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SRWP Studies on Pyrethroid Insecticides and Their Effects in the Sacramento River Watershed

The California Department of Pesticide Regulation’s Re-evaluation of Pyrethroid Insecticides

Monitoring by the California Department of Pesticide Regulation (CDPR), the SRWP-supported work, and other studies throughout California have documented the presence of pyrethroid insecticide residues in both agricultural and urban-affected waterbodies, and concentrations sufficiently high to represent a threat to sensitive aquatic life. These findings have sparked the recent entry of pyrethroids into a formal process known as “re-evaluation.” It is by far the largest re-evaluation effort CDPR has ever pursued and unusual in that it has been prompted by environmental, rather than human health concerns.

It is probably surprising to most Californians to learn over 1,200 pyrethroid-containing products are registered with CDPR for use in the State. About half of these products are used indoors, are in containerized baits, or for other reasons would be unlikely to contribute to the residues found in surface waters, and are therefore excluded from re-evaluation. However, the re-evaluation does encompass slightly over 600 products from over 120 companies that have registered pyrethroids with CDPR. The purpose of the re-evaluation is to obtain data from these manufacturers on the toxicity, environmental fate, sources, and potential mitigation practices for their products. This information will be provided over the next few months to next couple years, depending on the effort necessary to obtain specific data.

The re-evaluation is likely to result in modification of permissible application practices or allowable uses of specific products. While CDPR is certainly aware of the recent environmental findings regarding pyrethroids, they also view the compounds as preferable to many of the alternatives. Thus, it is unlikely that the re-evaluation will result in the withdrawal of a large number of pyrethroid products. Rather, it is hoped that the data provided will identify a narrow subset of products or practices that are contributing to the surface water impacts being observed and that these conditions can be corrected by specific and well-targeted regulatory actions.

In 2005, the SRWP was awarded a grant from the Pesticide Research and Identification of Source and Migration (PRISM) program of the State Water Resources Control Board for a study focusing on pyrethroid insecticides. Regulatory agencies and other stakeholders were aware of emerging pyrethroid use and the potential for environmental impacts, but lacked data by which to assess these risks. This study was intended to provide such data and help address uncertainties surrounding pyrethroids and identify management methods to reduce environmental impacts. The work was led by UC Berkeley, and included collaboration with Southern Illinois University, Pacific EcoRisk, UC Davis, and the California Department of Fish and Game.

When the SRWP pyrethroid study began, there was remarkably little data on the environmental fate or effects of pyrethroids on aquatic systems despite their widespread and rapidly growing use. In particular, there was no data at all on their presence in urban creeks, though pyrethroids have become the dominant insecticide used in urban settings. The SRWP study, focused on three main topics: 1) pyrethroids in urban creeks; 2) their persistence in farm soils: and 3) determining thresholds for toxicity.

Urban Creek Studies

At the outset of the study, available pyrethroid monitoring data was limited to agricultural water bodies; thus the impact on the environment was often perceived as an agricultural problem. However this perception ignores the fact that agricultural use of pyrethorids pales in comparison to urban use. It is used in the urban environment that is growing rapidly as pyrethroid-containing products replace those with diazinon and chlorpyrifos, compounds that can no longer be sold for use in urban environments.

The SRWP-supported study examined seven creeks in Sacramento. Toxicity in the water of many of these creeks had been previously reported, and found to be toxic due to diazinon or chlorpyrifos insecticides. However, pyrethroids are different in that they bind quickly to sediments; therefore, it is bottom-dwelling animals, rather than those in the water column, that are at the greatest risk. Our sampling of sediment in the Sacramento creeks showed pyrethroids to be present in all of them; every one of 28 samples had measurable concentrations of the compounds. Of greater concern was the fact that at least some portions of six of the seven creeks contained concentrations high enough to be toxic to sensitive aquatic life. Using a crustacean nationally used for sediment toxicity testing, two-thirds of the sediment samples were found to be toxic to the animal. Subsequent work in several other communities has shown these findings are not unique to Sacramento.

The one pyrethroid that contributed about two-thirds of the toxicity was bifenthrin. This compound is used around the exterior of homes by professional pest control applicators, and is also available for retail purchase by consumers, often as an insecticide for lawns and sometimes mixed with fertilizer. Whether it is professional or homeowner use of bifenthrin that is leading to appearance of the compound in creek sediments remains an open question, but the answer is likely to emerge from studies done over the next couple years.

In an interesting follow-up study, it has been shown that the pyrethroids in Sacramento creeks even played a role in increasing the aquatic impacts of the aerial spraying for mosquitoes over the City. A substance in the product applied, known as PBO, was intended to enhance the toxicity of the insecticides being sprayed, but appearance of the compound in Sacramento creeks after the spraying may also have slightly increased the toxicity of the pyrethroids already present in the sediments.

Persistence in Farm Soils

Though there is some dormant season application of pyrethroids in orchards, the majority are applied to crops during the growing season. The key to minimizing pyrethroid movement off the fields and into nearby creeks is to minimize erosion of soil particles and the insecticides they carry. While minimizing erosion is clearly in the interest of growers for reasons even beyond pesticide concerns, the strategies for accomplishing it will depend on whether control of dry season irrigation runoff or wet season stormwater runoff is the objective. For pyrethroids applied to crops in the summer, transport by irrigation runoff is of concern, but it is more readily managed by irrigation management practices than is control of runoff from winter storms. A key question that exists is how long pyrethroids remain in soils in a toxic form, and whether they would persist from summer application until winter rains.

The SRWP studies examined three farms: a pear orchard that applied the pyrethroid esfenvalerate, a tomato farm that applied lambda-cyhalothrin, and a rice farm that applied lambda-cyhalothrin. Environmental persistence of chemicals is measured using the concept of half-life; the length of time it takes half of any given amount of the compound to degrade. For the pyrethroids in the farm soils, the half-lives were relatively short, averaging 40 days for esfenvalerate in the orchard soil, and 23 and 54 days for lambda-cyhalothrin at the tomato farm and rice farm, respectively. The message from these results is control of sediment loss via irrigation runoff is critical for controlling off-farm movement of pyrethroids, but winter storms are likely to be of less significance because much of the compound will have degraded by that time.

Toxicity Thresholds

Most currently used pyrethroids were registered with the Environmental Protection Agency (EPA) 20-30 years ago with little or no testing of the compounds on species living in the sediment. Regulatory emphasis was placed on toxicity to water column animals, despite the fact that pyrethroids quickly adhere to sediments and thus do not stay in the water column. Despite decades of use, the amount of pyrethroids toxic to the organisms that are widely used for sediment testing remained unknown.

The SRWP study determined the levels of pyrethroids needed to cause toxicity to Hyalella azteca, a small, shrimp-like crustacean that is used in virtually all sediment toxicity testing in California. The results showed that toxicity varied 20-fold depending on the particular pyrethroid, and the most used pyrethroid (permethrin) was also the least toxic. However, several other pyrethroids were toxic at about 3 parts per billion; roughly equal to the weight of a few grains of dry rice in a dump truck load of soil.

Data on the amount of pyrethroid needed to cause toxicity has been critical in developing monitoring programs for the substances. Before the SRWP work, much of the State’s monitoring was done with a detection limit of 10 parts per billion. Thus, concentrations could be three times lethal levels and would still be chemically undetectable. The data provided by this study has been critical in encouraging laboratories to improve their analytical capabilities for the compounds.

A great deal has been learned in the past few years about pyrethroids in the environment through the research done under this SRWP-supported PRISM project. Further details on studies described above can be found at www.sacriver.org. The urban work in particular, has provided important lessons on how lawn care or structural pest control practices in residential areas can have adverse consequences in nearby creeks that homeowners rarely consider when they make choices for pest control. The results have had considerable impact in designing monitoring programs, in State pesticide regulatory efforts (see sidebar The California Department of Pesticide Regulation’s Re-evaluation of Pyrethroid Insecticides), and in ongoing federal re-registration of pyrethroid compounds. Pyrethroids offer a number of advantages to the organophosphate insecticides they have replaced for some uses, though the challenge remains to keep residues from moving from the site of application and affecting non-target animals. Efforts are underway to provide more precise identification of sources and develop measures to minimize these unintended effects.