Insecticides: Definition, Types, Classification and Disadvantages

Insecticides are chemicals that are used to control insects by killing or preventing them from engaging in undesirable or destructive behaviours. Insecticides are classified based on their structure and mode of action. There are many insecticides that act upon the insect’s nervous system, while others act as growth regulators or endotoxins. Such substances are generally used in order to control pests that infest cultivated plants or to reduce disease-carrying insects in specific areas. Let’s learn more about the topic and discuss some important questions.

Keyterms: Insecticides, Indsects, Nervous system, Endotoxins, Plants, Growth regulator, Agriculture, Medicine, Industry, Food chain

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What are Insecticides?

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The substances that are used to kill insects are called insecticides. They have a wide application in the field of agriculture, medicine, and industry and have the potential to alter ecosystem components primarily and are toxic to animals and humans as well. Few of the insecticides become concentrated as they spread in the food chain.

Insecticides

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Classification of Insecticide

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• Insecticides are classified as organic and inorganic depending on the chemical composition.
• They are classified as stomach poisons, contact poisons, fumigant poisons, and systemic poisons based on the mode of entry in the insects.
• Insecticides are classified as physical poisons, nerve poisons, respiratory poisons, protoplasmic poisons, general poisons, and chitin inhibitors based on the mode of action.
• Depending on toxicity, they are classified into four types:
  1. Extremely toxic – Colour: red, oral LD50: 1-50, symbol: skull and poison
  2. Moderately toxic – Colour: blue, oral LD50: 501 – 5000, symbol: danger
  3. Highly toxic – Colour: yellow, oral LD50: 51 – 500, symbol: poison
  4. Less toxic – Colour: green, oral LD50: >5000,symbol: caution
• They are classified as ovicides, pesticides, larvicides, and adulticides based on the stage of specificity.

Classification of Insecticide

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Types of Insecticide

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Below mentioned are the three different types of insecticides. 

1. Systemic – Systemic insecticides are the type of insecticide that is introduced into the soil to get absorbed by the plant roots and once the insecticide enters the roots, it moves to external areas such as fruits, twigs, leaves and branches. It acts as a poison to any insect that comes to chew the plant and forms a layer on the plant surface area.
2. Ingested – Few of the examples of ingested pesticides are roaches and rats.
3. Contact – Contact insecticides are the type of insecticides that act like bullets as they aim only at a particular target to kill insects by its application and usually, household insect spray works like contact insecticides as it is directly hit at the insect.

Types of Insecticide

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Classification of Insecticides based on Chemical Nature

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Insecticides are classified into four groups based on their chemical nature.

1. Organic insecticides
2. Synthetic insecticides
3. Inorganic insecticides
4. Miscellaneous compounds

Synthetic Insecticides and Natural Insecticides

A major organic chemistry's emphasis is given to developing chemical tools to enhance agriculture's productivity and insecticides can also represent a main area of emphasis. Along with this, various major insecticides are inspired by biological analogues, where many others are not found in nature.

Below listed are some important Synthetic and natural insecticides. Let us briefly understand these.

Organochlorides

One of the best-known organochlorines is given as DDT, which was created by the Swiss scientist named "Paul Müller" and for this, he was awarded the 1948 Nobel Prize for Medicine or Physiology, and it was introduced in 1944. Organochlorides also function by opening the sodium channels in the nerve cells of the insect and the contemporaneous chemical industry rise, which is facilitated by the large-scale production of DDT and related to the chlorinated hydrocarbons.

Organochlorides

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Carbamates and Organophosphates

Organophosphates are the other large class of contact insecticide types and they can also target the nervous system of the insects. These interfere with the acetylcholinesterase enzymes and other cholinesterases, disrupting the nerve impulses and disabling or killing the insect. 

Carbamates and Organophosphates

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Chemical Warfare

Nerve agents such as tabun, sarin, soman, VX and Organophosphate insecticides work in the same way and they have a cumulative toxic effect on wildlife. Hence, the multiple exposures to the chemicals amplify toxicity and Organophosphate use declined with the rise of substitutes in the US.

Carbamate Insecticides 

Such have a much shorter action duration and are somewhat less toxic and contain similar organophosphates.

Pyrethroids

Pyrethroid pesticide types mimic the natural compound pyrethrum's insecticidal activity, which is the biopesticide found in pyrethrins and are less toxic compared to carbamates and organophosphates. Along with this, these compounds are the nonpersistent sodium channel modulators, Compounds present in this group are often applied against household pests.

Pyrethroids

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Disadvantages of Insecticides

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1. Non-target organisms – Insecticides can kill more than intended organisms and are risky to humans and when these insecticides mix with water sources through drift, leaching, or runoff, they harm aquatic wildlife. Birds when drinking such contaminated water and eating affected insects then they die. Few examples of insecticides such as DDT were banned in the US as it affects the reproductive abilities of predatory birds.
2. Resistance – Insects when repeatedly exposed to insecticides build up resistance until finally, they have little or no effect at all and the reproduction in insects is so quick that they produce a new generation every three to four weeks and because of this, the resistance builds up rapidly.

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Things to Remember

• The substances that are used to kill insects are called insecticides. They have a wide application in the field of agriculture, medicine, and industry and have the potential to alter ecosystem components primarily and are toxic to animals and humans as well. 
• Systemic insecticides are the type of insecticide that is introduced into the soil to get absorbed by the plant roots and once the insecticide enters the roots, it moves to external areas such as fruits, twigs, leaves and branches.
• Contact insecticides are the type of insecticides that act like bullets as they aim only at a particular target to kill insects by its application and usually, household insect spray works like contact insecticides as it is directly hit at the insect.
• One of the best-known organochlorines is given as DDT, which was created by the Swiss scientist named "Paul Müller" and for this, he was awarded the 1948 Nobel Prize for Medicine or Physiology, and it was introduced in 1944. 

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