CLASS OF BIOPESTICIDE PRODUCTS
Biopesticide products fall into two major categories: microbial and biochemical. Within each of these, there are various types of products, each with its own mode of action.
In this category, the active ingredient is a microorganism that either occurs naturally or is genetically engineered. The pesticidal action may be from the organism itself or from a substance it produces. The following microorganisms are used in microbial biopesticides:
- Bacteria- Biopesticides based on bacteria have been used to control plant diseases, nematodes, insects and weeds. They control pests in a number of ways: by producing toxins outcompeting the damaging pathogen, producing anti-fungal compounds and by promoting root and top growth. Bacillus thuringiensis (Bt), which targets larvae and Pseudomonas syringae, which controls bacterial spot are examples of microbial.
- Fungi – Fungal biopesticides are relatively new. They may be used to target nematodes, mites, insects, weeds or other fungi. Like bacteria, they may act by out-competing the targeted pathogen or producing toxins. They may also attack and parasitize plant pathogens or insects. Trichoderma harzianum is a fungi that is also a fungicide, targeting Pythium, Rhizoctonia and Fusarium.
- Nematodes – Nematodes are colourless roundworms. Many are parasitic to plants and cause serious damage to crops. However, some are actually beneficial, attacking insect pests. The two main nematodes used for biopesticidal purposes are Steinernema spp. and Heterorhabditis spp.
- Protozoa- Protozoa are single-celled organisms that live in both water and soil. While most protozoa feed on bacteria and decaying organic matter, many species are insect parasites. In particular, one species of protozoa, Nosema locustae, is used to control grasshopper, locust and crickets on rangeland.
- Viruses – Microbial biopesticides known as baculoviruses are a family of naturally occurring viruses known to infect only insects and some related arthropods. Most are so host-specific that they infect only one or a few species of Lepidoptera larvae, which makes them ideal for management of crop pests with minimal harm to beneficial. The granulosis virus of Cydia pomonella, the codling moth, and the nuclear polyhedrosis virus of Heliothis/ Helicoverpa spp., the corn earworm, are two examples.
- Yeast – Some yeast species that naturally occur in plants have been developed into products that help to control postharvest decay and/or stimulate the plant’s immune system. For example, Candida olephila Strain O, first isolated from Golden Delicious apples, has been developed into an effective biopesticide for the control of post-harvest fruit rots.
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Biochemical pesticides are naturally occurring or synthetically derived compounds that are structurally similar (and functionally identical) to their natural counterparts. Unlike conventional chemicals, which usually directly attack and kill the pest, biochemical biopesticides are characterized by a non-toxic mode of action that may affect the growth and development of a pest, its ability to reproduce, or pest ecology. Biochemicals might also be used to effect the growth and development of treated plants and their fruit, including during the postharvest period. Biochemicals fall into the categories below:
- Plant Growth Regulators – Plant growth regulators (PGRs) are compounds that effect major physiologic functions of plants, such as growth rate, seed germination, bolting, fruit set and ripening, branching and many others. They may be naturally-occurring or synthetically manufactured to mimic the function of the natural substance. Many are plant hormones, but there are also other compounds that have an impact on plant growth, including a variety of secondary metabolites produced by plants and some plant-associated microbes. As the benefits of PGRs have become better understood by growers, their use has increased. They provide value through their ability to maximize productivity and quality, improve consistency in production and to overcome genetic and abiotic limitations to plant productivity. There are five major classes of natural plant hormones. Under each class there are a number of PGR products that play specific roles in optimizing crop yield and quality. Gibberellins, Cytokinins, Abscisic acid, Ethylene and Auxins .
There are a variety of concrete examples of how PGR’s meet the needs of growers in the fresh produce market. These include:
- Improved fruit quality: Gibberellins reduce russet on apples, improve firmness in cherries and rind quality in citrus. Abscisic acid enhances the red colour on red grapes and ethylene aids in ripening of fruit crops.
- Improved yield: Gibberellins increase the fruit size of cherries, table grapes, pineapple and bananas.
- Gibberellins also improve fruit set on citrus and many other crops and the leaf size and yield are increased on spinach and other leafy vegetables. Cytokinins also increase the berry size of table grapes and ethylene modulation increases fruit set on walnuts.
- Overcoming genetic limitations: Gibberellic acid improves seed germination of dwarf rice varieties and increases berry size on seedless table grapes. Both gibberellins and cytokinins can also increase fruit size in small apple varieties.
- Reduce labour costs: Cytokinins and auxins induce thinning of flowers and fruits. Ethylene modulation allows growers to manage the timing of fruit maturity, and therefore, the harvest. Gibberellins also allow growers to delay the harvest on citrus and cherries.
- Extend post-harvest life: Ethylene management and ethylene receptor blockage can enhance the shelf life of fruit. Gibberellins extend the green life of bananas and lemons during shipping and storage.
- Insect Growth Regulators
Most chemical insecticides work by killing insects outright, often targeting the nervous system. Often, beneficial insects are killed as well. Insect growth regulators (IGRs) use a different and more selective mode of action; they disrupt the growth process of insects, preventing them from the reaching reproductive stage. The direct impact of IGRs on target pests combined with the preservation of beneficial insects and pollinators aids growers in maximizing yield and product quality. IGRs can be divided into two broad categories: those that disrupt the hormonal regulation of insect metamorphosis and those that disrupt the synthesis of chitin, a principle component of insect exoskeletons. Agricultural applications currently focus on the first category of compounds, also known as “hormone mimics.” The most widely used IGR is azadirachtin, which structurally mimics the natural insect molting hormone ecdysone. Immature insects exposed to azadirachtin may molt prematurely or die before they complete a properly timed molt. Insects that survive exposure are likely to develop into a deformed adult incapable of feeding or reproducing. Since beneficial insects do not feed on the treated foliage, biopesticide insect growth regulators are considered “soft” on beneficial insects such as honeybees, ladybugs, green lacewings and parasitic wasps.
Organic Acids – Peracids are highly effective sanitizing agents used for controlling algae and pathogens. Peracids can be used for sanitation of greenhouse surfaces, shock applications for tanks and piping, continuous application at a low concentration and as a bacterial or fungicidal application to plant foliage or roots. A further advantage is that when peracids degrade, the by product is oxygen, which is safe and beneficial.
Plant Extracts – In order to protect themselves from insect, animal and fungal predators, plants have devised numerous biochemical defences. Some discourage feeding by insects and herbivores, some have anti-bacterial or anti-fungal activity that provides protection or even immunity from some pathogens, and others have a detrimental effect on nearby plants in order to reduce competition for resources. By studying the diverse chemistries of many different plant species, scientists have discovered many useful compounds that can be used as biopesticides. These are called plant extracts and provide pest control in a variety of ways:
- Insect growth regulators – Prevent insects from reaching the reproductive stage.
- Feeding deterrents – are compounds that, once ingested by the insect pest, cause it to stop feeding. Crop damage is inhibited and the insect eventually starves to death.
- Repellents – are typically compounds that release odour’s that are unappealing to insects. Examples include garlic or pepper based insecticides.
- Confusants – imitate food sources and are used as traps or decoys to lure insects away from crops. They can also be formulated as concentrated sprays designed to overwhelm insects with so many sources of stimuli that they cannot locate the crop.
- Allelopathy – Some plants naturally produce biochemicals to prevent competition from neighbouring plants. Juglone, the allelochemical produced by black walnut trees (Juglans nigra), is toxic to many other plants. Many recently discovered allelochemicals have potential for development as natural product herbicides.
- Plant Growth Regulation – Some plant extracts can act as effective contact herbicides through a variety of mechanisms such as disrupting cell membranes in plant tissue, inhibiting amino acid synthesis or enzyme production.
- Mechanical Control – Some plant extracts are powerful natural agents that act directly on weeds. D-limonene, for example, is a degreasing agent that strips the waxy cuticle from weed leaves, causing necrosis, dehydration and weed death.
- Fungicidal Control – By disrupting cell membrane integrity, deactivating key enzymes and interfering with metabolic processes, plant extracts can act as contact fungicides.
- Induced Resistance – Crops treated with some plant extracts produce and accumulate elevated levels of specialized proteins and other compounds that inhibit the development of fungal and bacterial diseases. In effect, the crop’s immune system is triggered to defend against destructive diseases.
Pheromones – Insects release chemical signals, called pheromones, to communicate with others in their species for a variety of reasons. These might include finding a mate, warning others of potential danger or indicating the location of a food source. By using synthetic pheromones that mimic the action of the pests’ natural chemical, growers can disrupt mating cycles or lure pests away from crops. Each year, more than one million acres worldwide are treated with pheromones to control insect damage through mating disruption. Pheromones are also used in traps, allowing growers to predict insect populations and time application of controls.
- Minerals – Minerals play a key role in a wide range of biopesticide applications that can be divided into three categories:
- Barriers – act to keep pests away from plant tissues and/or impact pathogen water supply. An example is kaolin clay, which acts as a repellent that coats the plant surface, making it unsuitable for insect feeding or egg laying.
- Smothering and/or abrasion – one example is diatomaceous earth, which contains fossilized microscopic plants, giving the compound a sharp surface that cuts through insects’ exoskeletons, a process that leads to desiccation of the insect. Mineral oils are often used to smother insects in the nesting or crawler phases.
- Carrier for other biopesticides – minerals are also used as inert carriers for companion biopesticides. In these applications, minerals are included in formulations to deliver or enhance pest control agents, but the mineral itself is considered inert. Talc, kaolin, montmorillonite and attapulgite are just a few.
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