Pesticides are poisons designed to destroy unwanted life forms. Used properly, modern pesticides can perform their functions without causing significant hazards to humans or the environment. Pesticides have many uses in homes, gardens, farms, forests and public health. It is difficult to imagine what life would be like without modern pesticides. This discussion is continued from BioTalk 13.

Insecticides
By far, insecticides are the largest group of pesticides. Insecticides are chemicals used to kill, repel, alter the growth patterns, or manipulate the behaviour of insects, other arthropods and nematodes. Insecticides include a wide variety of chemical compounds ranging from highly toxic nerve poisons to practically non-toxic pheromones. Other pesticides that kill animals are the rodenticides for rodents, molluscicides for slugs and snails, piscicides for fish, avicides for birds, and predacides for predators. These are not as widely used as insecticides, but some of them have similar properties. Most insecticides applied to agricultural crops and in urban areas break down after a given time. However, some are very persistent and may remain in the environment for a long time. Persistence is a good quality for some insecticides, because it makes them effective in killing pests for a long time. However, persistent insecticides are more apt to find their way into water supplies at a level of toxicity that can cause problems. These substances can build up in invertebrates and fish. They can pass through the food chain to fish, birds, mammals, and even humans.
Insecticides have varying toxicity for aquatic organisms. Some can kill fish; some disrupt the food chain by killing aquatic insects and other organisms upon which fish depend for food.


Insecticide Leaching and Solubility
Several systemic insecticides that are applied to the soil are water soluble to allow them to be taken up through plant roots. Many of these are highly toxic to mammals. Soluble systemic insecticides such as aldicarb and oxamyl are used at heavy rates for nematode control. They persist for weeks, sometimes months, in the soil. Erosion can carry them into surface waters where they dissolve readily. Leaching can drive them into ground water.


Herbicides
Herbicides are among the most widely used chemicals in the U.S. They account for more than 70 percent of the total volume of pesticides applied in agriculture. Herbicides generally work by altering one or more of the following processes: seedling growth, transport of water and nutrients, production of plant foods (photosynthesis), plant cell development, and plant protein or lipid synthesis. Most herbicides are not very toxic to mammals. The range of plants affected by a particular herbicide may be broad or very narrow. Some herbicides are toxic to almost all plants. These chemicals are appropriately named non-selective herbicides. Non-selective herbicides are useful for controlling vegetation along roadsides or around petroleum storage facilities and electric power stations. Selective herbicides also can be used to control weeds when the physical characteristics of the target weeds are different from those of desirable plants nearby.  Many herbicides are designed to kill only certain plants. These are called selective herbicides. Selective herbicides may affect only a few weeds or a wide variety of plants. Most selective herbicides are very broad-spectrum plant killers. The persistence of some herbicides can be looked upon as either a detriment or advantage. Obviously, the longer these materials remain active in the soil, the less appealing they are environmentally. However, to the farmer, weed control throughout the crop growing season (generally 3 to 6 months) is essential to ensure a good quality, profitable crop. Sometimes the herbicide's active ingredient is not as toxic as its inert ingredients. Therefore, the formulation may have more impact on the toxicity of the product than the active herbicide ingredient.