Pest Control—Pesticides

Worldwide, only about 100 species of weeds, insects, fungi, and microbes cause about 90% of the damage to the crops we grow.

To help control pest organisms, a variety of pesticides are developed. Pesticides (or biocides) are chemicals used to kill or control organisms we consider undesirable. Pesticides are classified according to the target organisms they are designed to control. Common types of pesticides include insecticides (insect killers), herbicides (weed killers), fungicides (fungus killers), nematocides (roundworm killers), and rodenticides (rat and mouse killers). 

Many of the earlier chemical pesticides were broad spectrum, and remained active for long periods of time. These are called persistent pesticides. One of the earliest pesticides used was arsenic, a chemical element toxic to all life, including people.

Development of more sophisticated pesticides began in 1939 when Paul Muller discovered DDT. DDT soon became the world’s most used pesticide and Paul Muller received the Nobel Prize in 1948 for his discovery.

Since then, over 60,000 different compounds that have potential as pesticides have been synthesized. However, most of these have never been put into production because of cost, human health effects, or other drawbacks.

At first, DDT was thought to be the long-sought magic bullet.  It appeared to have no short-term effect on people and seemed to kill only insects.

Eventually, three facts about DDT were discovered: (1) It has long-term effects on other, desirable organisms because of its long persistence; (2) It is stored in oils and fats and is concentrated as it is passed up food chains, so that the higher an organism is on a food chain, the higher the concentration of DDT it contains, a process known as biomagnification; and (3) The storage of DDT in oils and fats allows the chemical to be transferred biologically. As a result of these problems, DDT was considered as the most notorious chemical of the last century, and as a result was banned in most developed nations.

Fig.: Different ways of pesticide application

Alternatives to DDT include organophosphates and carbamates. These chemicals are more specific and decay rapidly in the soil. They too, are toxic to people and must be handled extremely carefully by those who apply them.

A perfect pesticide would have the following characteristics:  

1. It would be inexpensive.
2. It would affect only the target organism.
3. It would have a short half-life.
4. It would breakdown into harmless materials.

However, the perfect pesticide has not been invented.