I remember when I first learned about pyrethrins. I had read Rachel Carson’s Silent Spring and was damned if I’d use anything on my garden. Let insects eat what they will—we can all share. Artichokes became a sacrificial plant that protected my peach trees from aphids, though deer proved a larger—much larger—problem than insects.
But I was neither bringing goods to market nor depending upon the income. Where livelihoods are at stake, the very effective pyrethrins, naturally derived from the chrysanthemum plant, struck me as a miracle. “It seemed like a really good solution to organophosphates,” recalls Dr. Inge Werner, director of the Aquatic Toxicology Lab at UC Davis.
But pyrethrins had a drawback: though they will readily kill the pest, they don’t stick around to gun it out a couple weeks down the road. Farmers wanted something long-lasting that they wouldn’t have to spray on every week. Enter pyrethroids, the synthetic cousins of pyrethrin. Pyrethroids have an extra molecule or two that adds to their stability and toxicity, to protect crops from insect damage more effectively. “Their trait of being hydrophobic—they don’t like to go into solution, into water—made them good candidates to counter aquatic toxicity,” Werner says. “They tend to stick to whatever they come into contact with.”
But that very stickiness means the pesticide clings to soil particles, to mud, to potting mixes from commercial nurseries. Storms and drip or overhead irrigation can wash the particles into drainage ditches and nearby creeks, where they can be toxic. Werner’s group applied organophosphate on one side of an orchard and pyrethroids on the other. By measuring the stormwater runoff, they discovered that the pyrethroid side was far more deadly to fish than the organophosphate side.
Pyrethroids are extremely toxic to fish and smaller organisms. A nursery might use 25 parts per million pyrethroid in its potting mix to discourage ants; 0.007 parts per billion can kill aquatic invertebrates. “They are toxic at such low concentrations that sometimes we can’t measure it, but it still affects the organism,” Werner says. “People say, ‘Oh, well, we didn’t detect anything.’ That doesn’t mean it isn’t there.”
Werner worries about the misinformation spread by nurseries and hardware stores. “These compounds are super toxic, but we’re told, `This is natural; this is from chrysanthemum.’ Most people don’t know the difference between pyrethrins and pyrethroids.” Brand names for pyrethroids include the all-too descriptive Capture, Talstar, Warrior, Ammo, Fury, Ambush, and Pounce. If you see bifenthrin or permethrin on the label, you know you’re dealing with a synthetic pyrethroid.
More studies are needed to understand the extent of the problem. “We’re still not sure how much gets into the water and dissolves,” Werner says. Once the pyrethroids are in the water, they remain sticky. “They do bind and then settle out,” she says. “They don’t remain so toxic. So we don’t know how much of these compounds fish encounter. And we don’t know anything about the effects on organisms who feed on algae or plants that have pyrethroids stuck to them.”
As usual, the precautionary principle is invoked in the rearview mirror.
“We have a chance to do it in a smart way,” Werner says. “If you’re using pyrethrins that will kill your pests and then degrade, and you’re not spraying them on right before a storm, then it can be safe. But if you’re talking pyrethroids, you have to be more aware of the fact that they could be washed into a nearby creek or a storm drain.”