More on Caffeine as a Molluscicide

Following up on our brief reports in September 2002 (page 98) and
February 2003 (page 19), here is additional information based on a
detailed report (referenced below) by the researchers in Hawaii who
discovered that caffeine repels and is toxic to slugs and snails.
The discovery was made during trials conducted to test caffeine as
a frog poison; sprays containing 1-2% caffeine were observed to be fatal
to slugs, and so tests to determine the molluscicidal potential of
caffeine were done. The mollusc species in these tests were small orchid
snails (Zonitoides arboreus) and large brown slugs (Veronicella
cubensis). In one experiment, soil containing slugs was made
³thoroughly² wet by spraying water containing various concentrations of
caffeine. Subsequently, slugs that exited from the soil and slugs that
exited from the soil and died were counted. In a second experiment, Napa
cabbage leaves immersed in water containing various concentrations of
caffeine were allowed to drip-dry and placed in containers on a bed of
coir (shredded coconut husk). For choice trials, two leaves were placed
in each container, one of which was dipped in distilled water. For
no-choice trials, only one leaf was placed in each container. There were
five slugs in each container. Changes in the weights of the leaves were
tracked for several days. In a third experiment, water containing
various concentrations of caffeine was applied directly to snails, and
the snails¹ heartbeat rates were observed at various times. Also, the
numbers of dead snails were tracked over time. And in a fourth
experiment, snail-infested potted orchids were sprayed or drenched with
water containing various concentrations of caffeine (plus, in some
cases, a surfactant), and the numbers of dead and living snails were
counted 12 days after treatment. Some of the drenches contained the
conventional molluscicide metaldehyde at its maximum allowable
concentration (0.195% active ingredient) rather than caffeine.
EXPERIMENT 1 RESULTS: Few slugs exited the soil over a period of
seven days following spraying with a caffeine concentration of 0% or
0.1%, but more than 70% of the slugs exited the soil within one day
following spraying with a caffeine concentration of 0.5% or 1%, and 100%
of the slugs exited the soil within two days (86% within one hour)
following spraying with a caffeine solution of 2%. Mortality rose to
around 90% two days following spraying with 2% caffeine.
EXPERIMENT 2 RESULTS: Leaf consumption was reduced by 39% with 2%
caffeine and by 29% with 0.5% caffeine in the no-choice trials. Leaf
consumption relative to that of leaves dipped in distilled water was
reduced by 64% with 2% caffeine and by 23% with 0.5% caffeine in the
choice trials.
EXPERIMENT 3 RESULTS: Heartbeat rates of snails treated with 0.1%,
0.5%, or 2% caffeine were significantly reduced relative to the
heartbeat rates of untreated snails. More than 90% of the snails treated
with 2% caffeine died within two days; all snails treated with 0.5% or
2% caffeine died within four days. No untreated snails or snails treated
with 0.01% caffeine died within four days.
EXPERIMENT 4 RESULTS: Spraying with 1% caffeine resulted in 55-65%
snail mortality, while spraying with 2% caffeine gave approximately 95%
mortality. Drenching with 2% resulted in significantly higher mortality
than drenching with metaldehyde.
Some foliar phytotoxicity symptoms have been reported due to 2%
caffeine applications, especially if the leaves were not rinsed off
within a week. Severe damage to cabbage and lettuce leaves resulted from
dips in 2% caffeine after the leaves had been harvested, but foliar
sprays of 2% caffeine did not damage the foliage of growing lettuce
plants.
Currently, the U.S. Food & Drug Administration considers caffeine to
be a GRAS (generally recognized as safe) chemical; it is allowed in soft
drinks at a concentration of 0.02%. However, pure caffeine is quite
toxic to humans, and its legal use as a pesticide in the U.S. has been
approved by the Environmental Protection Agency only in Hawaii (to
control frogs, NOT molluscs), where pure caffeine powder is considered a
³restricted use² pesticide and must be applied only by Certified
Applicators. As we noted in February 2003, instant coffee (as drunk)
typically contains approximately 0.05%, and brewed coffee has a bit
more. It would take a REALLY STRONG BREW of coffee to approach the
percentages of caffeine found by the Hawaiian researchers to be highly
effective against molluscs.
Reference: Robert G. Hollingsworth (U.S. Pacific Basin Agricultural
Research Center, U.S.D.A. Agricultural Research Service, P.O. Box 4459,
Hilo, HI 96720), John W. Armstrong, and Earl Campbell, ³Caffeine as a
Novel Toxicant for Slugs and Snails,² _Annals of Applied Biology
142(1)_, 2003, 91-97. (Association of Applied Biologists, c/o
Horticulture Research International, Wellesbourne, Warwick CV35 9EF,
UNITED KINGDOM.)
-----------------------------------------------------------------------
Reprinted with permission from the April 2003 _HortIdeas_. Copyright
2003 by Greg and Pat Williams. HORTIDEAS (ISSN 0742-8219) is published
monthly by Gregory and Patricia Y. Williams, 750 Black Lick Road, Gravel
Switch, KY 40328 U.S.A. Annual subscription rates: U.S., $25.00
periodicals or $27.00 first class; Canada and Mexico, $32.00; Overseas,
$30.00 surface mail or $42.00 air mail. Single issues: North America,
$2.50 each; Overseas, $3.00 each, surface mail, or $4.00 each, air mail.
The email address for HORTIDEAS is: . _HortIdeas_ is now on
the world wide web at http://www.users.mis.net/~gwill/hi-index.htm

--
Bob Batson


"Eagles may soar, but weasels don't get sucked into jet engines"