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Wisconsin Cranberry School 2004 proceedings

Mahr, Dan
How long do insecticide residues persist?,   pp. 16-19

Page 16

Dan Mahr
Extension Entomologist
University of Wisconsin - Madison
A question that I frequently get asked is "How long do insecticides last after spraying?"
Usually this question relates to the insecticidal effectiveness of the spray. But with some growers
experiencing abnormally high acephate residues in 2003, the question is equally relevant to
residues at the time harvest in relation to federally-allowable limits. The question is not an easy
one to answer, as a diversity of factors impact insecticide persistence. Further, in many cases,
there has been little research on specific pesticides against specific pests on specific crops. Once
the Environmental Protection Agency (EPA) determines what the allowable residues will be on a
crop at harvest, chemical companies must then do field trials to determine how long residues last
after application. But much of this information is held by the companies and never published in
accessible locations. Further, for pests of minor crops such as cranberry, there has been relatively
little research done on the effective life of pesticides after spraying, or the factors that lengthen
or shorten the period of effectiveness.
Insecticides begin to break down as soon as they are mixed in the spray tank. Indeed,
many products start to break down while still in the original container, and most products have
an acknowledged shelf life. Once mixed and applied, factors that influence rate of breakdown
*  the chemistry of the insecticide,
*  chemical and physical properties of spray additives,
*  chemistry (pH, hardness) of the spray water,
*  a multitude of environmental factors (temperature, humidity, rainfall),
*  factors relating to the plant (surface chemistry, waxiness, etc.).
All of these factors can influence (1) the insecticidal effectiveness of a product, (2) its fate and
persistence in the environment, and (3) the amount of residues left in or on the berries at harvest.
Defining some terms. The following terms relate to pesticide residues.
Residue. Any quantity of the originally applied pesticide chemical. This can refer to
residues in or on the plant, or in the environment, such as soil residues.
Residual effectiveness. The period of time the the applied material remains as an effective
Half-life. The amount of time it takes for V2 of the original material to be broken down or
removed. For each additional half-life period, 50% of the remainder will be lost.
Breakdown product. Insecticides disappear in a variety of ways. They may be washed off
and end up in the soil or water. They may evaporate. Or they may decompose. Decomposition
can be caused by light (photodecomposition), chemical reactions, or other factors. When the
molecules break down, smaller molecules of various types remain; these are called breakdown
products. These in turn can further decompose into yet additional breakdown products.
Breakdown products may be either more or less toxic (e.g. to mammals) than the original
insecticide. For example, a breakdown product of acephate is methamidophos, which is itself

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