E X T O X N E T
Extension Toxicology Network
A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Assessment Program.
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Pesticide
Information
Profile
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Pyrethrins
Publication Date: 3/94
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TRADE OR OTHER NAMES
Several trade names associated with these compounds are Buhach,
Chrysanthemum Cinerariaefolium, Ofirmotox, Insect Powder, Dalmation Insect
Flowers, Firmotox, Parexan and NA 9184.
INTRODUCTION
Pyrethrins are natural insecticides produced by certain species of the
chrysanthemum plant. The flowers of the plant are harvested shortly after
blooming and are either dried and powdered or the oils within the flowers are
extracted with solvents. The resulting pyrethrin containing dusts and
extracts usually have an active ingredient content of about 30%. These active
insecticidal components are collectively known as pyrethrins. Two pyrethrins
are most prominent, pyrethrin-I and pyrethrin-II. The pyrethrins have another
four different active ingredients, Cinerin I and II and Jasmolin I and II.
Pyrethrin compounds have been used primarily to control human lice,
mosquitoes, cockroaches, beetles and flies. Some "pyrethrin dusts," used to
control insects in horticultural crops, are only 0.3% to 0.5% pyrethrins, and
are used at rates of up to 50 lb/A. Other pyrethrin compounds may be used in
grain storage and in poultry pens and on dogs and cats to control lice and
fleas.
The natural pyrethrins are contact poisons which quickly penetrate the
nerve system of the insect. A few minutes after application, the insect
cannot move or fly away. But, a "knockdown dose" does not mean a killing
dose. The natural pyrethrins are swiftly detoxified by enzymes in the insect.
Thus, some pests will recover. To delay the enzyme action so a lethal dose is
assured, organophosphates, carbamates, or synergists may be added to the
pyrethrins.
Semisynthetic derivatives of the chrysanthemumic acids have been
developed as insecticides. These are called pyrethroids and tend to be more
effective than natural pyrethrins while they are less toxic to mammals. One
common synthetic pyrethroid is allethrin.
In this report, the term "pyrethrins" refers to the natural insecticides
derived from chrysanthemum flowers; "pyrethroids" are the synthetic chemicals,
and "pyrethrum" is a general name covering both compounds. The EPA classifies
pyrethrin-I as a Restricted Use Pesticide (RUP). Restricted Use Pesticides
may be purchased and used only by certified applicators.
TOXICOLOGICAL EFFECTS
ACUTE TOXICITY
Synthetic pyrethroid compounds vary in their toxicity as do the natural
pyrethrins. Pyrethrum carries the signal word CAUTION. Inhaling high levels
of pyrethrum may bring about asthmatic breathing, sneezing, nasal stuffiness,
headache, nausea, incoordination, tremors, convulsions, facial flushing and
swelling, and burning and itching sensations (5). The most severe poisonings
have been reported in infants, who are not able to efficiently break down
pyrethrum. The lowest lethal oral dose of pyrethrum is 750 mg/kg for children
and 1,000 mg/kg for adults (5). Oral LD50 values of pyrethrins in rats range
from 200 mg/kg to greater than 2,600 mg/kg (4). Some of this variability is
due to the variety of constituents in the formulation. Mice have a pyrethrum
oral LD50 of 370 mg/kg (5). Animals exposed to toxic amounts may experience
tongue and lip numbness, nausea, and diarrhea. Symptoms may also include
incoordination, tremors, convulsions, paralysis, respiratory failure, and
death.
Pyrethroids can cause two quite different responses at near lethal doses
in rats; aggressive sparring and a sensitivity to external stimuli progressing
to tremors is the one response and pawing and burrowing behavior, and
salivation leading to chronic seizures is the other (8). Human response to
these two different types of pyrethroids has not yet been evaluated. Recovery
from serious poisoning in mammals is fairly rapid.
Rats and rabbits are not affected by large dermal applications (4, 5).
On broken skin, pyrethrum produces irritation and sensitization, which is
further aggravated by sun exposure.
CHRONIC TOXICITY
Absorption of pyrethrum through the stomach and intestines and through
the skin is slow. However, humans can absorb pyrethrum more quickly through
the lungs during respiration. Response appears to depend on the pyrethrum
compound used. Overall, pyrethrins and pyrethroids are of low chronic
toxicity to humans and the most common problems in humans have resulted from
the allergenic properties of pyrethrum (7). Patch tests for allergic reaction
are an important tool in determining an individuals sensitivity to these
compounds.
Many of the natural and synthetic compounds can produce skin irritation,
itching, pricking sensations and local burning sensations. These symptoms may
last for about two days (8).
Reproductive Effects
Rabbits that received pyrethrins orally at high doses during the
sensitive period of pregnancy had normal litters. A group of rats fed very
high levels of pyrethrins daily for three weeks before first mating had
litters with weanling weights much lower than normal (4). Overall, pyrethrins
appear to have low reproductive toxicity.
Teratogenic Effects
The one rabbit reproduction study performed showed no effect of
pyrethrins on development of the offspring (3). More information is needed.
Mutagenic Effects
No information was found.
Carcinogenic Effects
No carcinogenic status has been established for pyrethrins or
pyrethroids.
Organ Toxicity
In mammals, tissue storage has not been recorded. At high doses,
pyrethrum can be damaging to the central nervous system and the immune system.
When the immune system is attacked by pyrethrum, allergies can be worsened.
Animals fed large doses of pyrethrins may experience liver damage. Rats
fed pyrethrin at high levels for two years showed no significant effect on
survival, but slight, definite damage to the livers was observed (4).
Inhalation of high doses of pyrethrum for 30 minutes each day for 31 days
caused slight lung irritation in rats and dogs (5).
Fate in Humans and Animals
Pyrethrins, pyrethroids, and their metabolites are not known to be stored
in the body nor excreted in the milk (2). The urine and feces of people given
oral doses of pyrethrum contain chrysanthemumic acid and other metabolites (2, 4).
These metabolites are less toxic to mammals than are the parent compounds
(3). Pyrethrins I and II are excreted unchanged in the feces (2). Other
pyrethrum components undergo rapid destruction and detoxification in the liver
and gastrointestinal tract (4).
ECOLOGICAL EFFECTS
Pyrethrin is extremely toxic to aquatic life, such as bluegill and lake
trout while it is slightly toxic to bird species, such as mallards. Toxicity
increases with higher water temperatures and acidity. Natural pyrethrins are
highly fat soluble, but are easily degraded and thus do not accumulate in the
body. These compounds are toxic to bees also.
Because pyrethrin-I, pyrethrin-II, and allethrin have multiple sites in
their structures that can be readily attacked in biological systems, it is
unlikely that they will concentrate in the food chain (2).
ENVIRONMENTAL FATE
Two pyrethroid synthetic insecticides, permethrin and cypermethrin, break
down in plants to produce a variety of products (6). Pyrethrins have little
residual effect. In stored grain, 50% or more of the applied pyrethrins
disappear during the first three or four months of storage. At least 80% of
what remains is removed by handling, processing, and cooking (3).
Pyrethrins alone provide limited crop protection because they are not
stable. As a result, they are often combined with small amounts of
antioxidants to prolong their effectiveness. Pyrethrum compounds are broken
down in water to nontoxic products.
Pyrethrins are inactivated and decomposed by exposure to light and air.
Pyrethrins are also rapidly decomposed by mild acids and alkalis. Stored
pyrethrin powders lose about 20% of their potency in one year.
As the pyrethrins are purified, their stability decreases; thus, pure
pyrethrin-I and pyrethrin-II are the least stable of the pyrethrins (4).
Purified pyrethrins are very expensive and are only available for laboratory
uses.
PHYSICAL PROPERTIES AND GUIDELINES
The pyrethrins are viscous brown resins, liquids, or solids which
inactivate readily in air. Due to differences in the types and amounts of
esters in the pyrethrum mixture, its molecular weight ranges from 316 to 374.
Exposure Guidelines:
| NOEL: | 10 mg/kg bw/day (rats) (3) |
| ADI: | 0.04 mg/kg body weight (humans) (3) |
| PEL: | 5 mg/m3 |
| TLV-TWA: | 5 mg/m3 |
| STEL: | 10 mg/m3 |
Physical Properties:
| CAS #: | 8003347 |
| Solubility in water: | considered to be insoluble in water. |
| Solubility in solvents: | soluble in organic solvents like: alcohol, kerosene, nitromethane, petroleum ether, carbon tetrachloride, and ethylene dichloride. |
| Boiling point: | for pyrethrin I: 146-150 degrees C (1); for pyrethrin II: 192-193 degrees C (1) |
| Vapor pressure: | about 0 mm/Hg |
BASIC MANUFACTURER
There are several manufacturers of products in this category.
REFERENCES
Windholz, M., et al., eds. 1983. The Merck Index. 10th ed. Merck &
Co., Inc.
Elliot, M; Janes, N.F. Kimmel, E.C.; and Casida, J.E. 1972.
Metabolic Fate of Pyrethrin I, Pyrethrin II, and Allethrin Administered
Orally to Rats. J. Agr. Food Chem. 20: 300-312.
Vettorazzi, G. 1979. International Regulatory Aspects for Pesticide
Chemicals. pp.89-90. CRC Press.
Hayes, Jr., W.J. 1982. pp. 75-81. Pesticides Studied in Man.
Williams & Wilkins.
Occupational Health Services, Inc. "Pyrethrum." Material Safety Data
Sheet. 1 April 1987. New York: OHS, Inc.
Roberts, T.R. and A.N. Wright. 1981. The metabolism of a pyrethroid
metabolite, in plants. Pest. Sci. 12:161.
Ecobichon, Donald J. 1991. Toxic Effects of Pesticides. In
Casarett and Doull's Toxicology, The Basic Science of Poisons, Third Edition.
Curtis D. Klaassen, Mary O. Amdur, and John Doull editors. Macmillan
Publishing Company, NY.
Aldridge, W.N. 1990. An assessment of the Toxicological Properties of
Pyrethroids and their Neurotoxicity. Toxicology, Vol 21, Number 2. pp. 89-104.
