultural Control of Insect Pests
- Biological Control of Insect Pests
- Microbials for Controlling Insect Pests
- Botanicals for Controlling Insect Pests
- Spray Oils for Controlling Insect Pests
- Insecticidal Soaps for Controlling Insect Pests
- Minerals for Controlling Insect Pests
- Pheromones for Controlling Insect Pests
- Insect Growth Regulators