I. Yamamoto

Fumigants and Nematicides

It has been estimated that plant-parasitic nematodes are as important economically as fungi, bacteria, or viruses. Nematodes distribute about 0.3 m below the soil surface. Meloidogyne, Heterodera, Tylenchulus, Rotylenchulus, and Protylenchulus are important agricultural pest nematodes. To be effective, chemicals must have high volatility at room temperature. Hence, nematicides are generally gases or volatile liquids of comparatively low molecular weight enabling them to penetrate through soils. Nonvolatile nematicides diffuse in soil by watering or mixing in soil. Fumigation is an important method of soil sterilization for the control of soil insects, nematodes, fungi, and weed seeds. It is also very important in the control of stored product insects and fungi provided that the fumigants do not leave toxic residues on the fumigated commodities. Chemical fumigants, because of their high volatility, must be used in enclosed spaces such as greenhouses, food stores, warehouses, and the like. For soil treatment, the surface of the soil must first be covered with a plastic sheet to prevent loss of the fumigant. One of the earliest fumigants used commercially probably was hydrogen cyanide which was used for the control of scale insects on citrus trees in California.

Function and Classification

The term pesticide is an all-inclusive word meaning killer of pests (the ending “cide” comes from the Latin “cida”, meaning killer). Pesticides are legally classed as economic poisons and are defined as any substance used for controlling, preventing, destroying, repelling, or mitigating any pest. According to this classification, many chemicals such as attractants, repellents, chemosterilants, hormonal agents, etc., are also designated as pesticides, although from a toxicological viewpoint they may not be directly involved in killing the pest (Table 2.1). Recently, terms like agrochemicals or bioregulators have been proposed to describe pesticides. The main classes of pesticides are shown in Table 2.2.

Compounds Interfering with ATP Synthesis

In a respiratory system, the mitochondrial electron transport coupled with oxidative phosphorylation is the important step to produce ATP which is vital to any organism and vulnerable to pesticides. During various steps in the glycolysis and the tricarboxylic acid cycle, NAD is reduced to NADH. Also, flavin adenine dinucleotide (FAD) is reduced to FADH2 when succinate is converted to fumarate during the tricarboxylic acid cycle. These two cofactors are reoxidized by passing their reducing power ( H+ + electron) through the mitochondrial electron transport system, eventually to oxygen, reducing it to water (Fig. 17.1). The electron transport system divided into four complexes ( I, II, III, IV) and flavin mononucleotide (FMN), FAD, FeS, coenzyme Q, and cytochromes (b, c1, c, a, a3), constitutes a respiratory carrier chain. The reactions produce energy at the indicated sites, which is utilized for ATP formation from ADP and inorganic phosphate.

The Organochlorine Insecticides

DDT is the common name given to the technical mixture of compounds in which the major component is p, p′-DDT [dichlorodiphenyl trichloroethane; 2, 2-bis (p-chlorophenyl)- 1, 1, 1 -trichloroethane). Pure p, p′-DDT is a white, tasteless, almost odorless crystalline solid. It is practically insoluble in water but soluble in many organic solvents such as acetone, benzene, carbon tetrachloride, kerosene, diesel oil, etc. It is slightly soluble in alcohol (about 2%) . For field application, DDT has been formulated as solutions, emulsifiable concentrates, wettable powders, dusts, and aerosols.

Evaluation of Toxicity in Insects

The susceptibility of an insect population to a poison is assessed by constructing a dosage-mortality curve in which the dosage is plotted against the percentage mortality at a given period of time. Such a plot produces a sigmoid curve whose asymptotic approaches at the regions of zero and 100% mortality are difficult to define without extensive testing. The use of probit transformation in which the sigmoid curve is converted to a straight line by plotting the logarithm of the dosage against the probit value of percent mortality (Log-Probit paper is available commercially for making these plots) is recommended. This method of computation yields a straight line which greatly facilitates the determination of the LD50 and other values on the plot (Fig. 6.1). Detailed discussions of this method are available in specialized books (Finney 1949, 1952; Swaroop and uemura, 1956; Busvine 1971).

Attractants, Repellents, and Antifeedants

The earliest record of an attractant used for economic purposes was an attempt in 1885 to control grasshoppers in California by means of attractive poisoned baits. Later, wine growers in Europe used traps baited with stale beer, sugar water, and old cider to control the grapevine moth. Other baits included lemons and oranges in the formulation, later changed on a chemical basis by substituting amyl alcohol for citrus fruit. A fly bait consisting of a mixture of old casein, brown sugar, and water was considered an excellent attractant. Molasses-yeast baits for peach moth control were added to the list. In the late 1920s, hundreds of aromatic compounds were screened for their attractant properties. Additional impetus was given by the discovery of geraniol as an attractant for the Japanese beetle. A list of many of the old type attractants is given by Dethier (1947).

Methods of Testing Chemicals on Insects

There are several ways of administering a chemical to an insect. A commonly employed method is topical application, where the insecticide is dissolved in a relatively nontoxic solvent, such as acetone, and small, measured droplets are applied at a chosen location on the body surface. Topical application as is practiced today was made possible by the invention of the micrometer-driven syringe (Trevan 1922) and the use of continuous carbon dioxide anesthesia (Williams 1946). The advantages of this method are: n n1. n nThe high degree of precision and reproducibility that can be attained. n n n n n2. n nThe large number of tests that can be performed in a relatively short time. n n n n n3. n nThe small number of insects (10-20) required per replication. n n n n n4. n nThe simple and inexpensive equipment needed. n n n n n5. n nThe very small amount of chemicals and solvents used. n n n n n6. n nThe fact that the LD50 values obtained for any species are reasonably constant and reproducible from laboratory to laboratory, provided that identical conditions of testing are maintained.

Hazards Associated with Pesticide Use

There are several types of potential hazards associated with the use of pesticides. People exposed to some highly toxic compounds may suffer short-term or long-term health problems. Excessive residues in the environment may contaminate water supplies and lead to lower water quality. They may contaminate our food through excessive residues on sprayed crops. Pesticides may cause injury to nontarget organisms such as bees, birds, other wildlife, and natural enemies of pest insects. Improperly applied pesticides may cause damage to treated surfaces, or through drift to surfaces adjacent to treated areas. Some pesticides may be phytotoxic, that is, injurious to crops and ornamental plants. The mode of action of pesticides, the symptoms of poisoning they manifest, and therapy are discussed in the text under each functional group of pesticides.

Formulations of Insecticides

The majority of insecticides are applied as sprays, dusts, granules, microcapsules, vapors, aerosols, or seed dressings. After an insecticide is manufactured in a relatively pure form ( technical grade), it must be formulated before it can be applied. Formulation is the processing of a compound by such methods that will improve its properties of storage, handling, application, effectiveness and safety to the applicator and the environment, and profitability. Formulation is the final physical condition in which the insecticide is sold commercially. In most cases, it must be diluted according to the formulator’s instructions before use. The price for a given weight of chemical depends largely on the type of formulation, the most expensive being the pressurized aerosol.

Insecticides Acting as GABA-Ergic Agents

The GABA receptor-chloride ionophore contains the GABA recognition site and binding sites for a variety of agonists and antagonists (Fig. 10.1).