Insecticides usually work like nerve gas agents used in warfare. They are applied at low doses so that – at least in theory – only the insects will be directly affected. Yet there are major concerns within the scientific and environmental communities that some insecticides will become more and more concentrated as they move up the food chain. Bald eagles, for example, can show higher concentrations of an insecticide than the small song birds the eagles eat, and the small song birds show a higher concentration than the insects they eat who were initially treated with the insecticide.
Again, insecticides can be divided into different classes based on how they affect the target insect. Some are “contact” insecticides that kill when they come into direct contact with the insect. Others are “systemic” and will be incorporated into a plant’s cell structure. Systemic insecticides kill insects when they eat the plant and ingest the insecticide chemical.
Insecticides can also be divided into inorganic compounds, organic compounds and naturally occurring chemicals. Inorganic compounds are the oldest insecticides and were manufactured with heavy metals like arsenic, copper, fluorine or sulfur. Organic insecticides were manufactured beginning in the 1940s and 50s and are based carbon molecules (hence “organic”). These are the largest class of insecticides in use today. Natural insecticides are based on chemicals occurring in nature, like nicotine, pyrethrum and neem extracts. Most recently, scientists are trying to isolate natural chemicals from the insects themselves that prevent juvenile organisms from developing into adults.
Insecticides can also be classified by their mechanism of action.
- Organochlorine compounds work on insects by opening what’s known as the sodium ion channel in the neurons or nerve cells of insects, causing them to fire spontaneously. The insect will go into spasms and eventually die. DDT was the earliest of these chlorinated hydrocarbons, but DDT and many others in this class have been banned from general use in most countries. (See the following story for more about DDT.
- Organophosphates also work on the nervous system, but they keep the nerve cells from communicating with each other. Normally, nerve cells in the brains or muscles of humans or insects send tiny electrical pulses down tendril to the end ot the cell where the pulse has to jump across a gap – known as a synapse – to another nerve cell. A chemicals known as ACh moves from one cell to the other and binds with the new cell, sending the electrical pulse down the new cell. These insecticides – and nerve gas agents that are closely related – prevent the ACh from coming loose from the new cell, so it can’t receive any more impulses. The insects can’t function and die. Malathion is a common insecticide in this class and was famous for treating infestations of the Mediterranean Fruit Fly and West Nile Virus-carrying mosquitoes.
- Carbamates have similar properties to the organophosphates, but last in the environment for a much shorter time period. They are thought to be less toxic.
- Pyrethroids are synthetic compounds that mimic the action of chemicals in the Chrysanthemum flower. They are considered to be among the safest insecticides because they break down when exposed to light. They are used particularly against lice and other household pests.
- Neonicotinoids are the synthetic versions of nicotine and make insects jumpy, with leg tremors, rapid wing motion, disoriented movement, paralysis and death. But they are not as toxic in mammals (including humans) because they work on a neural pathway that is more abundant in insects. But some of these chemicals have recently been banned in places because they might be contributing to the collapse of colonies of honey bees that are vital to the pollinating many crops.
- Bt insecticides are chemicals produced by natural bacteria that kill insects in their larval stage. Because these are natural compounds and because there are several species that kill different species of insects, these Bt insecticides have become very popular with organic farmers. In addition, Bt genes have been isolated and introduced into the DNA of various crops so that the plants themselves produce their own insecticide.
- IGRs or Insect Growth Regulators are new chemicals that are isolated from the insects themselves, and will interfere with the normal growth or development of the pest. One class of chemicals prevents insects from molting, or shedding their skill as they grow. Another class blocks the chemical signal that tells juvenile insects to move on to an adult stage. These insecticides keep the insects in a larval or nymph stage.
- Antifeedants are a new class of chemical that will attack the insect’s ability to digest food. It dies of starvation slowly. These chemicals are still being developed to make them cheap enough for commercial use.