The Pesticide Threat

The reader now has some of the basic concepts necessary to objectively assess the real risk of consuming fruits and vegetables contaminated with residue levels of pesticides. There are a few more tools needed before we can solve the puzzle. Prior to addressing the true toxicology of these chemicals, we should become familiar with the actual culprits. 

What are pesticides The Merriam Webster s Collegiate Dictionary (1994) defines a pesticide as an agent used to destroy pests.  

An epidemic disease associated with high mortality spec. Plague 

Something resembling a pest in destructiveness esp. a plant or animal detrimental to humans or human concerns (as agriculture or livestock production) 

Thus we focus on chemicals used to treat plant and animal pests that are detrimental to agriculture and food destined for human consumption. The term pesticide is an umbrella for numerous compounds used to eliminate many types of pests. The bulk of the toxicology literature is concerned with the following  

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Carsel RF, Mulkey LA, Lorber MN, et al. 1985. The pesticide root zone model (PRZM) A procedure for evaluating pesticide leaching threats to groundwater. Ecological Modeling 30 49-69. 

Several aspects of the problem of herbicides being contaminated with nitrosamines, and the resulting inadvertent introduction of nitrosamines into the environment, will be discussed in other papers in this symposium. Unrecognized until less than five years ago, the situation has inspired intense debate and prompted several of the environmental chemistry studies mentioned in this paper. Like the presumed threat from the in vivo nitros-ation of pesticide residues, discussions sometimes lack the type of anticipated dose and effect calculations just mentioned. Unlike the active ingredients, whose benefits can justify residue tolerances and acceptable daily intakes, nitrosamine contaminents afford no known benefits, and the desirability of minimizing their levels is undisputed. 

Lawrence, A.R., Foster, S.S.D. (1987) The Pollution Threat from Agricultural Pesticides and Industrial Solvents. Hydrogeological Report No. 87/2, British Geological Survey, Wallingford, Oxfordshire. 

Based on results published so far, a threat to water quality by the pesticides permitted in organic fanning can not be assumed. Together with the fact of the complete absence of synthetic pesticides, however, a conclusive... 

Soil erosion continues to be one of the greatest threats to the sustainability of agriculture around the world. Erosion caused by water and wind reduces rich topsoil and crop yields. Soil erosion also produces a variety of adverse off-site impacts, including increased sedimentation of lakes and streams and transport of nutrients and pesticides to surface waters (Ribaudo and Johansson, 2006) (Table 1.5). 

Define the potential threat in terms of the toxicity of the pesticide to be handled and the conditions under which it will be handled (i.e., the exposure scenario). 

When a pesticide product has been found in groundwater or has characteristics that may pose a threat of contamination of ground water, the pesticide product labehng may contain statements to alert you to the concern. Typical pesticide labeling statements include ... 

So what is the future of agricultural aviation The author remains confident that aircraft provide a viable and efficient means of pesticide delivery in a wide range of environments and situations. How then should aerial application be managed to reduce the potential threat of off-target damage and spray drift ... 

Carsel, R. F., L. A. Mulkey, M. N. Lorber, L. B. Baskin, "The Pesticide Root Zone Model (PRZM) A Procedure for Evaluating Pesticide Leaching Threats to Groundwater," accepted for publication, Ecological Modeling 1985. 

An examination of the pesticide data in the report points out that the four Lindane detects were in the southern states and the three methoxychlor detects were in the western states (10). The maximum value found for either product was never more than 2% of the established MCL. Yet, the report, while stating that neither Lindane nor methoxychlor pose health threats at the levels found, goes further and concludes that since these products are poisons, even the presence of these small amounts is undesirable. Hasn t it been known for centuries that all substances can be poisons More troublesome is an extrapolation reported. From the 1% detects in the subsamples analyzed, the statement is made that 224,000 samples (IX of the 22,000,000 rural homes) had methoxychlor. While perhaps statistically appropriate, this extrapolation seems to ignore the fact that methoxychlor was only found in western state samples. 

Pesticides are first and foremost a serious threat to the quality of drinking water resources. The Pesticides Action Plan II is operating now in Denmark. This action plan was drafted on the basis of a report published by The Bichel Committee, which thoroughly investigated all of the aspects involved in the use of pesticides in agriculture. The comminee concluded that the consumption of pesticides could be halved in Denmark without causing any reduction in crop production. Besides a special tax was introduced in Denmark to reduce the pesticide consumption. This tax is acmally used to support research studies for developing alternatives to pesticides. 

Data from measurements of environmental matrices demonstrated that the environmental burden of some pesticides was increasing and that action was necessary to alleviate the potential threat of increased human and animal exposure from this source. From the regulatory point of view, an initial step was taken when the PR 70-15 notice was issued by the USDA requesting that pesticide registrants provide basic information on the environmental fate of pesticides as part of the data submitted for registration. 

A strong adsorption of the major part of a biocide on the soil clay minerals reduces its bioavailability and must be compensated by addition of increased amounts. On the other hand, dangerous biocide cations adsorbed on the clay minerals constitute a long-term health hazard when slowly released to the ground water. Coadsorption of the pesticide together with a non-phjrtotoxic compound or displacement of the pesticide (for instance diquat by 4-pyridyl pyridinium chloride [32]) reduce this threat and increase the bioavailability. As shown by Narine and Guy [20], the divalent cations paraquat and diquat are more easily displaced by washing with salt solutions than monovalent cations such as methylene blue and thionine. 

Chlorinated pesticides differ substantially in their toxicity to aquatic plants. Endrin is among the most toxic, inhibiting growth and carbon uptake at concentrations as low as 0.1-1.0 [xg (Menzel etal., 1970). Equally low levels of dieldrin and aldrin may reduce growth, whereas DDT and DDE generally have little inhibitory effect below 1.0 /xg (Luard, 1973 Powers etal, 1975). Development of tolerance to DDT and its derivatives is frequent, and there are several species that are apparently unaffected by DDT levels > 1000 jug L (Luard, 1973). Chlordane generally elicits toxic responses at concentrations < 50 ju.g L and is therefore more toxic than either heptachlor or toxaphene. However, this order is subject to considerable variability, due to the development of resistance, effects of other species-related variables, and differences in environmental conditions. Lindane and its isomers are generally the least toxic of the pesticides and probably pose little threat to aquatic plants in natural waters the majority of species tolerate concentrations > 1000 /xg L (Luard, 1973). 

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