CHEMICAL CONTROL OF INSECTS
Introduction
Chemical control refers to the use of chemicals, known as pesticides, to manage insect pests. Pesticides are substances that kill or repel pests, including insects, animals, mites, diseases, and weeds.
Types of Pesticides Based on Target Organism
- Insecticides – Kill insects
- Nematicides – Kill nematodes
- Miticides (Acaricides) – Kill mites
- Rodenticides – Kill rodents (e.g., rats and mice)
- Weedicides (Herbicides) – Kill weeds
- Fungicides – Kill fungi
Importance of Chemical Control
- Chemical insecticides are the most powerful tools in pest management due to their:
- High effectiveness
- Rapid curative action
- Adaptability to various agronomic and ecological conditions
- Flexibility in meeting changing pest populations
- Economical nature
Additionally, insecticides are:
- Reliable for emergency action when pest populations exceed the economic threshold.
- Essential in integrated pest management (IPM) strategies.
- Not necessarily harmful when used based on ecological principles.
General Properties of Insecticides
- Most pesticides are concentrated and require dilution before use, except for ready-to-use dusts and granules.
- They are highly toxic and are available in different formulations.
Toxicity Terms and Effects on Mammals
Type of Toxicity | Definition |
Acute Toxicity | Effect produced by a single dose of a toxicant. |
Chronic Toxicity | Effect produced by long-term accumulation of small amounts of a toxicant. |
Oral Toxicity | Toxic effect when consumed orally. |
Dermal Toxicity | Toxic effect when absorbed through the skin. |
Inhalation Toxicity | Toxic effect caused by inhaling fumes or vapors (fumigants). |
Classifications of Insecticides
- Based on Origin and Source of Supply
A) Inorganic Insecticides Derived from mineral sources, such as:
- Arsenate compounds
- Fluorine compounds
- Sulfur (used as an acaricide)
- Zinc phosphide (used as a rodenticide)
B) Organic Insecticides
- Insecticides of Animal Origin: Nereistoxin (extracted from marine annelids). Fish oil rosin soap (derived from fish)
- Plant-Origin Insecticides (Botanical Insecticides) Nicotinoids. Pyrethroids. Rotenoids
- Synthetic Organic Insecticides. Organochlorines, Organophosphates, Carbamates
- Hydrocarbon Oils. Coal tar oil, Mineral oils
ii) Based on Mode of Entry
Type | Description | Examples |
Contact Poisons | Enter through the cuticle or spiracles. | Phosalone, HCH |
Stomach Poisons | Ingested through food. | Bacillus thuringiensis, Trizophos, Quinalphos |
Fumigants | Enter as gases through spiracles. | Aluminium phosphide, Carbon disulfide, Ethylene dibromide (EDB), Sulfur dioxide (SO₂) |
Systemic Insecticides | Move through plant vascular systems to kill feeding insects. | Methyl demeton, Phosphamidon, Acephate |
🔹 Non-systemic insecticides lack systemic movement.
🔹 Translaminar insecticides move from one leaf surface to another (e.g., Malathion, Spinosad).
III. Based on Mode of Action
Category | Mechanism | Examples |
Physical Poisons | Cause asphyxiation or dehydration. | Heavy oils, Inert dusts (Aluminium oxide, Charcoal) |
Protoplasmic Poisons | Destroy cellular protoplasm. | Arsenicals, Mercury, Copper |
Respiratory Poisons | Block respiration by inhibiting enzymes. | Hydrogen cyanide, Carbon monoxide, Rotenone |
Nerve Poisons | Affect the nervous system by blocking acetylcholinesterase (AChE). | Organophosphates, Carbamates |
Chitin Inhibitors | Prevent normal molting and development. | Novaluron, Diflubenzuron, Lufenuron, Buprofezin |
General Poisons | Induce neurotoxic effects. | Chlordane, Aldrin, Toxaphene |
iv) Based on Toxicity Levels (LD₅₀ values)
Category | Symbol | Oral LD₅₀ (mg/kg) | Dermal LD₅₀ (mg/kg) | Label Color |
Extremely Toxic | ☠ Poison | 1 – 50 | 1 – 200 | Red |
Highly Toxic | ⚠ Poison | 51 – 500 | 201 – 2000 | Yellow |
Moderately Toxic | ⚠ Danger | 501 – 5000 | 2001 – 20,000 | Blue |
Less Toxic | ⚠ Caution | >5000 | >20,000 | Green |
v) Based on Stage Specificity
- Ovicides – Kill insect eggs.
- Larvicides – Kill larvae.
- Pupicides – Kill pupae.
- Adulticides – Kill adult insects.
vi) Based on Generation of Insecticides
Generation | Category of Insecticide |
First Generation | Inorganics and Botanicals |
Second Generation | Synthetic Organics |
Third Generation | Insect Growth Regulators (IGRs) |
Fourth Generation | Anti-JH, Synthetic Pyrethroids |
Toxicity Evaluation of Insecticides
LD₅₀ (Lethal Dose 50%)
- Introduced by Finney in 1952.
- It is the amount of toxicant required to kill 50% of the test population.
- Expressed in mg of toxicant per kg body weight (mg/kg).
- Lower LD₅₀ = More toxic chemical.
LD₅₀ Examples (Oral Toxicity in Rats)
- Phosphamidon – 28 mg/kg
- Parathion – 3.6 to 13 mg/kg
- Malathion – 2800 mg/kg
- Hydrogen Cyanide – 1.0 mg/kg
Acute vs. Chronic Toxicity
- Acute toxicity – Effect from a single dose.
- Chronic toxicity – Effect from long-term accumulation of small doses
