🌼 Botanical 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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PRODUCTS, NICOTINE AND PYRETHRUM

Botanical insecticides

Toxicants derived from plants are used in insect control and among them neem

products, nicotine and pyrethrum are well known.

Insecticidal butylamides

Several members of the Compositae and Rutaceae groups of

plants contain insecticidal unsaturated butylamides. Example: Pellitorine

and Fagaramide. These compounds like several of the pyrethrins, have a

rapid knock down effect of flying insects; their practical application as

insecticides is unfortunately, limited by chemical instability. However a

number of synthetic analogues have been examined and several show

promise as insecticides.

Azadirachtin

Neem – derived from Persian word Asad-dilakt-I-hind, which means free

tree of India - Azadirachta indica A. Juss.

In India 14 million trees (1959 survey) of which 50 % in UP. Though not a

forest tree it grows wild I the forests of AP, Tamil Nadu and Karnataka.

Full-grown neem tree yields 50 kg fruit annually and 350 kg of leaves.

India probably produces 0.7 million tones of fruits and 5 million tones of

leaves every year.

Almost every part of the tree is bitter and finds its application in

indigenous medicine. Oil and cosmetic industries are the users of neem.

The refined and purified neem seed oil has many therapeutic properties.

Considerable quantities of oil are used in cosmetic preparations. The

neem cake after oil recovery is used for slow release of nitrogenous

fertilizers.

Neem oil contains limonoids, a class of compounds that act as

antifeedants or growth regulators in insects. They do not kill instantly but

wipe out a whole generation of insects by preventing the young ones from

maturing and the adults from reproducing.

The most effective of the limonoids is a compound called azadirachtin.

It is similar to the insect hormone ecdysone, which is needed for moulting

during insect development. It works at a concentration of 1-10 ppm by

blocking ecdysone’s action, thereby preventing the larvae from shedding

their external skeletons and maturing. It also prevents feeding in about

200 insects at a concentration of 10-100 ppm.

The knowledge about neem in India is at least a few centuries ago. Indian

scientists had been studying neem for about 70 years and isolated some

active compounds by 1960’s.

A number of neem formulations are being produced by small-scale

formulators and marketed as insecticides. (Neemguard, Margocode,

Nimbicidine, Neemplus, Sukrina, Achook etc.).

Neem oil has some of the less known antifeedants like., salanin, nimbin

and epoxyazadiradione.(4 International conferences were held discussing

regarding production, structure elucidation, isolation of ai, residue

analysis).

Nicotine (C10 H14 N2 ).

Its chemical name is I-methyl-2,3 (pyridyl) pyrrolidine. It is the principal alkaloid in

tobacco (An alkaloid may be defined in general as a naturally occuring heterocyclic,

optically active nitrogenous base of relatively high molecular weight and having marked

physiological activity). Nicotine is found in the leaves of Nicotinana tabacam and N.

rustic a in the range of 2 to 14 per cent. Among the twelve alkaloids present in tobacco

nicotine is the most important one contributing about 97 per cent and the other two of

insecticidal value are (i) Nornicotine (C9 H12 N2) 2-(3-pyridyl pyrrolidine) (ii) Anabusine

(Neonicotine, 3-(2-piperdyl) pyridine.

Pyrethrum

The insecticidal principle in pyrethrum is found in the flower heads of certain plants

of chrysanthemum genus, family compositeae. Only a few species like c.roseum,

c.cineravieflium, c.marshalli and C.tamrutene have been found t<' be valuable sources

of this insecticide. It is used as dusts and sprays.

The flowers are dried at 54.4°C. These flowers are ground to a fine powder and

extracted with the solvents. The solvent is repeatedly percolated through the ground

flowers and the weak extracts. are evaporated in a vacuum still to recover the solvent.

The extracted powder (pyrethrum) containing traces of pyrethrins is sometimes used as

carriers in dust preparations. Dust concentrates are made from concentrated pyrethrum

extracts with a non-volatile solvent and a suitable absorbent carrier and then diluted

before use with an inert diluent. Antioxidants such as tannic acid or hydroquinone are

used to stabilise the pyrethrins in dust preparations. Concentrated extracts of pyrethrum

in aectone, alcohol or a hydrocarbon solvent together with an emulsifier is sold ill the

market.

Miscellaneous compounds

Picrotoxinin (PTX) has been isolated from the seeds of Anamirta

cocculus and is moderately toxic to Insects. E.g. Cockroaches, but is more

toxic to mice. PTX, like avermectins act on GABA regulated chloride ion

channels and functions as a GABA receptor antagonist.

Other compounds with a similar mode of action include the

trioxabicyclooctanes. Eg. The phosphorus ester and

bicycloorthocarboxylates: all are nerve poisons, which are not

cholinesterase inhibitors. The compounds were generally more toxic to

mammals than insects; the bicycloorthocarboxylates, however, can

sometimes show selective activity to house flies and cockroaches and

consequently, have potential for development as novel insecticides.

Insect neurapeptides (INPs) generally containing 5 – 10 amino acid

residues play vital role as circulating neurohormones and

neurotransmitters. They control many aspects of insect’s growth,

development and reproduction together with important physiological and

metabolic processes. Twenty five such processes are thought to be

mediated via these neuropeptides. By 1988, some 27 INPs have been

identified and their structures elucidated. For instance, in the locust, the

adipokinetic hormones AKH I and II, stimulate lipid metabolism during

periods of sustained activity. Eg. Migratory flight. Increased knowledge of

the structures and physiology of INPs should provide new potential target

sites for the design of novel insecticides. Peptide synthesis is one possible

area of exploitation – if certain physiologically active peptides were

introduced to the insect at the wrong time. Severe disruptive effects would

result.

If the gene coding for INPs could be introduced into crop plants,

then insects attacking the crop could be killed. Another promising line of

research would be the design of novel peptides and other molecules to

block the INP receptors.

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Summary Cheat Sheet

Topic	Key exam point
Main class	Botanical insecticides
Common examples	Neem, pyrethrum, rotenone, and nicotine
Neem note	Major active principle is azadirachtin
Pyrethrum note	Botanical insecticide derived from chrysanthemum flowers
Nicotine note	Classical plant-derived insecticidal alkaloid
Role in IPM	Useful in integrated pest management because of botanical origin and selective application contexts
Product diversity	Includes extracts, oils, and naturally derived active fractions
Mode theme	Often act as antifeedant, repellent, contact poison, or growth disruptor depending on source
Exam distinction	Botanical source and active principle are often asked together
Trap	Do not assume all botanicals are harmless; botanical origin does not remove toxicity concerns

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