Pyrethroids: 6 Percent of the Weight, 99.9 Percent of Aquatic Toxicity
Key result: Weighted by EPA’s benchmarks for freshwater invertebrates, five pyrethroids account for 98 percent of the aquatic toxicity of California structural pest control’s insecticide use, while making up 4.1 percent of its pounds. Bifenthrin alone is 2.2 percent of the weight and 44.7 percent of the toxicity-weighted total in 2022.
Why weight matters less than potency
Our analysis of what structural pest control applies showed that pounds are dominated by sulfuryl fluoride fumigant and borates. Neither says much about what a compound would do if it reached a stream: active ingredients differ in toxicity to aquatic life by more than a million-fold.
A standard way to account for that is toxicity-weighted use: divide the pounds of each active ingredient by a benchmark concentration, so that a compound ten times more toxic counts ten times more. We used the US Environmental Protection Agency’s aquatic life benchmarks for freshwater invertebrates, the organisms most sensitive to insecticides, taking for each compound the lower of its acute and chronic benchmark.
The result is not an exposure or risk estimate. It answers a narrower question: if every pound reached water in the same way, which compounds would carry the toxicity?
The ranking
| Active ingredient | Pounds, 2022 | Share of all structural pounds | Benchmark (micrograms per litre) | Type | Share of toxicity-weighted use |
|---|---|---|---|---|---|
| Bifenthrin | 87,853 | 2.25% | 5e-05 | chronic | 44.7% |
| Deltamethrin | 24,289 | 0.62% | 2.6e-05 | chronic | 23.8% |
| Lambda-cyhalothrin | 24,182 | 0.62% | 4e-05 | acute | 15.4% |
| Cypermethrin | 15,741 | 0.40% | <5e-05 | chronic | 8.0% |
| Esfenvalerate | 7,635 | 0.20% | 3.09e-05 | chronic | 6.3% |
| Gamma-cyhalothrin | 1,088 | 0.03% | 4e-05 | acute | 0.7% |
| Permethrin | 75,401 | 1.93% | 0.0033 | acute | 0.6% |
| Cyfluthrin | 1,414 | 0.04% | 0.00012 | chronic | 0.3% |
| Alpha-cypermethrin | 2,699 | 0.07% | 0.00059 | chronic | 0.1% |
| Beta-cyfluthrin | 7,094 | 0.18% | 0.003 | chronic | 0.1% |
| Imidacloprid | 21,395 | 0.55% | 0.01 | chronic | 0.1% |
| Fipronil | 20,658 | 0.53% | 0.011 | chronic | 0.0% |
Sources: California Department of Pesticide Regulation, Pesticide Use Report 2022, site code 10; US Environmental Protection Agency, Aquatic Life Benchmarks, last updated 2 February 2026, freshwater invertebrates. “<” means EPA lists the benchmark as below the value shown, so the compound’s true share would be larger. Author calculation.
What it means. Bifenthrin, deltamethrin, lambda-cyhalothrin, cypermethrin and esfenvalerate together account for 98.1 percent of toxicity-weighted use, from 4.1 percent of structural pounds. All pyrethroids together are 6.4 percent of pounds and 99.9 percent of the toxicity. Permethrin, the second-heaviest pyrethroid by weight, contributes under 1 percent, because its benchmark is 66 times higher than bifenthrin’s. Fipronil and imidacloprid, each about half a percent of the pounds, contribute about 0.05 percent each.
What limits it. 966,602 of 3,907,203 structural pounds in 2022, about a quarter, are compounds with an EPA freshwater invertebrate benchmark. Sulfuryl fluoride, a gas, and disodium octaborate tetrahydrate have none and are excluded; neither is likely to reach streams in the way sprays can. Weighting by the most sensitive benchmark emphasises invertebrates over fish and plants.
The paradox with what is found in streams
Our analysis of stream samples against EPA’s benchmarks found imidacloprid, not a pyrethroid, exceeding its benchmark most often: in 30.1 percent of US stream water samples. Bifenthrin was detected in 2.6 percent.
The two results are consistent. Pyrethroids bind tightly to soil and sediment, so they are rarely measured in the dissolved water most programmes sample, and their benchmarks are below what laboratories can detect. Imidacloprid dissolves and travels. A compound can dominate potential toxicity and be rarely seen in water samples, which is why toxicity-weighted use and water monitoring answer different questions.
Eight years of toxicity-weighted use
What it means. Toxicity-weighted use does not follow total pounds, which are dominated by fumigant. It follows pyrethroid pounds, and those swing widely: the index fell to 35 in 2019 and rose to 141 in 2021 before ending at 87 in 2023. Bifenthrin was the largest contributor every year from 2016 to 2022; deltamethrin overtook it in 2023, at 31.8 percent against 29.7.
What limits it. Pounds can swing with formulation changes; a more concentrated product applied at the same rate appears as more pounds. There is no clean trend to report.
What would change the total
Because five compounds carry almost all of it, toxicity-weighted use is sensitive to a handful of product choices. In 2022, replacing every pound of bifenthrin with permethrin at the same weight would have cut the total by about 44 percent, on these benchmarks. We do not suggest such a substitution is practical or effective; the point is that the measure is concentrated, not diffuse. Our chemistry trends analysis shows the industry’s pyrethroid mix shifting toward lambda-cyhalothrin and alpha-cypermethrin.
Checks
1. Benchmark mapping
Result. EPA’s table was read by column; imidacloprid (0.01), fipronil (0.011) and bifenthrin (0.00005 micrograms per litre, chronic) match the values used in our stream analysis.
2. Sensitivity to the benchmark choice
Method. Recomputed using chronic benchmarks only.
Result. The same five pyrethroids remain the top five, with bifenthrin at 51.6 percent, and pyrethroids are 99.86 percent of the total.
Download the data
The tables in this report are published as CSV files under a Creative Commons Attribution 4.0 licence. You may republish, remix and build on them, including commercially, with attribution to The Pest Control Brief and a link to this page.
- Pounds, applications, EPA benchmarks and share of toxicity-weighted use by active ingredient, 2016 to 2023 (CSV, 925 rows)
How to cite this report
The Pest Control Brief, “Pyrethroids Are 6 Percent of Pest Control's Pesticide Weight and 99.9 Percent of Its Aquatic Toxicity”, 2 October 2026. https://thepestcontrolbrief.com/pest-control-aquatic-toxicity-weighted-use/
References
- US Environmental Protection Agency. Aquatic Life Benchmarks and Ecological Risk Assessments for Registered Pesticides. Last updated 2 February 2026. epa.gov aquatic life benchmarks. Accessed 2 October 2026.
- California Department of Pesticide Regulation. Pesticide Use Report annual archives, 2016 to 2023. files.cdpr.ca.gov/pub/outgoing/pur_archives/. Accessed 2 October 2026.
How the measure was built
Use. Pounds of active ingredient by compound, site code 10, Structural Pest Control, from the Pesticide Use Reports.
Benchmarks. For each compound with an entry in EPA’s table, the lower of the freshwater invertebrate acute and chronic benchmarks, in micrograms per litre.
Formula. Toxicity-weighted use = pounds divided by benchmark. Each compound’s share is its toxicity-weighted use divided by the total across compounds with a benchmark.
Limitations. Not exposure: it ignores where and how products are applied, indoors or outdoors, and how much reaches water. Compounds without benchmarks are excluded. Benchmarks listed as “less than” or “greater than” are used at the stated value.
Last reviewed: 4 October 2026.
For environmental and trade reporters
Most quotable statistics
- Pyrethroids are 6.4 percent of California structural pest control’s pesticide pounds and 99.9 percent of its aquatic-toxicity-weighted use.
- Five pyrethroids account for 98 percent of toxicity-weighted use.
- Bifenthrin is 2.2 percent of the pounds and 44.7 percent of toxicity-weighted use.
- Fipronil and imidacloprid contribute about 0.05 percent each.
Caveats that travel with them
Toxicity-weighted use measures potential, not exposure or harm. California structural pest control only.
Press summary
Almost all of the aquatic toxicity in professional pest control’s chemistry comes from five pyrethroid insecticides, according to an analysis by The Pest Control Brief that weights California’s structural pesticide use by EPA’s aquatic life benchmarks. Pyrethroids make up 6.4 percent of the pounds applied but 99.9 percent of the toxicity-weighted total, with bifenthrin alone at 44.7 percent. Fipronil and imidacloprid, the insecticides most often found in stream water, carry a tiny share, because pyrethroids bind to sediment rather than staying in water.
Headlines editors could use
- Five insecticides carry 98% of pest control’s aquatic toxicity
- Bifenthrin: 2% of the weight, 45% of the toxicity
- Why pounds of pesticide are the wrong measure for streams
Questions about this measure
Which pest control chemical is most toxic to aquatic life?
Weighted by amount used in California structural pest control in 2022, bifenthrin, at 44.7 percent of toxicity-weighted use, followed by deltamethrin and lambda-cyhalothrin.
What is toxicity-weighted use?
Pounds of each pesticide divided by a toxicity benchmark, so that more toxic compounds count for more. It shows where potential toxicity is concentrated.
Are pyrethroids worse for streams than neonicotinoids?
Pound for pound they are far more toxic to aquatic invertebrates, but they are rarely found in stream water because they bind to sediment. Neonicotinoids are less toxic per pound but are found more often in water.
Does this mean pest control pollutes streams?
No. This measure does not estimate how much reaches water, which depends on where and how products are applied.
Where can I get the data?
The CSV above has every compound with a benchmark, for each year from 2016 to 2023.
