Insecticides: Types

 Classification of Insecticide:

Based on mode of entry

A) Stomach poisons: Those insecticides that reach through their food into the target insect's body are considered stomach poisons. These insecticides are sprayed on the plants or dusted, and when the insects eat parts of these plants including leaves, tender stems, seeds, etc., they enter the stomach and kill them. Stomach insecticides are very effective against insect pests having biting and chewing types of mouth parts. Example- Malathion.

 

B) Contact poisons: Such insecticides that enter the insect's body through their cuticles or spiracles. The pest may absorb the poison when walking on the treated ground, flying through a mist or fine droplets, or when the insecticides are directly hit during spraying or dusting. Example- Fenvalerate.

 

C) Fumigants: The insecticides that penetrate the insect's body through the spiracles and tracheae in a gaseous state. In closed spaces, they are most effective and as such are commonly used to manage the stored grain pest. Example- Aluminium phosphide.

 

D) Systemic poisons: Chemicals are absorbed by vegetation (or) roots when added to plant or soil and translocated through the vascular system, resulting in the death of insect feeding plant. Example - Dimethoate.

 

Based on mode of action

A) Physical poison: Poison which kill the insect by exerting a physical effect are known as physical poisons. Heavy oils and tar oils kill the insects through asphyxiation i.e. exclusion of air. Inert dust causes cuticular abrasion or removes moisture from the insect's skin. Due to loss of body moisture insect dies in both cases.

 

B) Protoplasmic poison: Toxicants that kill the insect by disrupting the cellular protoplasm of the midgut epithelium are referred to as protoplasmic poisons.  Example - Mercury, Copper, and Arsenic compounds.

 

C) Respiratory poison: A toxicants that block the cell respiration and inactivate the respiratory enzymes are termed as respiratory poisons. Example- HCN, CO etc.

 

D) Nerve poison: Poison which affect the nervous system and render the insect to behave abnormally leading to death are called nerve poisons. The toxicant is soluble in tissue lipoids and inhibits the production of acetylcholinesterase enzymes in insects and mammals. Example - Organophosphate and carbamate insecticides.

 

Based on chemical nature

 

A) Inorganic insecticides: Inorganic compounds having insecticidal property.

- Arsenicals: Lead arsenate (PbHAsO₄), Calcium arsenate [Ca (AsO₄)₂], Paris green etc.

- Fluorides: Sodium fluoride (NaF), Sodium fluoaluminate (Na₃AlF₆).

 

B) Organic insecticides:

(i) Hydrocarbon oils: The presence of the unsaturated compounds makes them phytotoxic. Example- Petroleum oil and Coal tar oil.

 

(ii) Animal origin:

a) Neristoxins (NTX): It was isolated from a marine annelid, Lumbrinereis heteropoda by Nita in 1934 but its insecticidal properties were discovered by Sakai in 1964.

b)The salivary secretion of Assassin bug species causes failure of heart in Cockroach.

 

c) Cantharidin: It is obtained from the blood and accessory glands of the male genitalia of several species of meloid beetles.

 

 

(iii) Plant origin:

 

(a) Pyrethrum: It is obtained from the dried flowers of Chrysanthemum cinerarifolium. The insecticidal property of pyrethrum is due to presence of pyrethrin, which is an ester formed by two acids and three alcohols.

Acids: Chrysanthemic acid and Pyrethric acid.

Alcohols: Pyrethrolone, Cinerolone and Jasmolone.

 

(b) Azadirachtin: It is obtained from seeds and leaves of Indian neem tree (Azadirachta indica). The active compounds present in neem tree is Azadirachtin, melantriol, salannin, nimbin and nimbidin. Among them Azadirachtin is the main active compound having insecticidal properties.

 

(c) Rotenone: It is obtained from roots of leguminous plants Lonchocarpus spp. and Derris elptica. It is extremely toxic to fish.

 

(d) Sabadilla: Alkaloid obtained from seeds of tropical lily Schoenocaulon officinale. The alkaloid mainly contains, cevadine and veratridine which act as nerve poisons. Sabadilla is harmfull to pollinators (Honeybees).

 

(e) Ryanodine: It is obtained from woody stem of Ryania speciose. It acts as muscular poison by blocking the conversion of ADP to ATP in striated muscles.

 

(f) Nicotine: It is mainly isolated from Nicotiana tobacum and Nicotiana rustica. It was first used as an insecticide in 1763 in France but it was synthesized in 1904 and appeared in the market in 1910 under the name of ‘Black leaf 40’.It is effective against soft bodied sucking insect like thrips, leaf hopper, mealy bugs and leaf minor (Reddy, 2010).

 

(iv) Synthetic organic compounds:

 

(a) Organochlorine:These were the first synthetic organic insecticides ever discovered. The group includes DDT and its relatives (e.g., methoxychlor and kelthane), BHC, toxaphene, and cyclodiene insecticides (e.g., aldrin, dieldrin, endrin, mirex, chlordane, heptachlor, and endosulfan). Most organochlorines (they are also known as chlorinated hydrocarbons) are relatively stable compounds that act as nerve poisons.  These are more soluble in fat (lipid) than in water several thousand times. It means that they tend to accumulate in fatty tissues and concentrate at the top of the food chain (bioaccumulation) of a population of humans. Because of environmental problems associated with their long persistence and bioaccumulation, many organochlorine uses in India have been prohibited or discontinued. Lindane is still used as a seed dressing and sold by prescription for control of human lice; endosulfan and kelthane are still used for agricultural pest control.

 

(b) DDT (Dichloro Diphenyl Trichloroethane): It was first synthesized by Othmar Ziedler in 1874, but its insecticidal property was first discovered by Paul Herman Muller in 1939. It is safe to all crops except cucurbits.

 

(c) BHC (Benzene Hexachloride):It was first prepared by Michal faraday in1825, but its insecticidal property was discovered in 1942 by Dupit and Raucourt in France and Salade in England. It is a mixture of six chemically distinct isomers and if the prepared product contains 99% of gamma isomer then it is known as Linden.

 

(d) Cyclodines: It is a group of synthetic cyclic hydrocarbons first synthesized by Kurt Alder and Otto Diels, which includes Alderin, Dielderin, Endrin, Isoderin, Mirex and Endosulfan (Thiodan). Cyclodines are prepared by Diels Alder reaction. Endosulfan is mixture of two isomers alfa and beta and is very toxic to fish.

 

(e) Organophosphate:These compounds were first discovered as a by-product of nerve gas experiments by German researchers in the late 1930s. These are much less prevalent and do not accumulate in fatty tissues than organochlorines. The group includes a wide range of general-purpose insecticides with mammalian toxicity (e.g. malathion, parathion, diazinon, chlorpyrifos, azinphosmethyl, acephate, phorate, and phosmet) as well as compounds that work as fumigants (e.g., DDVP) and as systemics (e.g. dimethoate, disulfoton, demeton, and ronnel). All organophosphates are nerve poisons. They block the active site of an enzyme that breaks down (acetylcholinesterase) and eliminates a neurotransmitter (acetylcholine) from the synapse of the nerve. The prolonged accumulation of acetylcholine results in hyperactivity, including trembling, seizures and eventually death (https://genent.cals.ncsu.edu).Organophosphorus insecticides are considered as derivatives of the corresponding acids and are classified as follows-

*Phosphates acid: It includes Dichlorovos, Monocrotophos, Naled etc.

*Phosphorothiolate:It includes Oxydemeton-methyl, Vamidothion, Profenophos etc.

*Phosphorothionate: It includes Quinalphos, Diazinon, Chlorpyriphos etc.

*Phosphoroamidate: It includes Acephate, Isofenphos, Phosphoran etc.

 

(f) Carbamate:They are synthetic derivatives of physostigmine, commonly known as Eserine. It is the principal alkaloid of the plant calabar bean (Physostigmine venenosum). Physostigmine are unsuitable as insecticides as they are too polar and unable to penetrate the insect cuticle. They are less persistent than organochlorines and organophosphates and likewise the latter inhibit acetyl cholinesterase.  Carbamate insecticides are the derivatives of carbamic acid and dithio-carbamic acid.

 

Derivatives of carbamic acid	Derivatives of dithio- carbamic acid
Heterocyclic carbamate: Isolan	The compound of this group are mostly Nematicides with weak insecti-cidal property. Example: carbothion.
Napthyl carbamate: Carbaryl
Phenyl carbamate: Carbofuran
Oxime carbamate: Aldicarb

(g) Synthetic Pyrethroids: Such insecticides, modeled after the natural product found in chrysanthemums, are highly toxic to insects, but relatively safe for humans.  Natural pyrethrum is almost useless outdoors as it quickly breaks down in sunlight. The first light-stable analog was synthesized in 1967 (Resmethrin) and was commercially available as agricultural insecticides by 1976 as Permethrin and Fenvalerate. All of these compounds act as nerve poisons; they appear to disrupt sodium transport in axons in much the same way as some of the organochlorines. Example: Fenvalerate, Novaluron, Cypermethin etc.