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January 19, 2006
Chrysanthemum Flowers
... and other mysteries of pest control
by
AMY STEWART
If you read this column regularly, you're probably
an entirely organic gardener, or a mostly organic gardener or,
at the very least, I imagine that you, dear reader, would like
to cut back on the chemicals you use in your garden if you only
knew how.
You've probably figured out that organic gardening
is not just about what you don't use in the garden. It's not
only about eliminating synthetic pesticides and fertilizers.
It's also about what you do use, like fertilizers and pesticides
made from naturally occurring ingredients, and techniques such
as cover cropping and composting that keep the soil healthy.
But just because a product comes directly from
nature doesn't make it safe under all circumstances, all the
time. Poison oak, after all, is entirely natural. There are any
number of toxic mushrooms, seeds, leaves and shoots growing all
over Humboldt County. Most organic pesticides and fertilizers
are blended in high concentrations, making it even more important
that you use them carefully, no matter what they're made of,
and only when you can be sure that you've found the right product
for the job.
In particular, 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 all 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.
Send garden news to [email protected], or write
in care of the Journal at 145 G St., Suite A, Arcata, 95521.
Send garden news to [email protected], or write in care
of the Journal at 145 G St., Suite A, Arcata, 95521.
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