Development enhanced degradation

The exact mechanisms for microbial adaptation to the pesticide molecule in soils that develop enhanced degradation capacity are not completely understood. These processes could be viewed from the ecological and population aspects, from their biochemical and enzymatic reactions, or from the genetic aspects, in which extrachromosomal elements may be involved as part of the process. 

In the laboratory, some of the diphenamid-treated soils lost their activity after 1 or 2 years, but the enhanced degradation could be restored in these soils much faster than it took to develop enhanced degradation in the first place in the previously non-treated soil. Handling and storage conditions of the soil in the laboratory are critical for preserving the activity. 

Artificial Aging by Heat. The procedure developed earlier (11) was followed. Fabric pieces (15 X 15 cm) were placed on racks covered with a Fiberglas screen (7- X 3-cm mesh) in a forced convection oven preheated to 150 °C. The screen was used to prevent any enhanced degradation that might be caused by direct contact with the metal rack. The control silk fabric was heated for up to 4 days in 6-h increments, and then immediately placed in a desiccator that contained silica gel to keep the silk dry while cooling. 

In an effort to determine the criteria that should be used to invoke cases of enhanced degradation, an experimental approach for Its study was developed that focused on laboratory investigations with field-collected soils. It was obvious that Insecticide control failures were common occurrences and certainly not all due to enhanced degradation, as Investigations of faulty application methods and unusual environmental conditions have shown (18). The ideal approach to the study of enhanced degradation would Involve controlled field research in which pesticide persistence and control efficacy were both measured at many locations over a number of years. However, the tremendous cost In time and effort and confounding of results by environmental variables make a controlled laboratory approach desirable. The limitation of laboratory efforts focused exclusively on the soil-lnsecticlde Interaction is that they cannot fully address the additional insect-insecticide and Insect-crop interactions present in the field. This means that caution must be excercised when proof of enhanced degradation is discovered In the laboratory, for this does not necessarily mean that Insect control and crop yield will be adversely affected under field conditions. 

Therefore, although little mineralization of the carbofuran ring occurred in any soil, there was a tremendous accumulation of soil-bound residues in the soil in which carbofuran was rapidly degraded. The enhanced degradation of carbofuran has been extensively documented (18). The behavior of cloethocarb was similar to that of carbofuran, and it too was extensively degraded in the history versus the nonhistory soil with substantial accompanying production of soil-bound residues. Cloethocarb is a carbamate insecticide that was under development for the rootworm insecticide market but was withdrawn about the same time that decreased persistence in soil after repeated use was noted (28). 

It is important to remember that, for insecticides, enhanced degradation in Boil is not a new phenomenon, but only a newly observed one. It iB unreasonable to think that the effect of the first encounter between an insecticide and a soil microbial population in 1983 or 1989 would differ from that same encounter occurring in 1965 or 1970. It follows that, some soil applied insecticides were subjected to "enhanced degradation" within a few weeks of their introduction. Therefore, it iB unscientific to describe a particular chemical as "once having been a good material but now is of questionable value because enhanced degradative activity developed after a history of uBe". A more scientific... 

Development of enhanced degradation of EPTC in a Brookston soil was rapid, being evident after only a single year treatment. The application of the soil rhodanese assay to identify soils with enhanced EPTC capabilities was not successful. Studies with Brookston soil also indicated that addition of only a small amount of an enhanced soil (0.1%) to a non-enhanced soil was capable of increasing EPTC degradation rates in the non-enhanced soil. 

This summary presents data on two pesticides with. different molecular structures and biological activities, with the emphasis on the microorganisms associated with the development of enhanced degradation and the possible enzymatic reactions that are taken place in these processes. 

Several factors that influence enhanced microbial degradation include nutrient value of the metabolite molecule, toxicity of the metabolite to soil microorganisms, and the availability of the metabolite to soil microbes. Comparisons of several pesticides, and their respective degradation products provide insight into the question of why soil microbial populations can develop rapid degradation capabilities for some pesticides but not others. 

What I hope to do in this paper is to give the reader a brief introduction to bacterial genetics and then to describe the research that has demonstrated the involvement of plasmids in carrying genes which encode enzymes that degrade pesticides. I will then discuss the possible role of plasmids in the development of the enhanced degradation phenomenon and the types of research that may lead to the delineation of the molecular events that lead to the development of rapid pesticide degradation in soils. 

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