🐛 Carbamate Insecticides

This lesson explains key concepts in a structured way and connects them to practical agricultural applications and exam-oriented understanding.

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CARBARYL, CARBOFURAN, CARBOSULFAN AND ALDICARB

Mode of action – carbaryl, carbofuran, methomyl, aldicarb and bendiocarb

characteristics and use persistence in plant, soil and water.

Carbamates are derivatives (esters) of carbamic acid. Several carbamates are

systemic, transported in the xylem. It is possible to control pests on shoots and roots

which are otherwise difficult to reach. Hence, they are used as soil insecticides and

nematicides (aldicarb, carbofuran, oxamyl etc.). Commercial products available are

grouped into three groups.

1. N, N-dimethyl carbamates of enols and hydroxy heterocycles

2. Phosphocarbomates

3. Oxinecarbomate

Mechanism of action

The mechanism of action of the insecticidal carbamates is identical to that of the

organophosphates, viz., inhibition of the enzyme cholinesterase. This enzyme has the

function of hydrolyzing the pod synaptic effector, acetylcholine into chlolire and acetic

acid.

Inhibition of acetylchlinesterase (ACHE) leads to a buildup of acelylcholine in the

post synaptic membrane and hence to a permanent nerve stimulation with lethal results.

This stimulation of insects manifests itself in uncountrolled movements and paralysis.

A) Carbaryl (Sevin, Hexavin, Ravyon)

(Naphthyl carbamate, 1-Naphthyl-N-methyl carbamate)

LD50: 850 mg.

Broad spectrum contact insecticide, non-systemic used in cotton, fruits,

vegetables, forage crops etc. Also to control of earth worms.

Carbaryl, introduced by American Union Carbide Company in 1956 was the first

successful commercial carbamate. Carbaryl is produced by any of the general methods

of preparation of carbamates, like reacting 1-naphthol with methyl carbamoyl chloride at

room temperature.

The rate of reaction is increased by removing the HCl that is formed with a strem

of air or nitrogen.

Pure compound of carbaryl is obtained by reacting 1-naphthol with methyl

isocyanate.

Carbaryl is also synthesized by reacting 1-naphthylchlorocarbonate with

methylamine in the presence of HCl acceptors.

Carbaryl is a white crystalline compound with M.P. 142° C. it is highly soluble in

organic solvents; resistant to the action of water at room temperature and also to light

and oxygen of air.

In alkaline medium, it is rapidly hydrolysed and so it is not compatible with

compounds of alkaline nature like Brodeaux Mixture.

B) Carbofuran (Furadan)

(2, 3, dihydro 2, 2 dimethyl 7, benzofuranyl)

Broad spectrum insecticide, nematicide and miticide.

LD50: 8-14 mg/kg rat.

Can be incorporated in soil at 6-10 kg/ha for control of soil insects and nematodes. It

has high mammalian toxicity but is rapidly metabolized to non-toxic products in plants

and animals.

It is a systemic carbamate, broad spectrum insecticide. It is stable in acid and

neutral media but unstable in alkaline medium. Sparingly soluble in water; but soluble in

organic solvents. It is compatible with non-alkaline pesticides and fertilizers. It is not

phytotoxic to rice. Carbofuran when applied to soil is absorbed by plant roots and

distributed to stems and leaves and metabolized to non-toxic compounds in 30 days.

Carbofuran present in soil is degraded by hydrolysis depending on soil pH and clay

content. Toxic residues do not remain in the soil for long.

C) Carbosulfan

Carbosulfan, a sulphanylated derivative of carbofuran acts as a contact and

systemic insecticide. It can be applied to the foliage or soil as a nematicide. It has a

lower mammalian toxicity.

LD50: 209 mg

The carbomates, carbofuran, carbosulfan and aidicarb are valuable

nematicidesss. Carbosulfan formulated as granules is used in vegetables. eg.

brassicas, carrots and turnips.

D) Aldicarb (Temik)

2-Methyl-2-(methylthio) propanol 0-Methylamino carbonyl oxime)

Systemic insecticide, acaricide, nematicide for soil use; only available as

granules to reduce handling hazards.

LD50: 0.93 mg/kg rat.

Used for cotton, sugarbeet and ornamentals. Aldicarb is extremely toxic and is

absorbed through skin. It is therefore marketed as a granular formulation.

White crystalline substance m.p. 100°C. Sparingly soluble in water prepared by

reacting corresponding oxime with methyl isocyanate.

Aldicarb is a carbamate of carbamoyl oxime group. It is effective for control of

aphids, nematodes, flies beetles, leaf miners, thrips and white flies on a wide range of

crops. Aldicarb is readily translocated in plants after soil applicatikon where it is

metabolized to the sulphoxide and the sulphore which are also active.

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Summary Cheat Sheet

Topic	Key exam point
Main class	Carbamate pesticides
Common examples	Carbaryl, carbofuran, carbosulfan, aldicarb
Mode of action	Inhibit acetylcholinesterase, generally more reversibly than organophosphates
Carbaryl note	Common trade association: Sevin
Carbofuran note	Common trade association: Furadan
Use theme	Important synthetic insecticides/nematicides in crop protection
Toxicity caution	Several members are highly toxic and require careful handling
Classification distinction	Carbamates are separate from organophosphates though both affect the nervous system
Exam focus	Example matching and mechanism-of-action questions are common
Trap	Do not classify carbofuran or aldicarb under organophosphates

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website