Spring is underway, and across California, gardeners and farmers are combating the season’s annual pest problem: insects. But the pesticides that protect our roses and our lettuce may be harming more than just the bugs. Pesticides, in particular pyrethroids, are making their way from our gardens, farms, and houses into our watersheds, where they pose a serious threat to California’s aquatic populations. With annual pesticide use expected to peak in the coming months, now is the time to ask some questions. What are pyrethroids? How are they impacting our water systems? What is being done to mitigate the potential problems associated with their use? 

Since the late 19th century, pyrethroids have been around and are similar to some naturally occurring insecticides, called pyrethrins, found in chrysanthemum flowers. But unlike pyrethrins, pyrethroids are manufactured chemicals designed to quickly incapacitate insect nervous systems and simultaneously resist environmental degradation. Their low toxicity to humans, mammals, and birds makes them ideal for a variety of agricultural, public health, and domestic purposes. Pyrethroid compounds are effective insecticides employed on farms and in homes across the US. In particular, pyrethroids are an efficient mosquito killer and can help reduce cases of mosquito-borne illnesses like West Nile Virus. Many household products—such as lice-killing shampoos, household insecticides, pet sprays, and lawn care products—use pyrethroids as an active ingredient. Despite their overall low toxicity to humans, pyrethroids are not entirely safe. In high enough concentrations, pyrethroids can cause itching and burning skin, difficulty breathing, twitching, nausea, and fainting. The average US citizen is exposed to 3.2 micrograms of pyrethroids every day—levels which fall well below current health advisory levels.

Since the early 2000s, pyrethroid use has been on the rise. A survey of pesticide products available in retail stores found that pyrethroids are the most commonly available class of pesticides. Their growing prevalence across the US has sparked some concern among the scientific and water industry communities, especially in California’s Central Valley. As a major urban and agricultural hub, the Central Valley has studied pesticides and pyrethroids for decades. The findings suggest that pyrethroids are a potential watershed pollutant of increasing concern.

Whether sprayed over home gardens, washed down shower drains, or aerosolized over farmlands, pyrethroids end up in our stormwater, wastewater, and agricultural runoff. Although MS4s (Municipal Separate Stormwater Systems) and POTWs (Publicly Owned Treatment Works) cannot control all the chemicals entering their treatment systems, they invest considerable time in community outreach and education to help ameliorate the problem. Agency activities include efforts to encourage reduced and proper pesticide use, reduced runoff, and other pollution prevention activities. In spite of MS4 and POTW efforts, pyrethroids do ultimately enter the environment, particularly surface water. Once in the watershed, pyrethroids are not very mobile because they tend to bind tightly to soil and sediment. Since only a small percentage of pyrethroids dissolve freely and are susceptible to a certain level of natural degradation, overall water toxicity is somewhat limited. However, even very low concentrations of pyrethroids can have adverse effects on California’s aquatic populations. 

A multitude of studies have documented these impacts on aquatic life. In areas acutely affected by contamination, pyrethroids induce paralysis and death in invertebrates like amphipods or mollusks. Many of these invertebrates, such as H. azteca, are key food sources for aquatic species in the rivers and streams of the Central Valley. Other sublethal effects on aquatic populations include reduced organism growth, disruption of reproductive functions, and impaired swimming performance. In the Sacramento-San Joaquin Delta, several studies documented these sublethal effects in endangered fish populations like the Delta smelt, Chinook salmon, and Sacramento splittail. Unfortunately, indications suggest pyrethroids pose the greatest threat to some of California’s most vulnerable aquatic species.

To address these growing concerns about the impacts of pyrethroids on aquatic life, the Central Valley Regional Water Quality Control Board (Central Valley RWQCB) amended the Central Valley Basin Plan in 2017. The amendment prohibited pyrethroid discharges above certain concentrations, established Total Maximum Daily Loads (TMDL), placed a temporary memorandum on certain pyrethroid compounds, and proposed methods for monitoring and managing pyrethroid discharges. The amendment’s main objective was to reduce pyrethroid concentrations in the Sacramento and San Joaquin Valley watersheds to levels safe for aquatic life. The State Water Board approved the Basin Plan Amendment (BPA) on July 10, 2018; the Office of Administrative Law on February 19, 2019; and the United States Environmental Protection Agency on April 22, 2019. With the US EPA’s approval, the amendment and TMDL were passed into full effect.

For regulated communities, a new mandate, effective this year, requires certain municipal water systems to create a Baseline Monitoring Plan or Pyrethroid Management Plan. MS4 permittees are now required to sample and test their water for pyrethroids and, should their samples exceed pyrethroid TMDLs, create a plan to prevent more pyrethroids from getting into surface waters. Although the monitoring and management plans create additional financial stress for municipal water systems, the Central Valley RWQCB considers it a necessary economic burden to safeguard beneficial uses and aquatic life. This year the board also plans to complete a Pyrethroid Research Plan that will incorporate feedback from stakeholders and other agencies. 

As a leading environmental lab, Babcock Laboratories, Inc. stays abreast of potential water quality concerns to help safeguard public health and the environment. Contact us to learn more about California’s regulations and our testing services.