Pesticides field conditions

Field studies are required to provide a more reaUstic picture of the dissipation of the parent compound and those degradates determined to be significant. Under field conditions pesticides are exposed simultaneously to the individual dissipation processes that were examined separately in the laboratory studies. Thus, in field studies, some dissipation processes may be altered due to competition and interaction. Requirements for spray drift data were outlined in draft Subdivision R, but the EPA agreed that data generated on a generic basis by an industry consortium could represent the potential for drifting of individual pesticides. 

Stork A, Ophoff H, Smelt JH, et al. 1998. Volatilization of pesticides Measurements under simulated field conditions. In Fuhr F, Hance RJ, Plimmer JR, et al, ed. The Lysimeter Concept Environmental behavior of pesticides. Washington, DC American Chemical Society, 21-39. 

In recent years greater attention has been given to nitrogen containing pesticides and the possibility of their nitrosation in soil. The N-nitrosamines that form may arise from the parent pesticide or from a pesticide metabolite. The reaction calls for favourable pH conditions (pH 3-4) and excess nitrite. Under field conditions, the nitrosable residues are usually present in traces and only small quantities of these will actually be nitrosated in soils. However, the possibility exists that the small amounts of N-nitrosamines could be assimilated by plants. 

Assuming proper soil surface preparation (i.e., smooth with no soil clods or crop debris) and test substance application, the diameter of the soil probe does not generally impact observed pesticide residue concentrations in soil or associated variability. Nevertheless, a minimum diameter of 5 cm for the upper soil probe is recommended to improve sampling under less than ideal conditions. Increasingly, researchers are using probes having diameters >5 cm with good results under a variety of field conditions. 

The pesticide component of SWRRB takes into account the fate of the chemical applied under field conditions For example, the amount of pesticide actually reaching the ground after application over a plant canopy is calculated. Further, field dissipation of the chemical by photolysis on leaf surfaces as well as degradation in the soil is accounted for with the pesticide component of SWRRB. Leaching of the pesticide below the top 1cm of soil is also computed and runoff corrected for such losses. Further, adsorption of the pesticide to soil surfaces and sediment is taken into account by SWRRB. 

Davey, R.B., M.V. Meisch, and F.L. Carter. 1976. Toxicity of five ricefield pesticides to the mosquitofish, Gambusia affinis, and green sunfish, Lepomis cyanellus, under laboratory and field conditions. Environ. Entomol. 5 1053-1056. 

Ruzicka JH, Thomson J, Wheals BB. 1968. Gas-chromatographic determination of organophosphorus pesticides. Part V. Studies under field conditions. J Chromatogr 33 430-434. 

The manufacturer typically petitions the EPA to establish a tolerance at, or slightly higher than, the maximum residue encountered under the field studies. As such, one could consider the tolerance to represent a crude indicator of compliance with pesticide use conditions it is possible that blatant misuse of a pesticide could result in residues being detected in excess of the established... 

Andrawes, N.R., Bagley, W.P., and Herrett, R.A. Fate and carryover properties of Temik aldicarb pesticide [2-methyl-2-(methylthio)propionaldehyde 0-(methylcarbamoyl)oxime] in soil, J. Agric. Food Chem., 19(4) 727-730, 1971. Andrawes, N.R., Romine, R.R., and Bagley, W.P. Metabolism and residues of Temik aldicarb pesticide in cotton foliage and under field conditions, J. Agric. Food Chem., 21(3) 379-386, 1973. 

Dependencies among the input variables of a risk model can have pronounced effects on the output distribution, especially in the tails (Warren-Hicks and Moore 1998 US SAP 1999). Rainfall is fully independent of the intrinsic chemical properties of the pesticide, so that neither one depends on the other. But field conditions will most certainly affect the fate and transport of a pesticide once it is applied to the field. For example, the evaporation of the chemical from the field or plant surface depends on ambient temperature. Types of dependency include the familiar cases of independence and linear correlation, but also more complex relationships (Figure 2.4). 

The apparent rate of excretion was slower after dermal exposure than after oral administration, probably due to slower absorption of the 2,4,5-T ester from the skin than 2,4,5-T acid from the gut. This is in agreement with observations made by Feldmann and Maibach for 2,4-D and other pesticides applied to the forearm of human volunteers (13). Calculations by Ramsey et al. using three methods showed that 97% of the 2,4,5-T absorbed by forest workers would be excreted in urine within 7 days following dermal exposure under typical field conditions (16). 

Volatilization of pesticides is an important pathway for their loss from treated agricultural lands. The importance of volatilization in the forest environment has not been established by direct measurement, but can be inferred from volatilization rates of the same pesticides under agricultural conditions and from other data on their behavior in the forest environment. In recent years, several studies of actual volatilization rates of pesticides under field conditions have provided an assessment of the rate of input to the air under typical conditions of use (1). 

With so many opportunities, one cannot be surprised that many pesticides are found to undergo photochemical reactions. However, few photochemical investigtions have been made with forestry pesticides under practical field conditions, and illustration of the possible consequences requires a certain amount of extrapolation. 

Taylor, A.W. Post-application volatilization of pesticides under field conditions. J. Air Pollut. Contr. Assoc., 1978, 28, 922. 

Parmele, L.H. Lemon, E.R. Taylor, A.W. Micrometeorolog-ical measurement of pesticide vapor flux from bare soil and corn under field conditions. Water, Air, Soil Pollut., 1972, 1, 433. 

Pesticides are subject to considerable loss by evaporation when they are thinly spread over large areas of crop exposed to moving air. In this situation they are subject also to biochemical, photochemical, and solution losses which make it difficult to assess directly evaporation under field conditions. The rate of evaporation of water is easily determined and has been the subject of much experiment. The relationship of loss of pesticide to loss of water from the same surface can be verified by laboratory experiments. Crystallization and solution in leaf substances exert some effect also. When the pesticide is distributed in the soil, evaporation of water can accelerate that of the water-soluble pesticide the mechanism lies in capillary flow of solution and not in the evaporation process itself. 

Under field conditions water evaporation normally occurs from the same surfaces as evaporation of pesticide. Any temperature effect caused by the former will therefore affect the latter, and in this repect the state of affairs is simpler. On the other hand, leaves can restrict water evaporation, and the soil surface is very complex hence, other complications appear under field conditions. All these factors, however, will make the pesticide evaporation rate (if the pesticide is fully exposed on an outer surface) faster than that calculated from the water rate. 

In field conditions this complication would not occur. We are not likely to be concerned with a pesticide with as heavy a vapor as xylene (except in fumigation work), and convection from wind and local heating will always be more important than that produced by the vapor itself. This experiment is quoted as an example of a complication to avoid in laboratory work. 

Simple field observations may prove useful in situations where official sources of information are deficient. For example, it may be difficult to obtain a full picture of pesticide use from official sources because there may be widespread illegal use of pesticides, or use of unregistered pesticides in some instances also, official records may be incomplete due to smuggling of pesticides for purposes of evading taxation or other controls. In such situations, judgements need to be made about which pesticides are likely to be applied under observed field conditions. 

The ability of certain plants to absorb and store organochlorine pesticides has also been observed, the most notable examples being certain varieties of carrots, soybeans (6), and roots of forage crops (26). With carrots, for example, in a series of papers Lichtenstein and coworkers (12,13,14) have reported many conclusive experiments conducted under both controlled and field conditions. Marth (15) gives a comprehensive review of this type of work on numerous vegetables. 

J. Espinoza, Fate and Effects of Pesticides under Tropical Field Conditions Implications for and Research Needs in a Developing Country, in Proceedings of a Symposium Environmental Behavior of Crop Protection Chemicals, IAEA-SM-343/32, Vienna, Austria, 1997, pp 93-110. 

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