Natural and Green Insecticides and Integrated Pest Management Pesticides from Natural Sources

As pointed out earlier, many plants themselves manufacture certain molecules for their own self-protection that either kill or disable insects. Chemists have isolated some of these compounds so that they can be used to control insects in other contexts. Examples are nicotine, rotenone, the pheromones, and juvenile hormones.

One group of natural pesticides that has been used by humans for centuries is the pyrethrins. The original compounds, the general structure for which is illustrated on the next page, were obtained from the flowers of a species of chrysanthemum.

general Pyrethrin structure general Pyrethrin structure

In the form of dried, ground-up flower heads, pyrethrins were used in Napoleonic times to control body lice; they are still used in flea sprays for animals. They are generally considered to be safe to use. Like organophosphates, they paralyze insects, though they usually do not kill them. Unfortunately, these compounds are unstable in sunlight. For that reason, several synthetic pyrethrin-like insecticides that are stable outdoors—and therefore can be used in agricultural and garden applications—have been developed by chemists. Semisynthetic pyrethrin derivatives, called pyrethroids, are usually given names ending in -t/mn to denote their nature (e.g., permethrin). Pyrethroids are now also a common ingredient in domestic insecticides, as any visit to a garden center will testify. They have also been used in Mexico to spray houses in which malaria control is still necessary and to spray neighborhoods in New York City to reduce mosquito populations carrying the West Nile virus. In order to make them more effective as insecticides, pyrethroid formulations are usually mixed with piperonyl butoxide, a semisynthetic derivative of the natural product safrole, which can be extracted from sassafras plants. The piperonyl butoxide interferes with the enzyme that insects use to detoxify the pyrethroids, thus making them more potent in destroying the insects.

Pyrethroids are now so widely used that their metabolites were detected in the urine of most of the elementary schoolchildren in the Seattle study group mentioned previously. Interestingly, the pyrethroid levels did not decrease when the children adopted an organic diet. However, the most important contribution to high pyrethroid levels was not dietary but correlated with the use of pyrethrin insecticides by their parents.

Rotenone, a complex natural product derived from the roots of certain tropical bean plants, has been used as a crop insecticide for over 150 years and for centuries to paralyze and/or kill fish. The compound enters fish via their gills and disrupts their respiratory system. It is also highly efficient against insects but is decomposed by sunlight. Rotenone is widely use in hundreds of commercial products, including flea-and-tick powders and sprays for tomato plants. Notice in Table 10-4 that the "natural" insecticides pyrethrins and rotenone are classified as moderately hazardous, since they have about the same acute toxicity as some synthetic ones such as malathion, even though they are often marketed as safer, natural pesticides. There is epidemiological and toxicological evidence that chronic exposure to rotenone can contribute to the onset of Parkinson's disease. Indeed, there is some evidence that exposure to pesticides in general contributes to its incidence.

Integrated Pest Management

In recent years, integrated pest management (IPM) strategies have been developed. They combine the best features of various feasible methods of pest control—not just the use of chemicals—into a long-range, ecologically sound plan to control pests so that they do not cause economic injury. Generally speaking, a unique plan is developed for each area and crop, with chemicals used only as a last resort and when the monetary cost of their use will be more than recovered by increased crop yield. The six pest control methods that can be combined are

• chemical control—the use of chemical pesticides, both synthetic and natural;

• biological control—reducing pest populations by the introduction of predators, parasites, or pathogens;

• cultural control—introducing farm practices that prevent pests from flourishing;

• host-plant resistance—using plants that are resistant to attack, including plants adapted by genetic engineering to have greater resistance;

• physical control—using nonchemical methods to reduce pest population; and

• regulatory control—preventing the invasion of an area by new species.

Green Chemistry: Insecticides That Target Only Certain Insects

Insecticides such as the organophosphates and carbamates interrupt the function of specific enzymes that are common to most insects (and to humans). They are thus toxic to a wide array of insect species and are known as broad-spectrum insecticides. Although it may be an advantage to kill more than one species with a single pesticide, this often leads to the demise of beneficial insects such as pollinators (bees) and natural enemies (lady bugs and praying mantises) of insects that are pests.

An approach to limiting the environmental effect of an insecticide is to develop insecticides that are toxic to only certain species, i.e., the target organism. One way to accomplish this is to find a biological function that is unique to the target organism and develop an insecticide that interrupts only that function. The Rohm and Haas Company of Philadelphia, Pennsylvania, won a Presidential Green Chemistry Challenge Award in 1998 for the development of Confirm, Mach 2, and Intrepid. The active ingredients in these insecticides are members of the diacylhydrazine family of compounds (Figure 10-8a) and are effective in controlling caterpillars.

Caterpillars are the larval stage of insects such as moths and butterflies, and during the larval stage they must shed their cuticle to grow. The concentration

FIGURE 10-8 (a) Diacyl-hydrazine pesticides; (b) 20-hydroxyecdysone,

FIGURE 10-8 (a) Diacyl-hydrazine pesticides; (b) 20-hydroxyecdysone, of 20-hydroxyecdysone (Figure 10-8b), which is produced by the caterpillar and is a member of the steroid family of compounds, increases during the molting process. As a result of its presence, the caterpillar ceases to feed and sheds its cuticle. The concentration of this natural compound then decreases and the caterpillar resumes feeding. The diacylhydrazines present in the commercial products Confirm, Mach 2, and Intrepid mimic 20-hydroxyecdysone; however, their concentrations do not decline, and consequently the insect never resumes feeding. The insect thus dies of starvation or dehydration. These insecticides target only insects that go through molting stages during their growth; most other insects will be unaffected.

Confirm and Intrepid are classified as reduced-risk pesticides by the U.S. EPA. This classification program was started in 1993. To be placed in this category, a pesticide must meet one or more of the following requirements:

1. it reduces pesticide risks to human health;

2. it reduces pesticide risks to nontarget organisms;

3. it reduces the potential for contamination of valued environmental resources; or

4. it broadens the adoption of IPM (integrated pest management, discussed in the preceding section) or makes it more effective.

The diacylhydrazene insecticides certainly meet the first two of these requirements. In order to encourage the development of lower-risk pesticides, the EPA rewards the developers of pesticides that contain active ingredients that meet the EPA reduced-risk criteria with expedited review. (http://www.epa.gov/opppmsdl/PR_Notices/pr97-3.html)

Green Chemistry: A New Method for Controlling Termites

Termites invade over 1.5 million homes in the United States annually and cause about $1.5 billion in damage. Traditional treatments for termites involve treating the soil around the affected structure with 100-200 gal of pesticide solution to create an impenetrable barrier. This process may result in groundwater contamination, accidental worker exposure, and detrimental effects to beneficial insects.

FIGURE 10-9 (a) Sentricon monitoring/baiting station; (b) hexaflumuron structure, [Source: Photo by Michael Cann.]

FIGURE 10-9 (a) Sentricon monitoring/baiting station; (b) hexaflumuron structure, [Source: Photo by Michael Cann.]

Dow AgroSciences in Indianapolis won a Presidential Green Chemistry Challenge Award in 2000 for its development of Sentricon. In contrast to the traditional control of termites, Sentricon employs monitoring stations to first detect the presence of termites prior to the use of any insecticide. The monitoring stations (Figure 10-9a) consist of pieces of wood (1) contained in perforated plastic tubes (2), which are placed in the ground around the structure. If termites are detected in any of the monitoring stations, the wood pieces are then replaced by a perforated plastic tube (3) containing the bait. Bait stations may also be placed in the structure. The bait consists of a mixture of cellulose and the pesticide hexaflumuron (Figure 10-9b). Hexaflumuron interrupts the molting process of termites and thus is not harmful to most beneficial insects. Termites that have ingested the bait return to their nests and share the bait by trophallaxis, thus spreading the insecticide throughout the colony. Once the colony has been decimated, the bait is replaced with wood and monitoring resumes.

Hexaflumuron was the first pesticide to be classified as a reduced-risk pesticide by the U.S. EPA. Hexaflumuron is significantly less toxic and is used in quantities 100 to 1000 times smaller than traditional pesticides (see Table 10-5) employed for termite control.

TABLE 10-5

TABLE 10-5

Acute Oral LD, Pesticide (Compound Type) (mg/kg)

Acute Dermal LD, (mg/kg)

Chlorpyrifos (organophosphate) Permethrin (pyrethroid) Irnidacloprid (chloronicotinyl) Fipronil (pyrazole) Hexaflumuron

135-163

430-4000

424-475

100 >5000

2000 >4000 >5000 >2000 >2000

* Typical quantity applied: traditional pesticides, 750- 7000 g; hexaflumuron, 2-5 g.

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