Food Uptake And Utilization

these compounds show no symptoms for 1 or 2 hours after treatment, in contrast to insects 509

treated with other insecticides that react within a matter of minutes. In contrast to DDT, these insecticides block chloride channels in inhibitory neurons, by binding to the GABA-receptor protein, causing hyperexcitation of the nervous system. The resistance shown by certain strains is not because they no longer convert an insecticide to its toxic form as might be anticipated. Further, the toxic derivatives appear to have great stability, remaining unchanged even in resistant insects for several days. Resistance is due to a simple change in the structure of the GABA-receptor protein, specifically, substitution of alanine to serine or glycine (ffrench-Constant et al., 1993, 2000).

Organophosphates (e.g., parathion, malathion, diazinon, and dimethoate) bind cova-lently with and inhibit the action of cholinesterase, the enzyme that normally degrades acetylcholine at excitatory synapses, though there are reports that their toxicity is partially related also to inhibition of other tissue esterases. As with chlorinated hydrocarbons, resistance to organophosphates may be developed as a result of physical change, but generally is metabolic. Like cyclodienes, many organophosphates are "activated" (rendered more toxic) as a result of oxidation; for example, parathion is converted to paraoxon. Thus, resistance may be caused by a decrease in the rate of activation and/or an increase in the rate of conversion of the compound to a non-toxic form. (Apparently, resistance does not develop as a result of decreased sensitivity of the cholinesterase to an insecticide.) Many reports have shown that resistant strains are more able to carry out conjugation, especially with glutathione, or hydrolysis of the insecticide than are susceptible insects. This ability results from the presence of either greater quantities of esterifying enzymes or enzymes that, through mutation and natural selection, have become more specific for an insecticide.

Carbamates, for example, furadan, sevin, pyrolan, and isolan, are substituted esters of carbamic acid, which, like organophosphates, attack cholinesterase. Resistance to these insecticides also is very similar to that for organophosphates. Some resistance can be achieved by physical changes, but most is the result of increased rates of degradation, especially through oxidation and hydrolysis.

Shortly after the discovery that most resistance is metabolic, that is, results from increased quantities or specificity of particular enzymes that cause more rapid breakdown of an insecticide, it was realized that the phenomenon of synergism might be explored to advantage in the use of insecticides. Synergism describes the situation in which the combined effect of two substances is much greater than the sum of their separate effects. In practical terms, in the present context, it means that appropriate substances (synergists), when mixed with an insecticide, would increase the latter's effectiveness by combining with (and inhibiting) the enzymes that normally degrade the insecticide. The synergists used may be quite unrelated chemically to the insecticide but, most often, are analogues. The principle of synergism has been applied with limited success in the case of pyrethrin insecticides and DDT. For example, in the early 1950s DMC, the ethanol derivative of DDT, was found to be an effective synergist for DDT in DDT-resistant house-flies. However, perhaps not surprisingly, by 1955 the flies had developed resistance to the combination!

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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