Short-Term Effects of
by Victoria Mundy, Extension Educator
Whether you love them or hate them, pesticides
are fascinating when you come right down to it:
Every one is different. This article gives a
couple of examples of the short-term effects of
herbicides and insecticides on target organisms
Paraquat (GramoxoneŽ) kills a plant by
interfering with the plant's ability to
photosynthesize. Paraquat also uses a byproduct
of photosynthesis to disrupt plant cell
membranes, so that the plant more or less turns
into a puddle inside.
People, of course, don't photosynthesize. But
paraquat is similar in chemical structure to
compounds found naturally in the lungs. The lungs
can be tricked into accumulating paraquat and
soon will not function. The interesting thing is
that you actually have to drink a little paraquat
- or completely ignore safety measures during
spraying - for this to happen.
Paraquat has a chemical relative, diquat
(ZennecaŽ), which is slightly less toxic to
humans and other mammals than paraquat. The
mammalian body will metabolize, or break down,
diquat to some extent. Diquat is toxic to
mammals; the point is that these two herbicides
behave differently in the mammalian body even
though they are closely related.
Herbicides are usually less toxic to humans
than insecticides simply because mammals have
less in common with plants than with insects.
Many insecticides kill bugs by damaging their
central nervous systems (CNS). The CNS of mammals
is similar to that of insects, so substances
which are nerve poisons for insects are likely to
damage mammals too.
In the nervous system, tiny gaps exist between
nerve cells, and between nerve cells and the
muscle cells they control. Electrical impulses
carry messages across these gaps with the help of
substances called neurotransmitters.
One common neurotransmitter is acetylcholine
(AcCH). After a molecule of AcCH has helped an
electrical impulse to leap across the gap between
two cells, an enzyme called acetylcholinesterase
prevents the AcCH from helping another impulse to
Organophosphate insecticides such as LorsbanŽ
damage acetylcholinesterase. The enzyme becomes
unable to deactivate the neurotransmitter AcCH.
As a result, AcCH keeps shoving one electrical
impulse after another across gaps between cells.
Muscles contract because electrical impulses
stimulate them. If electrical impulses pass
continuously from a nerve to a muscle, the muscle
can never relax. Organophosphates, then, make
sure that muscles do not relax. Convulsions and
asphyxiation eventually occur.
Two naturally-occurring CNS disrupters are
nicotine and pyrethrum. Nicotine damages the
nervous system in almost the same way that
organophosphates do. Nicotine is incredibly toxic
to mammals and will cause illness or death if
either swallowed or touched. Nicotine is
water-soluble - which means that you can kill
roaches by soaking cigarettes in water and
pouring the resulting "tea" down the
Pyrethrum, or pyrethrin, is found in certain
chrysanthemum species. Natural pyrethrum is not
highly poisonous to mammals because mammals can
detoxify it in their bodies. Some synthetic
pyrethrins, though, are even worse for mammals
than nicotine is.
Pyrethrum produces an almost immediate
"knockdown" in insects, which is why it
is present in household insecticides. (It's so
satisfying to see that bug hit the floor!) But
insects can detoxify pyrethrum too, so they
recover quickly from knockdown. Other substances
are added to most pyrethrum insecticides so that
insects cannot detoxify the pyrethrum.
These are just a few stories from the
bewildering world of pesticides. Keep in mind
that the long-term effects of pesticides
are another chapter entirely!
Davidson, R.H., and W.F. Lyon. 1987. Insect
pests of farm, garden and orchard. John Wiley and
Sons, New York.
Hein, G.L., et al. 1995. Insect management
guide for Nebraska sugarbeets, dry beans,
sunflowers, vetch, potatoes, and onions. Extension
Circular 95-1561. University of Nebraska
Cooperative Extension, Lincoln, NE 68583.
McEwen, F.L., and G.R. Stephenson. 1979. The
use and significance of pesticides in the
environment. John Wiley and Sons, New York.