508 1976). Hydroxylation and conjugation are also important in making the normally fat-soluble insecticides water-soluble so that they can be excreted. Each of these processes is enzy-matically controlled, and it is not surprising to find, therefore, that metabolic resistance to insecticides most often results from qualitative or quantitative changes in the enzymes concerned so that the rate of detoxication is increased. For example, a variety of esterases bring about hydrolysis, mixed-function oxidases commonly induce hydroxylation, and glu-tathione S-transferase is responsible for promoting conjugation with this tripeptide. In fact, the involvement of these enzymes in insecticide detoxication appears to be an extension of a more general function. Thus, insects that encounter a broad range of naturally occurring plant-derived toxicants, for example, polyphagous Lepidoptera, have significantly higher mixed-function oxidase levels than do oligophagous species (Ronis and Hodgson, 1989). In some insecticide-resistant strains, an increase in the amount of detoxicant enzyme has been observed, which at the gene level results from either an increase in the rate of transcription (mRNA production) or gene amplification (the presence of many identical copies of the DNA coding for the enzyme) (Devonshire and Field, 1991). In others, it appears that the enzyme has changed so that it is now more specific toward its "new" substrate, the insecticide. In a few species, resistance seems to have developed as a result of an increase in the quantity, or decrease in the sensitivity, of the enzyme normally affected by the insecticide, specifically cholinesterase in the nervous system.

Mechanisms for detoxication vary among the different categories of insecticides. It is appropriate, therefore, to examine separately the metabolism of compounds in these categories. Three categories of insecticides will be considered: chlorinated hydrocarbons, organophosphates, and carbamates.

Among the chlorinated hydrocarbon insecticides are DDT, lindane (y-BHC), chlordane, heptachlor, aldrin, and isodrin.* The chlorinated hydrocarbons act on the nervous system, preventing normal impulse transmission by binding to sodium or chloride channel proteins in the axonal membrane (Bloomquist, 1996; Zlotkin, 1999). By binding to sodium-channel proteins, DDT and its analogues enable sodium ions to diffuse readily across the membrane of excitatory neurons; thus, the membrane becomes permanently depolarized. DDT was the first synthetic insecticide to be developed and "appropriately" was the first to which insects developed resistance. Resistance to DDT and its analogues is most often the result of the presence of an enzyme that dechlorinates the compound, forming the less toxic dichloroethylene derivative, DDE. Some species, however, convert DDT into other less harmful materials such as DDA (the acetic acid derivative), dicofol (kelthane, the trichloroethanol derivative), and DDD (the dichloroethane derivative). Many comparisons of the activity of the DDT-degrading enzyme in resistant and susceptible strains of a species have shown, however, that metabolic resistance alone is often insufficient to account for the full extent of resistance. Specifically, the known maximum rate of degradation of DDT measured in vitro is not high enough to account for the high tolerance shown by the resistant strain. In such cases, further work has usually shown the importance also of physical resistance mechanisms of the type outlined earlier.

The cyclodiene compounds, heptachlor, aldrin, and isodrin, are of interest from several viewpoints. In themselves they are not toxic but are oxidized within an insect's tissues to the highly toxic epoxy derivatives, heptachlorepoxide, dieldrin, and endrin, respectively, a process known as "autointoxication." Because of this conversion, insects treated with

* These are approved common names. For the chemical names, see Perry and Agosin (1974).

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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