Paraquat applicators

Paraquat applications to spruce plantations for grass control had no effect on the movement or density of field mice (Microtus arvalis) and voles (Microtus agretis), but shrews (Sorex sp.) migrated from treated areas to untreated ones (Summers 1980). 

Machado-Neto, J.G., T. Matuo, and Y.K. Matuo. 1998. Efficiency of safety measures applied to a manual knapsack sprayer for paraquat application to maize (Zea mays hf.Arch. Environ. Contam. Toxicol. 35 698-701. 

Paraquat is practically nontoxic to honey bees, only slightly toxic to fish, and moderately toxic to terrestrial animals. Hazard for birds and mammals is generally short lived after paraquat application. 

The herbicide paraquat (l,l -dimethyl-4,4 -bipyridinium ion) is used widely in vegetation control programs. It is estimated that paraquat has been used in the United States on 10-12 million acres annually and is registered on 63 crops. Recommended rates vary from 0.25 to 1.5 Ib/acre. Human exposure should be limited during application, due to the toxic nature of paraquat. Most worker exposure studies (1-3) have addressed the issue of occupational body burden idien engaged in paraquat application with conventional spray equipment. Precautions taken to reduce respiratory and dermal routes of entry substantially lower the risk of potential adverse health effects. 

After 8 h the treated Cannabis plants showed signs of substantial desiccation and browning, which probably further reduced the transfer of paraquat to the gloves of harvesters. The cumulative residues for both hands after 0, 1, 2, 4, 8, and 29 h was 1.45, 2.42, 3.07, 3.60, 4.01, and 4.24 mg, respectively. These cumulative residues would appear to be higher than those reported by Forbess et al. (1) on hands of either sprayer (0.081-1.324 mg) or mixers (nd-0.231 mg) engaged in paraquat applications to grape vineyards. 

The recommended field application rates for terrestrial weed control usually range between 0.28 and 1.12 kg paraquat cation/ha (0.25 and 1.0 pounds/acre), between 0.56 and 2.24 kg paraquat dichloride/ha (0.5 and 2.0 pounds/acre) — both applied as an aerosol — and between 0.1 and 2.0 mg/L for aquatic weed control, although sensitive aquatic plants may be affected between 0.019 and 0.372 mg/L (Ross etal. 1979 Summers 1980 Bauer 1983 Dial and Bauer 1984). Paraquat is frequently used in combination with other herbicides (Fletcher 1974 Summers 1980). Water solutions of the dichloride salt, which usually contain 240 g/L, have been successfully mixed with 2,4-D, substituted ureas, dalapon, amitrol, and various triazines (Anonymous 1963, 1974). 

Paraquat is not degraded significantly in soil during incubation periods up to 16 months at 25°C by chemical or microbiological vectors (Smith and Mayfield 1978). For example, paraquat dichloride applied once annually at 4.48 kg/ha, or 4 times annually at 1.12 kg/ha, remained essentially undegraded in the soil for 6 years (Fryer et al. 1975 Moyer and Lindwall 1985). Massive applications... 

Paraquat is used to control aquatic weeds. It also passes into aquatic environments through rain, where it is rapidly accumulated by aquatic organisms, especially fish (Gabryelak and Klekot 1985). Paraquat applied to control aquatic weeds is accumulated by aquatic macrophytes and algae, and it is adsorbed to sediments and suspended materials. Initial applications of 1 to 5 mg/L in the water column are usually not detectable under field conditions after 8 to 27 days (Summers 1980). The half-time persistence of paraquat in water column at normal doses for weed control (i.e., 0.5 to 1.0 mg/L) was 36 h less than 0.01 mg/L was detectable in 2 weeks (Calderbank 1975). In solution, paraquat was subject to photodecomposition and microbial metabolism, degrading to methylamine... 

Adverse effects of paraquat in sensitive species of terrestrial plants and soil microflora have been documented at application rates of 0.28 to 0.6 kg/ha (death, inhibited germination of seeds, reduced growth), at soil concentrations of 10 to 25 mg/kg (growth inhibition), and at soil-water concentrations as low as 1.6 mg/L (reduced growth, inhibited synthesis of protein and RNA). Among terrestrial invertebrates, certain species of mites were sensitive to paraquat at recommended rates of application, and the sensitive honey bee died when its diet contained 100 mg/kg. However, paraquat in soils was not accumulated by earthworms and other species of soil invertebrates after applications up to 112 kg/ha. These points, and others listed in this section, are discussed in greater detail later. 

Paraquat is embryotoxic to sensitive species of birds. Concentrations equivalent to 0.056 kg/ha applied in oil solution to the surface of eggs of the mallard (Anas platyrhynchos) inhibited development when applied in aqueous solution, paraquat was toxic at a dose equivalent to 0.56 kg/ha. In each case, adverse effects occurred below the recommended field application rate of about... 

Aqueous solutions 2.8 mg/kg egg, equivalent to 0.56 kg/ha or 0.5 pounds/ acre 8.3 mg/kg egg 23% dead, survivors stunted, 9% abnormal (Hoffman and Eastin 1982) 50% dead. The computed LC50 concentration was about 1.5 times the recommended field application rate. At 1.5-3.0 times the field level, paraquat produced abnormal development, including edema, stunting, and brain malformations (Hoffman and Albers 1984)... 

The degradation rate of paraquat in certain soils can be slow, and the compound can persist for years — reportedly in a form that is biologically unavailable. But data are missing or incomplete on flux rates of paraquat from soil into food webs and on interaction dynamics of paraquat with other herbicides frequently applied at the same time. It seems prudent at this time to keep under close surveillance the residues of paraquat in soils in situations where repeated applications have been made over long periods of time (Summers 1980). 

In Argentina, paraquat concentrations used to control aquatic weeds is set at 0.1 to 0.2 mg/L, with no more than four applications each year (Tortorelli etal. 1990). However, some aquatic... 

Eggs of migratory waterfowl seem to be especially sensitive to paraquat at recommended application rates in an oil vehicle, but were significantly more resistant to the same dose applied in water (Table 22.6). Application of paraquat in oil solution appears contraindicated in areas containing nesting waterfowl. 

Holmstedt (1959) and Brown and Muir (1971) have reviewed perocular absorption of pesticides. More recently, Sinow and Wei (1973) have shown that the quartemary herbicide paraquat can be lethal to rabbits if applied directly to the surface of the eyes. Parathion, in particular, is exceedingly toxic when administered via the eye, a concern that must be kept in mind for the protection of pesticide applicators. 

Electrophoretic and isotachoelectrophoretic techniques are gaining in popularity in soil analysis with applications to polyaromatic hydrocarbons, polychlorobiphenyls, tetrahydrothiophene and triazine herbicides, Paraquat and Diquat and growth regulators. Other lesser-used techniques include spectrophotometric methods (five determinants), spectrofluorimetric methods (two determinants), luminescence methods (one determinant), titration methods (one determinant), thin-layer chromatography (five applications), NHR spectroscopy (two applications) and enzymic immunoassays (one determinant). 

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