There is a group of pesticides that are often described as “derived from chrysanthemum flowers” cause confusion among organic and non-organic gardeners. These pesticides, called pyrethrins, may be the most-discussed, and least understood, of the home and garden chemicals in use today. I’ll try to sort it out for you here. This stuff gets a little technical, but bear with me. Your best weapon against pests and diseases in your garden is, after all, your own knowledge.
Pyrethrins are coming into wider use as some organophosphates like chlorpyrifos and diazinon, which were used to control roaches, ants, fleas, and other common pests, are being phased out due to health concerns. Recently, a synthetic version called pyrethroid (more about the synthetic and non-synthetic versions in a minute) has been used around California to control mosquitoes in an effort to eradicate West Nile Virus. The aerial spraying of pyrethroids in the Sacramento area last August drew protests from people who wondered, as a Sacramento Bee article put it, “If this stuff kills mosquitoes so well…what the heck is it doing to humans?”
Public officials were quick to point out that the droplets were designed to kill mosquitoes in the air but dissipate before, or shortly after, they hit the ground. A UC Davis researcher also said that exposure from aerial spraying would be significantly less than what a person would receive from a single application of delousing shampoo. Still, the EPA’s own data on pyrethroids show that, at high doses, they can affect the nervous system and are toxic to fish and bees.
In October, a study funded by the California State Water Resources Control Board showed that pyrethroids were accumulating in urban streams around Roseville in levels high enough to be toxic to organisms living in the sediment. The researchers speculated that the pesticides reached the streams through home pest control treatments and fertilizer/pesticide (“weed and feed”) combinations applied to suburban lawns.
Donald Weston, adjunct professor of integrative biology at UC Berkeley, was quoted in a UC Berkeley press release about the study as saying, “Our work should be of broad public interest, because the source of the toxicity we are finding in the creeks is just residential pesticide use in a typical suburban community. When people apply pesticides to their yards, or hire exterminators to do it, they just assume the pesticides will stay there. I think our work will increase awareness of the possible environmental dangers of pesticide overuse and maybe help people think twice before using pesticides ‘just to be safe’ when they do not have a clear pest problem.”
Even gardeners who want to reduce or eliminate their use of toxic pesticides can be confused by the various forms of pyrethrins available. Saying that they are “derived from chrysanthemums” makes them all sound sweet and old-fashioned, but it’s a little misleading. In fact, dried chrysanthemum flowers produce an oleoresin (that’s the kind of sticky mixture of oil and resin similar to what you might get from a pine tree, for example) called pyrethrum. The active insecticidal compounds within pyrethrum are generally referred to as pyrethrins, but that term is also loosely used to describe this general class of pesticides. So the term “pyrethrins” is applied to both natural pyrethrum-based products and pyrethroids, the synthetic version, which is engineered to last longer than the natural version.
To add to the confusion, pyrethrins are often mixed with a synergist called piperonyl butoxide that helps prevent insects from breaking down the pesticide before it kills them. The goal is to reduce the amount of pesticide that’s needed to do the job, but piperonyl butoxide by itself is considered by the Pesticide Action Network to be moderately toxic. The group classifies it as a possible carcinogen and a potential groundwater contaminant.
So are all pyrethrins bad? Not necessarily. Some organic gardeners will use a natural pyrethrum-based spray that doesn’t contain piperonyl butoxide to fight a major infestation of aphids, thrips, whitefly, caterpillars, beetles, and a broad range of other pests. Because it’s such a broad-spectrum pesticide, it’s tempting to use it everywhere, all the time, but that’s exactly how it should not be used. After all, even the natural version can be a skin irritant and can be toxic to pets and fish if it isn’t used and stored properly. Besides, insect populations can become resistant to the pesticide.
Instead, try all the other organic methods at your disposal first. Put up with a little damage—a few aphids here and there will attract predators. Cut off infested branches, or pull out infested plants, and throw them away. Use a blast of water from the hose, or a mixture of dish soap and water, to kill soft-bodied insects. And if you do need to use a heavy-hitter like a pyrethrum-based spray, start with a limited, careful spray and make sure you target pests you can actually see. And don’t worry if a few bugs come crawling back. After all, a garden is a living thing—bugs, weeds, and all.