Pesticide Toxicity
Background
The environmental movement was catalyzed by Rachel Carson's (1962)
book, Silent Spring. The title wasbased on her thesis that the
biomagnification of pesticides in avian food chains would lead to the
demise of song birds. Her concerns were based on studies which showed
that pesticides designed to exterminate insects (insecticides) were
being bioaccumulated in food chains to toxic levels in higher trophic
levels. This toxicity was first recognized in birds, since they were at
the top of most insect food chains.
However, she also suggested that tragedy could be averted with the
development of more responsible pesticide management practices:
Through all these new, imaginative and creative approaches to the
problem of sharing our earth with other creatures there runs a constant
theme, the awareness that we dealing with life - with living
populations and all their pressures and counter-pressures, their surges
and recessions. Only by taking account of such life forces and by
cautiously seeking to guide them into channels favorable to ourselves
can we hope to achieve a reasonable accommodation between the insect
hordes and ourselves.
The classic example of the adverse effects of pesticide
biomagnification is DDT in pelicans and peregrine falcons. There are
still concerns that DDT may be harming the Caspian tern population at
Moss Landing, which appear to be accumulating DDT from contaminated
sediments in Elkhorn Slough. These cases illustrate the inherent
complexity of determining the adverse ecological effects of any
pesticide.
The four primary factors determining the potential adverse effects of a
pesticide are:
inherent toxicity
stability
solubility
absorptivity
Characterization
Types of pesticides may be characterized by their "target"
organism
herbicide - weeds
insecticide - insects
arachicides - spiders
rodenticide - rodents
Types of pesticides may also be characterized by their chemical formulation:
- inorganic compounds
- used for decades to centuries
- examples - salt, sulfur, lead arsenate, copper sulfate
- toxic mechanism - varied
- biodegradability - compounds degrade, but stable (non-radioactive)
elements do not
- biomagnification - varied, usually less than organic compounds
- mammalian toxicity - varied (e.g., copper is low, lead is high)
- organochlorines
- synthesized in 1874, recognized as a pesticide in 1939
- Paul Mueller received the Nobel Prize in Medicine in 1948
- DDT - type - e.g., DDT, TDE, rhothane, methyoxychlor, dicofol
- chlorinated cyclodine compounds - e.g., aldrin, dieldrin, endrin,
heptachlor, chlordane
- other types - e.g., lindane, toxaphene, mirex, chlorodecone (kepone)
toxic mechanism - neurotoxin (open sodium channels in neurons)
- biodegradability - relatively long (e.g., t of DDT > 10 yrs)
- biomagnification - relatively high
- mammalian toxicity- relatively low (e.g., troops were doused with
DDT in WWII)
- organophosphates
- developed 1944
- esters of phosphates
- examples - malathion and parathion
- toxic mechanism - neurotoxicn (hyperstimulation of cholinergic pathways)
- biodegradability - relatively short
- biomagnification - relatively low
- mammalian toxicity - relatively high (toxic to nearly all animals)
- other - account for most of the 3,000 agricultural poisonings year-1
in the US
- carbamates
- developed 1956
- derivatives of carbamic acid (NH2CO2H)
- examples - aldicarb and carbofuran
- similar to organophosphates in biodegradability and nonpersistance
- not very toxic to fish, but highly toxic to bees and birds (Metcalf,
1981)
- pyrethroids
- first produced 1949
- synthetic formulations resembling pyrethrum (in Chrysanthemum flowers)
- examples - allethrin and dimethrin
- toxic mechanism - similar to DDT (i.e., open neuronal sodium channels)
- biodegradability - relatively short
- biomagnification - relatively low
- mammalian toxicity - relatively low
- other - some fish, crustaceans, and beneficial insects are very sensitive
to pyrethroids
- biological insecticides
- toxic mechanism - attack specific arthropod targets
- example - diflubenzuron is a chitin synthesis inhibitor
- biodegradability - varies
- biomagnification - varied
- mammalian toxicity - relatively low
References
Carson, R. 1962. Silent Spring. Houghton Mifflin, Boston, MA, pp.
368.
Chambers, J.E. 1994. Toxicity of Pesticides. In: Basic Environmental
Toxicology ( L.G. Cockerham and B.S. Shane, eds.), CRC Press, Boca Raton,
pp. 185-198.
Metaclf, R.L. 1981. Insect control technology. In: Encyclopedia of Chemical
Technology, Volume 13. Wiley-Interscience, New York, NY. pp. 413-485.
