Biochemical Methods Of Insect Control

The development of insect resistance has been an important factor prompting the search for methods of insect control that are not totally dependent on the use of pesticides. These methods are outlined briefly because their adoption by farmers will help both to minimize the amounts of synthetic pesticides in the environment and to slow the development of resistant strains of insects. It should be noted that the majority of U.S. farmers use synthetic pesticides, but the amounts used per acre have decreased as biological methods of control are used increasingly in conjunction with the application of synthetic pesticides.

8.13.1 Pheromones

Pheromones are chemicals secreted by one species to affect the behavior of another of its own species. Insect pheromones are used mainly to indicate the location of food or to attract a mate (sex pheromones) (Figure 8-7). In most instances the female secretes sex pheromones. Insect sex pheromones have been studied extensively with the goal of controlling the insect by disrupting mating behavior or by attracting the males to traps where a toxic agent kills them. Pheromones are usually species specific: that is one can selectively control a specific insect pest. Presumably, the insects will not become resistant because pheromones, being produced by their own species, should not trigger the production of protective enzymes.

Thirty years ago pheromones held great promise as species-specific, biodegradable agents for the control of insects which would not promote the development of resistant strains. Initially, this promise was not realized. It was

Insect Repellent Permission Form
FIGURE 8-7 From The Scientist, p. 3, March 30, 1998. Copyright © 2001 by Sidney Harris. Used with permission.

not possible to devise low-cost, large-scale syntheses of these compounds so that they could be manufactured in sufficient quantities for large-scale use. The supply problem was aggregrated by the pheromones rapid rate of degradation in the environment, making their effectiveness short-lived.

There has been a gradual resurgence in the use of pheromones because the problems associated with their manufacture and application were addressed. The problem of their rapid degradation in the environment was solved by encapsulation in capillary tubes from which the pheromone slowly diffuses into the field or orchard. Political and economic factors drove these innovations because of the desire to limit the use of pesticides, which persist in the environment, leave residues on fruits and vegetables, and lead over time to the development of resistant strains of insects. Pheromones are now an important part of the integrated pest management (IPM) approach (Section 8.14) for the control of insect predators on major crops.

Since pheromones are present only in trace amounts in insects, special techniques are required to determine the structures of these compounds. For example, only 900 p,g(9 x 10~4gm) of pure male sex attractant was isolated from 135,000 virgin female fall armyworm moths. The isolation and analysis of the pheromone is even more difficult if it consists of a mixture of compounds that are effective only when present in the proper ratio. The cotton boll weevil pheromone "grandlure" is an example of a pheromone that consists of four substances in the following proportions:

13 10 1 1

The chemical constitution of the boll weevil pheromone " grandlure"

This pheromone is unusual in that it is released by the male to attract females. The pheromone of the pink boll worm moth, an insect whose larvae eat cotton, consists of two substances:


CH3COCH2(CH2)5CH=CH(CH2)2CH=CH(CH2)3CH3 cis cis or trans

The pink bollworm moth pheromone "gossyplure"

Once the structure of the pheromone was determined, an efficient method of chemical synthesis was devised to make a sufficient amount available for large-scale agricultural use.

Pheromones have been used in at least four different approaches to insect control.

1. To monitor the population of a specific insect. The pheromone is placed in traps, and the number of insects trapped is recorded. The population density of the insect can be monitored to measure the effectiveness of the insecticides being used or the insects' migration into new areas. The traps must be designed to be compatible with the behavior of the target insect, which means that design and color can be chosen only after appropriate field tests. The migration, in a wind tunnel, of a male moth to a source of the pheromone produced by the female has been monitored and is shown in Figure 8-8.

2. To trap males (or females in some instances). A large number of traps containing the pheromone are used to attract the males in an area. To be effective, almost all the males must be trapped, so that they are not available for mating. If as few as 10% are not trapped, the local population will build up again within a few generations, which for insects may only be a matter of weeks.

3. For male (female in some instances) confusion. Capillary tubes are used to distribute large amounts of the pheromone in the field, causing the air to be permeated with the female sex pheromone. The male is surrounded by the attractant, which saturates the insect's pheromone receptors, preventing the male from locating the female. This techniques and technique 2 are effective only if there is a low population of the insects to be controlled. For

FIGURE 8-8 Track of male moth flying toward a source of female heromone in a wind tunnel. Marks on track at one-second intervals. Redrawn with permission from W. Booth, Science, 239, 136. Copyright © 1988 American Association for the Advancement of Science. Source: The Insects by R. F. Chapman. Use of Pheromones in Insect Control

FIGURE 8-8 Track of male moth flying toward a source of female heromone in a wind tunnel. Marks on track at one-second intervals. Redrawn with permission from W. Booth, Science, 239, 136. Copyright © 1988 American Association for the Advancement of Science. Source: The Insects by R. F. Chapman.

0.5 m example, male confusion is not effective if the population of insects is high enough for the male to see the female.

4. To attract (aggregate) or disperse both males and females. Some phero-mones attract both sexes to a particular area, resulting in a population large-enough to form colonies. This was observed with beetles, which require a certain number of conspecifics to launch a mass attack on a tree. Once that number has been reached, a dispersant pheromone is emitted that diverts potential new colony members. The aggregation pheromone is used in insect control to bring the target organisms together in one location, where they can be efficiently killed by a pesticide. The dispersant is used to divert the insects away from the area in which the crop is growing. These attractants and dispersants are species specific and have no effect on other, some potentially beneficial, insects.

The male confusion approach has proved to be useful in the control of insects that feed on grapes, apples, pears, nectarines, and currants. Studies are in progress on the application of the male confusion technique to insect control on about 10 other crops. One example is the control of the grape berry moth, the only insect pest that attacks Concord grapes. The pheromone is encapsulated inside thin, hollow polyethylene tubes that are tied to the wires used to hold up the grapevines. The release of the pheromone is controlled by the length of time it takes for its vapor to diffuse down this capillary tube into the atmosphere. Initial studies show that this approach keeps the damage to grapes at or below the 2% level, much lower than that observed with use of synthetic insecticides.

The juice from concord grapes is used in making wine, juice, and jelly, but poorer product is obtained if more than 2% of the grapes are damaged. The vineyards tend to be near homes, and thus synthetic pesticides cannot be used. Since the grape arbors are small, it is economically feasible to use the phero-mone, which is a 9:1 mixture of two compounds, to confuse the male moths.

CH3 CH2(CH2)6CH2OCCH3 9 parts

CH3 CH2(CH2)8CH2OCCH3 1 part

Grape berry moth sex pheromone

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