Question

Explain how DDT functions as an insecticide.

Short answer

DDT functions as an insecticide by disrupting the nervous system of insects, causing paralysis and death.

Step-by-step solution

Understanding DDT's Chemical Nature

DDT (dichloro-diphenyl-trichloroethane) is an organochlorine compound, which means it is made up of carbon, hydrogen, and chlorine atoms. Its chemical structure allows it to be highly stable and persistent in the environment.

Absorption into Insects

DDT is absorbed by insects primarily through contact with their body surfaces or through ingestion. This occurs as the compound is sprayed onto crops and insects come into contact with it.

Disruption of Nervous System

Once inside the insect, DDT affects the nervous system by interfering with sodium ion channels in the neurons. This disruption causes the neurons to fire spontaneously and repetitively, leading to convulsions and eventual paralysis of the insect.

Insect Mortality

Due to the continuous firing of neurons, insects become immobilized and eventually die. DDT's persistent effects can lead to the death of insects over time, even with minimum exposure.

Key concepts

Chemical Structure of DDT

Among its many qualities, DDT's chemical structure is vital to its prowess as an insecticide. DDT stands for dichloro-diphenyl-trichloroethane, indicating its composition with carbon, hydrogen, and chlorine atoms. This structure is classified as an organochlorine compound.
A key characteristic of this structure is its high stability, a property that ensures DDT remains unchanged in diverse environmental conditions. The stability comes from the strong carbon-chlorine bonds, which make the molecule resistant to breaking down easily.
Because of its robust chemical structure, DDT can persist in the environment for extended periods. This same stability that helps DDT act powerfully against pests also poses environmental challenges.

Absorption in Insects

Once DDT is introduced into the environment, its absorption into insects becomes a crucial step. The primary method of absorption is through contact with an insect's outer body surface. This can happen when insects crawl over residues of DDT on plants or other surfaces and through ingestion if they consume contaminated food.
Upon contact, DDT penetrates the insect's exoskeleton due to its lipophilic (fat-loving) nature. This property allows it to readily dissolve in the fatty tissues of insects, facilitating its entry into the body. This thorough absorption is critical for the effectiveness of DDT as it ensures maximum exposure to the pest’s internal systems.

Nervous System Disruption

After DDT is absorbed, it begins to affect the insect at a critical level: the nervous system. DDT particularly targets the neurons, crucial cells in the insect’s nervous network. It disrupts the normal function of sodium ion channels along the neuron membranes.
These channels are responsible for transmitting nerve impulses, and DDT interferes by forcing them to remain open for prolonged periods. This unnatural state leads to continuous firing of the neurons, making the insect experience uncontrolled nervous signals.
The result of this interference is chaotic: convulsions, loss of coordination, and, ultimately, paralysis of the insect, effectively neutralizing its threat to crops. This explains why DDT has been a potent insecticide, as it attacks pests at a fundamental physiological level.

Environmental Persistence of DDT

Despite its effectiveness as an insecticide, DDT's most concerning property is its environmental persistence. This term refers to how long DDT remains in the environment without breaking down. Its chemical structure, as mentioned, makes it highly stable, leading to long-term presence not just in soil and water, but also within living organisms.
Once DDT is released into the environment, it can accumulate in the tissues of plants and animals through a process called bioaccumulation. Over time, with each level of the food chain, DDT concentrations can increase, a phenomenon known as biomagnification. Therefore, even in areas where DDT use has ceased, its effects can continue to harm wildlife and influence ecosystems.

• Risks to non-target species, including fish and birds.
• Potential impacts on human health through consumption of contaminated food.
• Long-lasting contamination of natural resources.

These points emphasize the need to be cautious about the use of persistent chemicals like DDT.