Pesticide

A pesticide is a chemical used to control, to repel, to attract or to kill pests, for example, insects, weeds, birds, mammals, fish, or microbes, that are considered a nuisance. Pesticides are usually, but not always, poisons. 

Examples of pesticides 
herbicides for destruction or development prevention of weeds 
fungicides for destruction or development prevention of fungi (example, QoI) 
insecticides for destruction or development prevention of insects 
Growth regulators to prevent excess growth of a plant (prevention of lodging in wheat) 
molluscicides (against snails...) 
acaricides or miticides against mites 
products favoring plant resistance... 

Chemical engineers continually develop new pesticides to produce enhancements over previous generations of products. 

DDT is an example of a once heavily used pesticide, while glyphosate today is the world's most used herbicide. 

Some crops, such as BT Corn, are genetically engineered to create their own pesticides. 

In the US, all materials intended for sale and use as pesticides must be registered with the EPA. The process may be long, complex, and expensive, because research must prove that the material is effective against the intended pest, yet safe to use. During the registration process a pesticide label is created, which has directions for proper use of the material. Use not consistent with the label is pesticide misuse. 

Some pesticides are considered too hazardous for sale to the general public, and these are designated restricted pesticides. Only certified applicators, who have taken a course and passed an examination, may purchase and use restricted pesticides. Records of sales and use are kept, and can be audited by the EPA. 

"Read and follow label directions." is often quoted by county extension agents, garden columnists and others teaching about pesticides. This is not merely good advice; it is the law, for the USA. Similar laws exist in much of the rest of the world. The Federal Insecticide, Fungicide, and Rodenticide Act of 1972 (FIFRA) set up the current system of pesticide regulations. It was amended somewhat by the Food Quality Protection Act of 1996. Its purpose is to make pesticide manufacture, distribution and use as safe as possible. The most important point for users to understand is that it a violaton to apply any pesticide in a manner not in accordance with the label for that pesticide. It is a crime to do so intentionally. 

Most pesticides present some danger to humans when used to control weeds or insects on food crops. This is one basis for the organic food movement. Certain food crops such as apples, peppers, celery, cherries, grapes, nectarines, peaches, pears, potatoes, red raspberries, spinach, and strawberries contain residual pesticides after being washed or peeled, although they may still meet government limits which are set to minimize exposure not eliminate it. These crops are the so-called dirty dozen as designated by the Environmental Working Group which has issued a Shopper's Guide to Pesticides in Produce. 

Besides human health risks, pesticides also pose dangers to the environment. Non-target organisms can be severely impacted. In some cases where a pest insect normally has some controls from a beneficial insect predator or parasite, an insecticide application can kill both pest and beneficial. The control insect almost always takes longer to recover than the pest. Applications for adult mosquitoes, for example, may temporarily depress mosquito populations, but cause a larger population in the long run, by damaging controlling factors. Pesticides are also a factor in pollinator decline, which is a food supply issue.