The pesticides, called pyrethroids (pie-REE-throids), have been considered safe for fish and other organisms that live in the water column, but no one has studied their effect on sediment-dwelling organisms, such as midge larvae or shrimp-like amphipods, said University of California, Berkeley, biologist Donald P. Weston, adjunct associate professor of integrative biology. These two organisms are used by the U.S. Environmental Protection Agency (EPA) as indicators of the health of fresh water sediment.
Weston and colleague Michael J. Lydy (LIE-dee) of Southern Illinois University (SIU) in Carbondale collected sediment samples from 42 rivers, creeks, sloughs and drainage ditches in California's Central Valley and exposed amphipods and midge larvae to the sediments for 10 days. Twenty-eight percent of the sediment samples (20 of 71) killed amphipods at an elevated rate, and in 68 percent of these sediments, the pyrethroids were at levels high enough to account for the deaths. Thus, while other pesticides may well have contributed to the amphipod deaths in some sediment samples, pyrethroids alone explain the toxicity in the vast majority of the sediment samples, Weston said.
"About one-fifth of our Central Valley sediment samples are toxic to a standard testing species due to a class of pesticides no one has tested for before, for which there are little data on their toxicology when sediment-bound, and which are being promoted as an alternative to the increasingly restricted organophosphate insecticides," he said.
The study by Weston, Lydy and post-doctoral researcher Jing You in the Department of Zoology at SIU appeared in the April 8 online version of the American Chemical Society's journal Environmental Science & Technology and will be published later in hard copy.
In the tests, the midge larvae died at higher rates when exposed to sediment from 13 percent of 39 collection sites, and 40 percent of these sediment samples contained enough pyrethroids to account for the deaths. Weston notes that these midges (Chironomus tentans) are known to be about three times less sensitive to pyrethroids than are the amphipods (Hyalella azteca), which explains the difference between the species results.
"Since the levels are high enough to be toxic to the standard 'lab rat' species, the next question is: What's happening with the resident species?" Weston said. "The concern is that invertebrates, particularly crustaceans, could have reduced populations, and these organisms are an important food for a variety of bottom-feeding fish."
Alternatively, the amphipods and midge larvae from areas of intensive agricultural or urban pesticide use may have adapted to live with normally toxic levels of the pesticide. Weston and his colleagues now are sampling these organisms from the rivers, creeks, sloughs and ditches to determine if they respond the same way as lab-raised organisms.
Pyrethroids are a class of compounds represented by permethrin, first marketed in 1973, and various other chemicals usually ending in the suffix -thrin. Permethrin is found in home and garden pesticides ranging from RAID to flea killers and head lice creams, but permethrin and it's kin find broad use in agriculture, such as on cotton, fruit and nut orchards, and on lettuce and rice. California's Central Valley produces more than half the nation's fruits, vegetables and nuts.
Though pyrethroids are used far less than organophosphates like diazinon and chlorpyrifos, their use in California has risen rapidly in recent years because of increased regulation of the spraying of organophosphates, due to health threats to farm workers and increased toxic runoff from fields. According to Weston, pyrethroid use in California increased 58 percent from 2001 to 2002, if account is taken of the increased potency of newer pyrethroids such as cypermethrin. Over a quarter of a million pounds of pyrethroids were spread on California farm fields in 2002, while about 500,000 pounds were used for structural and pest control and landscape maintenance.
Despite this increased use, environmental monitoring still concentrates on organophosphates, he said. Monitoring also tends to focus on concentrations in the water column, under the assumption that sediment-bound chemicals like pyrethroids are unavailable. The current study shows that to be untrue.
"It's amazing that, after 20 years of use, there is not one published study on pyrethroids in sediments in areas of intensive agriculture," Weston said.
Part of the reason for a lack of data is that analytical methods to detect pyrethroids in sediment have not been broadly available or standardized. Lydy, an environmental toxicologist with SIU-Carbondale's Illinois Fisheries and Aquaculture Center, developed such a method.
"Prior to our study, scientists in area water-monitoring programs were seeing that if they placed aquatic invertebrates in their sediment samples, the animals would die, but they didn't know why - they'd attribute it to organophosphates or organochlorines (two pesticide ingredients being phased out because of environmental concerns), or they'd put it down to 'unknown causes,'" Lydy said.
"Where our study is unique is that we looked at the toxicity and tried to figure out what was actually causing it. We detected organochlorines, such as DDT and chlordane, in the sediments, but at concentrations not high enough to cause the toxicity we noted, whereas concentrations of pyrethroids were high enough to account for that toxicity."
The samples, over 70 in all, were obtained from two major rivers - the San Joaquin and the Feather - and 19 creeks or sloughs, 17 irrigation ditches and two tailwater ponds in 10 Central Valley counties, including the ones with the greatest pyrethroid use: Fresno, Madera, Stanislaus and Sutter. Each sample was placed in a jar and left with 10 test organisms for 10 days, and the death rate compared with similar organisms raised with pristine sediment. The levels of pesticides in each sediment sample also were measured, and 75 percent contained pyrethroids.
Weston, who focuses on freshwater and marine pollution and how it gets from sediments into creatures living on the bottom, noted that another chemical sometimes applied with pyrethroids may be making the situation worse. Piperonyl butoxide, or PBO, is a synergist that shuts down the enzymes that detoxify pyrethroids, making them last longer in an organism and increasing their killing potential.
He and his colleagues are now trying to measure the level of pyrethroid that kills amphipods, which is around 3 parts per billion in sediments, and whether levels of PBO need to be considered in order to estimate the true toxicity of pyrethroid pesticides. The latter study is being conducted by UC Berkeley post-doctoral researcher Erin Amweg.
"I don't want to give the impression that pyrethroids are destroying the streams, since that has not yet been shown, but if we are serious about maintaining stream health, we have to consider the sediments and not limit our sampling just to the water above," said Weston. "While pyrethroids may be preferable to the organophosphates that preceded them, our work shows that the environmental effects of pyrethroids can not be ignored and have had too little study for too long. We need to know more about pyrethroids, because if we don't, how can we regulate them?"
Even if pyrethroids turn out to be little better than the pesticide compounds they replaced, Lydy doesn't expect them to go away any time soon.
"Farmers don't have another class of compounds waiting in the wings," he said. "I'm realistic. You can't ban them without giving farmers another choice, so you have to find some way of making them work better under the conditions in which they're being used. 'Best management practices,' such as introducing buffer strips and wetlands, may reduce pesticide loads in aquatic systems, which would reduce the risk to non-target species."