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A pesticide is any chemical used to do harm to a target pest. These chemicals are purposely introduced into an environment to manage any weeds, insects, rodents, bacteria, or other pests. The term pesticide encompasses insecticides, herbicides, rodenticides, bactericides, fungicides, nematicides, antimicrobials, and more.

There are two main categories of pesticides: biorational and conditional.

Biorational Pesticides

Biorational pesticides are biochemical or microbial products that are relatively low in toxicity to humans and have few environmental side effects. Many are naturally occurring chemicals or have similar structure to naturally occurring chemicals. Some even qualify for use on organic farms.

Botanicals are naturally occurring plant-derived pesticides that generally have a short life in the environment because exposure to air and light breaks them down quickly. Pyrethrum is a botanical insecticide made from a mixture of four compounds found in certain species of tropical chrysanthemums. It is marketed under the name Pyrethrin. Neem oil and azadiractin are botanical insecticides and fungicides derived from the seeds of the neem tree. Garlic, capsaicin, vegetable oil, and manmade pheromones are other examples of botanical pesticides.

Microbial pesticides are created from microorganisms or their by-products which tend to be more advantageous than botanicals because they are safer to use and more selective in the pests they kill, reducing collateral damage to beneficials. Bt or Bacillus thuringiensis, a naturally occurring bacteria in soil, is the most used microbial insecticide. The chemical is toxic to the larvae of several insect species and is typically applied to foliage when the pest larvae are actively feeding. There are many different varieties of Bt which contain various crystals made by a bacterium that are toxic to specific types of insect pests like caterpillars, mosquitoes, blackflies, and Colorado potato beetles.

Mineral pesticides are made of minerals mined from the earth and minimally processed. These include Kaolin clay, iron phosphate, and insecticidal soaps. If minerals are heated, chemically reacted, or combined with surfactants, then they may be considered synthetics. Synthetics inhibit or interrupt the lifecycle of a pest.

Conventional Pesticides

Conventional pesticides are those pesticides that are neither biological pesticides nor antimicrobial pesticides. Their active ingredients are usually synthetic chemicals or agrochemicals. They are typically used in conventional agriculture.

For centuries, inorganic pesticides have been used. These are made from minerals such as sulfur, zinc, boron, copper, cryolite, tin, and lead.

Synthetic organic pesticides are manmade. They consist of a combination carbon, hydrogen, and other elements like nitrogen, phosphorous, and chlorine. Glyphosate or Roundup is one example of a synthetic organic pesticide.

Other Classifications of Pesticides

Another way to classify pesticides is based on how they work. There are several major groups of pesticides when classified in this manner.

Contact poisons are designed to be harmful, damaging, or lethal to the target pest when the pesticide is absorbed through direct contact. The often leave a residue to kill pests later after the pesticide dries. They can be organic, inorganic, or natural and often come in foggers or aerosols.

Protectants are applied to plant surfaces creating a chemical barrier between the plant and the pest preventing entry into the plant or damage by the pest.

Stomach poisons are pesticides that must be ingested by a pest and absorbed into its body before causing the pest’s death. These often contain arsenates or fluorides.

Translocated pesticides or systemic pesticides are absorbed by and circulated throughout the target pest before leading to its death.

Fumigant pesticides form gases that are toxic to their target pests.

Factors that Affect Pesticide Activity

There are many factors that can determine the effectiveness of pesticides. Soils rich in organic matter will typically require a higher quantity of pesticide than those with less organic matter to achieve the same level of pest control. Climate can affect pesticide activity in a variety of ways. Excessive rain or irrigation can cause the soluble pesticides to leach down into the soil or runoff rendering them ineffective. Sunlight can break down certain pesticides. Over time, pests can become resistant to a pesticide if it is applied repeatedly.

Pesticide Formulations

The chemical in a pesticide that kills the pest is known as the active ingredient. The active ingredient is rarely applied in pure form. They are typically mixed with other substances called inert ingredients, making the product easier and safer to apply. The active ingredient and inert ingredients together are known as a formulation.

Many of the same formulations covered in the previous chapter for herbicides are applicable to all forms of pesticides. These include formulations like emulsifiable concentrates (E or EC), granules (G), wettable powders (W or WP). Some common formulations not covered in the previous chapter relating to herbicides are covered below.

Dust (D) formulations are dry with their active ingredient typically bound to clay or some other fine powder, though they can be made entirely from pure active ingredient like borate or silica.

Aerosol (A) spray formulations contain the active ingredient in a solution or solvent. Because they are premixed and typically come in their own container that can dispense it, you do not need a separate sprayer. They can contain multiple pesticides but are relatively expensive on a per volume basis. They are best used in enclosed spaces like greenhouses, barns, and other structures.

Bait (B) formulations are created by mixing an active ingredient with food or another attractive substance.

Fertilizer mixtures can contain relatively small amounts of pesticides. This is most common in fertilizers for turfgrass. When this occurs, the mixtures are legally classified as pesticides.