Short-Term Effects of Pesticides Explained

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 and humans.

Paraquat (Gramoxone®) 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 (Zenneca®), 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 pass.

Organophosphate insecticides such as Lorsban® 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 sink.

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.