Dissolved insecticides

When rainfall intensity exceeds infiltration rate and surface-storage capacity has been reached, overland flow begins. The transfer of dissolved pesticides from the soil matrix to overland flow consists of several mechanisms desorption from soil organic matter, mineral surfaces and plant residues dissolution of insecticide crystals or granules and diffusive and turbulent transport of dissolved insecticide from soil water into overland flow (2, 62). The relative importance of each process depends on the physico-chemical properties of the chemical, formulation, initial placement, soil properties, recent hydraulic history and vegetation (62). 

Dissolved insecticides are transferred from soil solution to surface runoff through the concurrent mechanisms of molecular diffusion, raindrop impact induced turbulent diffusion, and shear stress induced mass transfer (63, 64). In addition, shallow interflow may contribute dissolved chemicals to surface runoff as it returns to the surface downslope or seeps into rills and furrows 65). Most studies of dissolved chemical transport into overland flow have employed inorganic tracers such as bromide, gypsum (CaS04 2H20) and 66, 67). The behavior of organophosphorus insecticides, however, is considerably more complex due to association with particulate and colloidal natural organic matter. 

Once dissolved pesticides have been extracted from the soil matrix into overland flow or reach the surface through shallow interflow, they are transported toward the field outlet with surface runoff For one-dimensional overland flow, dissolved insecticide transport can be expressed as 76) ... 

Agricultural Products. Pesticides are frequendy appHed as emulsiftable concentrates. The active insecticide or herbicide is dissolved in a hydrocarbon solvent which also contains an emulsifier. Hydrocarbon solvent selection is critical for this appHcation. It can seriously impact the efficacy of the formulation. The solvent should have adequate solvency for the pesticide, promote good dispersion when diluted with water, and have a dash point high enough to minimise dammabiUty ha2ards. When used in herbicide formulas, low solvent phytotoxicity is important to avoid crop damage. Hydrocarbon solvents used in post-harvest appHcation require special testing to ensure that polycycHc aromatics are absent. 

Approximately 5% of the U.S. consumption of is in agriculture. Boron is a necessary trace nutrient for plants and is added in small quantities to a number of fertilizers. Borates are also used in crop sprays for fast rehef of boron deficiency. Borates, when apphed at relatively high concentration, act as nonselective herbicides. Small quantities of borates are used in the manufacture of alloys and refractories (qv). Molten borates readily dissolve other metal oxides usage as a flux in metallurgy is an important apphcation. Other important small volume apphcations for borates are in fire retardants for both plastics and ceUulosic materials, in hydrocarbon fuels for fungus control, and in automotive antifreeze for corrosion control (see Corrosion and corrosion inhibitors). Borates are used as neutron absorbers in nuclear reactors. Several borates, which are registered with the Environmental Protection Agency (EPA) can be used for insecticidal purposes, eg, TIM-BOR. 

It is also necessary, from the military point of view, that the insecticides supplied to troops in the field be readily convertible into end-use items. It should not be necessary to go through an involved procedure in preparing insecticides for field utilization. For example, the earliest lots of DDT that were received in the North African theater were of the consistency of beeswax, and were extremely difficult to get into solution. It was necessary to process all this material through meat grinders, of the hamburg variety, requisitioned from the civilian economy before this material could be satisfactorily dissolved in Diesel oil. Such a situation complicates field operations unnecessarily, and should be avoided in the development of future insecticides. 

Still another requirement is that the concentrated form of the insecticide readily combines with various types of vehicles. At the present time, pure DDT can be dissolved only in organic solvents, and it has been necessary to provide a 50% water-dispersible DDT powder when wrater was to be used as the vehicle. With the advent of 90% water-dispersible DDT, it may be possible to utilize this material in the preparation of both water suspensions and organic solutions of the chemical. The ideal chemical in this respect would be one that could be shipped as 100% dust, and could be readily diluted with inert dust, water, and organic solvents in the field. 

DDT met all but one of Muller s ideal characteristics for an insecticide. It was a cheap, contact poison without objectionable odors. It was stable in air and light. Because it was so powerful, extremely small doses could be used. And finally, it dissolved so poorly in water that warm-blooded organisms absorbed only traces of it. The fact that DDT dissolves well in oils did not seem dangerous only later did scientists realize that, because DDT accumulates in animal fat and mammals milk, it becomes increasingly more concentrated in predator species as it moves up the food chain. DDT s only failing, as far as Muller s original conception was concerned, was that it did not kill immediately. 

Muir, D.C.G., B.R. Hobden, and M.R. Servos. 1994. Bioconcentration of pyrethroid insecticides and DDT by rainbow trout uptake, depuration, and effect of dissolved organic carbon. Aquat. Toxicol. 29 223-240. 

Zhou JL, Rowland S, Mantoura FC (1995) Partition of synthetic pyrethroid insecticides between dissolved and particulate phases. Water Res 29 1023-1031... 

Broadly speaking herbivorous animals (guinea-pig excepted) show cardiac symptoms and carnivores develop central nervous system convulsions or depression with omnivores both heart and central nervous system may be affected. Cold-blooded vertebrates are usually less sensitive to fluoroacetate, but frogs are more sensitive in summer than in winter.2 Fish appear to be insensitive to fluoroacetate dissolved in water.3 Insects are easily killed by fluoroacetate, and the use of sodium fluoroacetate as a systemic insecticide is described on p. 182. 

Extraction Procedure. We modified the extraction procedure of Nelson et al (10). Brie acidified with 2 ml of 5% trichloracetic acid (TCA) was extracted 3 times with 20 ml of petroleum ether. The combined extracts were reduced to 5 ml in a rotating evaporator, returned to the separatory funnel, and combined with 60 ml each of acetonitrile and distilled water. The acetonitrile-water-insecticide mixture was extracted twice with 60 ml of petroleum ether and anhydrous Na2S04 was added to the combined 120 ml extract. The extract was evaporated just to dryness and the residue was dissolved in benzene for analysis by gas-liquid chromatography (GLC). Extraction efficiencies in spiked experiments were 73% (aldrin) and 83% (dieldrin). 

DDT enters an insect by dissolving the thin layer of fatty substances that repel water from the waxy outer skin and then paralyzes vital nerve centers. Muller was disappointed to learn that he was not the first discoverer of DDT 65 years earlier, a graduate student Othmer Zeidler had synthesized the compound as part of his chemistry doctoral thesis. Zeidler described many of DDT s properties and developed the method used to make it commercially, but he did not discover its insecticide powers. When Geigy took out the basic Swiss patent in 1940, it was not for the composition matter, but for its use as an insecticide. This began the era of synthetic chemical pesticides. 

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