What are biopesticides? |

Bacillus thuringiensis

Hungry insects are the bane of gardeners, since their appetite results in defoliation of the crops. This problem is worsened for farmers, whose livelihoods depend on keeping fields free of destructive insects. Although effective, chemical pesticides have a variety of drawbacks that include contamination of surface and groundwater and adverse health effects on noninsect species, including humans. The increasing popularity of organically grown produce that is untreated by chemicals is indicative of the wariness of consumers concerning human-made pesticides. In response to consumer concern over the safety of chemical pesticides, agricultural biologists have turned to nature to solve pest problems. Biopesticides are naturally derived insecticides. The process of evolution has produced biopesticides that are very specific and effective in their activity. A biopesticide may be sprayed directly on crops or may be genetically engineered to be produced by a crop itself.

One of the oldest commercial biopesticides is a bacterium called Bacillus thuringiensis (Bt). Since the 1950s, Bt has been used on crops susceptible to destruction by insect larvae. Bt is a spore-forming bacterium, meaning it is capable of producing an environmentally hardy form that protects the genetic material from adverse conditions. When conditions improve, the spore can germinate to reestablish the normally growing and dividing form of the organism. The basis of Bt’s action is the ingestion of the spores by the feeding insect larvae. During the sporulation process, Bt produces a protein crystal. When the protein is synthesized by the bacteria, it is an inactive form known as a proenzyme. After being digested by a larva, enzymes in the larval gut cleave the proenzyme into an active version that kills the larva by binding to receptors in the insect’s midgut cells and blocking those cells from functioning. Only caterpillars (tobacco hornworms and cotton bollworms), beetles, and certain flies have the gut biochemistry to activate the toxin. The toxin does not kill insects that are not susceptible, nor does it harm vertebrates in any way. This makes Bt a very specific pesticide.

Initially, Bt was expensive and remained active following spraying for only a relatively short time. These obstacles were overcome in the early 1990s, when scientists utilized genetic engineering technology to produced transgenic cotton plants that generated their own Bt toxin. The toxin gene was first isolated from Bt cells and ligated (enzymatically attached) into a Ti plasmid. A Ti plasmid is a circular string of double-stranded DNA that originates in the Agrobacterium tumefaciens
bacteria. A. tumefaciens has the ability to take a portion of that Ti plasmid, called the T-DNA, and transfer it and whatever foreign gene is attached to it into a plant cell. Cotton plants were exposed to A. tumefaciens carrying the toxin gene and were transformed. The transgenic plants synthesized the Bt toxin and became resistant to many forms of larvae. This approach has become known as plant-incorporated bioprotectants.

Many crystal toxins have been isolated from various strains of Bt. These toxins make up a large collection of proteins active against pests from nematodes to aphids. Researchers are in the process of reengineering the toxin genes to improve upon their characteristics and to design better methods of transporting genes from one Bt strain to another.

Other Biopesticides

Several species of fungi are toxic to insects, including Verticillium lecanii
and Metarhizium anisopliae. Natural fungicides that have been discovered include oils of tea tree, cinnamon, jojoba, neem, and rosemary.

In the mid-1990s, a viral biopesticide called baculovirus became widely popular. Baculoviruses are sprayed onto high-density pest populations just like chemical pesticides. Baculoviruses have several advantages over conventional pesticides. The most important advantage is their strong specificity against moths, sawflies, and beetles but not against beneficial insects. Also, viruses, unlike bacteria, tend to persist in the environment for a longer period. Finally, baculoviruses are ideal for use in developing countries because they can be produced cheaply and in great quantity with no health risks to workers. One limitation of baculovirus is that it must be administered at a certain time and location to be effective. Rather than spraying onto a crop and killing the insects that subsequently feed, baculovirus needs to be applied directly to the target insect population. Knowledge of insect behavior after hatching, the insect population’s distribution within the crop canopy, and the volume of foliage ingested by each larva are essential. For example, moths usually do the most damage at the late larval stage. To minimize crop damage from moths, baculovirus needs to be sprayed as early as possible before the insects reach that late stage.

Another biopesticide approach has been to make transgenic plants that manufacture proteins isolated from insect-resistant plant species. Tomatoes naturally make an enzyme inhibitor that deters insects by keeping their digestive enzymes (trypsin and chymotrypsin) from functioning. These inhibitors were isolated by Clarence Ryan at the University of Washington. Ryan transformed tobacco plants with two different forms of inhibitor (inhibitors I and II from tomatoes). The tomato proteins were effectively produced in tobacco and made the transgenic plants resistant to tobacco hornworm larvae.

Biopesticides exist and have been refined for the control of insect pests even though they are not toxic to the insects. These biochemical pesticides include compounds that interfere with insect pheromones (chemicals that attract insects to a potential mate). Their use inhibits insect mating and so the production of the next generation of the particular insect.

Biopesticides are also being implemented to control the population of mosquitoes in some regions of the world that are susceptible to malaria. The idea is that by controlling the mosquito population, the spread of the microorganism responsible for the disease, which occurs when the mosquito takes a blood meal, will be lessened. The strategy has been tested in several malaria-prone countries in Africa.

Biopesticide Resistance

As with chemical pesticides, over time insect populations grow resistant to biopesticides. Bt-resistant moths can now be found around the world. Resistance arises when pesticides are too effective and destroy more than 90 percent of a pest population. The few insects left are often very resistant to the pesticide, breed, and with succeeding generations create large, resistant populations.

Entomologists have suggested strategies for avoiding pesticide-resistant insect populations. One strategy suggests mixing biopesticide-producing and nonproducing plants in the same field, thereby giving the pesticide-susceptible part of the insect population places of refuge. These refuges would allow resistant and nonresistant insects to interbreed, making the overall species less resistant. Other strategies include synthesizing multiple types of Bt toxin in a single plant to increase the toxicity range and reduce resistance, making other biological toxins besides Bt in a single plant, and reducing the overall exposure time of insects to the biopesticides.

Organizations including the US Environmental Protection Agency (EPA) and Department of Agriculture recommend using biopesticides as part of what is termed an “integrated pest management approach” that uses a number of control and crop growth strategies. The aim is to decrease the use of conventional pesticides while maintaining or even increasing crop yield. This approach also helps lessen the development of resistance, since the same biopesticide is not used constantly.

Key terms



Agrobacterium tumefaciens

:

a species of bacteria that is able to transfer genetic information into plant cells





Bacillus thuringiensis
(Bt)
:

a species of bacteria that produces a toxin deadly to caterpillars, moths, beetles, and certain flies




baculovirus

:

a strain of virus that is capable of causing disease in a variety of insects




transformation

:

the process of transferring a foreign gene into an organism





transgenic organism


:

an organism synthesizing a foreign protein, the gene of which was obtained from a different species of organism





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