Persistence of pyrethroid insecticides in farm soils
Erin L. Amweg and Donald P. Weston
Introduction
Pyrethroid insecticides are widely employed in California agriculture to control a variety of crop pests. Recently, pyrethroid residues have been detected in agriculture-affected drains and streams
in the Central Valley, indicating that off-site transport of these compounds is occurring (Weston et al., 2004). Due to the strong hydrophobicity of pyrethroids, this transport is mediated by movement
of fine particles in irrigation return flows or rainfall events (Gan et al., 2005). While irrigation in fields occurs throughout the growing season for many crops, the timing and volume of water applications
can be managed. Loss of suspended sediments and associated pesticide residues due to irrigation return flow can potentially be reduced through proper irrigation management. Storm events, however, typically
occur from December through March and are less easily managed due to the large volume of runoff water. Therefore, the timing of pyrethroid application relative to rainfall events, and the rate of pyrethroid
degradation in the field soils ultimately determine the amount of pesticide susceptible to off-site transport.
Nearly all studies on the persistence of pyrethroids in farms soils are unpublished proprietary data collected by pesticide manufacturers. These data have been compiled and summarized by Laskowski
(2002), with only summary information presented regarding methods and test conditions. Additionally, most degradation studies were conducted with spiked sediments in the laboratory or under controlled
greenhouse conditions. In these past studies, degradation rates in soils for nine pyrethroids ranged from half-lives of 3.3-96.3 d under aerobic conditions, and 5-425 d in anaerobic soil. These studies
have not addressed potential changes in pesticide bioavailability that may occur during these time periods. Many hydrophobic organic contaminants have been shown to become less bioavailable with increasing
contact time between the contaminant and the particles to which they are adsorbed. Thus, the risk of aquatic toxicity may be diminished with time, even if the concentration as measured by conventional
chemical means were to remain unchanged.
The current study was intended to provide data regarding pyrethroid persistence in soils under real-world conditions, as well as provide information on changes in bioavailability for residues remaining
in the soils. These data are needed to accurately assess the potential risks and environmental impact of increasing pyrethroid use in agriculture, as well as develop farm management practices that reduce
these risks.
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