[Chemistry Class Notes] Insecticides Pdf for Exam

The population of insects is estimated to be a lot more than all the big creatures that we can see easily. They are a very essential creation of nature that can be found in all the habitable regions of earth. Though all of them play a role in the balance of nature as we experience it, from the context of humans they can be either beneficial, harmful, or neutral. Like any other living being they also need food for their survival and are always searching for it. It can be either a herbivore consuming on materials available from plant source or can be a predator hunting down other smaller insects. 

They are also sometimes parasites that stay associated with their hosts for their survival. This is the diversity of nature but as humans what we are worried about is their conflict with us for the  access to natural resources for our survival. Or sometimes they can directly affect the health of our body or the environment surrounding us. Insects are notoriously known to destroy the crops that we create and affect the agricultural and economic system of our society. 

They also affect the livestock of farmers that we raise as a source of food products. Several diseases are also caused due to them which becomes a financial burden. So in order to prevent these mishappenings to occurring, we use various kinds of chemicals. These chemicals can be natural or artificial that can stop the growth of such insects in the crop field, animal houses or even inside our homes. Such types of chemicals are known as insecticide. There are various insecticides and pesticides available in the market under the name of various brands. Some insecticides are banned in different regions of the country after the discovery of harmful effects on our environment.

What are Insecticides?

The substances that can be used to kill insects are referred to as insecticides. Insecticides consist of a wide application in various fields such as agriculture, medicine, and industrials. They also have the potential to alter the components of the ecosystem primarily, and they are toxic to animals and humans as well. As they spread in the food chain, a few insecticides become concentrated.

Classification of Insecticides

Let us look at the classification of insecticide in detail.

• Depending on the chemical composition, it can be classified into 2 types – organic and inorganic.

• The mode of action can be classified into various poisons such as nerve poisons, physical poisons, protoplasmic poisons, respiratory poisons, chitin inhibitors, and general poisons.

• Depending on the mode of entry in insects, it can be classified into fumigant poisons, contact poisons, systemic poisons, and stomach poisons.

• Depending on the specificity stage, it can be classified as pupicides, ovicides, adulticides, and larvicides.

• Depending on the toxicity level, it can be classified into 4 types, as listed below:

• Less toxic – Symbol: caution, Color: green, oral LD50: >5000

• Moderately toxic – Symbol: danger, Color: blue, oral LD50: 501 – 5000,

• Highly toxic – Symbol: poison, Color: yellow, oral LD50: 51 – 500,

• Extremely toxic – Symbol: skull and poison, Color: red, oral LD50: 1-50

Types of Insecticides

There exist 3 different types of insecticides. They are listed as follows.

This type of insecticide has been introduced into the soil. Moreover, it is for the soil to get absorbed by the roots of plants. Once the insecticide enters the roots, it moves to the external areas such as fruits, leaves, branches, and twigs. It then forms a layer on the plant surface area and will act as a poison to any insect that comes to chew the plants.

A few examples of the ingested pesticides type can be given as roaches and rats.

These types of insecticides act as bullets that aim only at a specific target to kill the insects with the help of its application. In general, the household insect spray works like a contact insecticide because it must directly hit the insect.

Insecticides Classification based on Their Chemical Nature

Depending on the chemical nature, the insecticides are classified into 4 groups, which are listed below:

• Organic insecticides

• Synthetic insecticides

• Inorganic insecticides

• Miscellaneous compounds

Synthetic Insecticides and Natural Insecticides

A major organic chemistry’s emphasis is given to developing chemical tools to enhance agriculture’s productivity. Insecticides can also represent a main area of emphasis. Several major insecticides are inspired by biological analogues, where many others are not found in nature. Some important Synthetic and natural insecticides are listed below. 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.” He was awarded the 1948 Nobel Prize for Medicine or Physiology for this discovery, and it was introduced in 1944. It also functions by opening the sodium channels in the nerve cells of the insect. The contemporaneous chemical industry rise, which is facilitated by the large-scale production of DDT and related to the chlorinated hydrocarbons.

Carbamates and Organophosphates

Organophosphates are the other large class of contact insecticide types. These 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. 

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

Carbamate insecticides contain similar organophosphates, but they have a much shorter action duration and are somewhat less toxic.

Pyrethroids

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

Disadvantages of Insecticides

Let us look at a few disadvantages of insecticides, as listed below.

When the insects repeatedly exposed to the insecticides build up resistance until finally, they contain either a little or no effect at all. The insect’s reproduction is much quicker – they are capable of producing a new generation every 3 to 4 weeks. Thus, the resistance builds up rapidly.

Here, the insecticides will kill more than the intended organisms, which are risky to humans. Besides, when these insecticides get mixed with water sources through drift, leaching, or runoff, they result in harming the aquatic wildlife. When birds drink this contaminated water and eat the affected insects, they also die. A few examples of insecticides, such as DDT, were banned in the US because it affects the predatory birds’ reproductive abilities.