Spotters in "Environment and Health: Insecticides" for CFM & MPH students
Insecticides:
These are substances that are used to kill insects.
Pesticides:
It is a general term that includes insecticides, fungicides rodenticides, herbicides, disinfectants, repellents, and other chemicals used for the control of pests.
Insecticides are classified into 3 groups: Contact poisons, Stomach poisons and Fumigants.
CONTACT POISONS: These are those insecticides that kill insects primarily by contact e.g., pyrethrum, DDT, HCH, dieldrin.
STOMACH POISONS: These are those insecticides which when ingested cause the death of the insects, e.g., Paris green, sodium fluoride.
FUMIGANTS: These are those insecticides that give off vapours that have a lethal effect on the insects, e.g., sulphur dioxide.
1. DDT (Dichloro-diphenyl-trichloroethane):
It was first synthesized in 1874, by a German chemist, Ziedler. Its insecticidal properties were discovered by the Swiss scientist, Paul Muller in 1939.
Important points:
PROPERTIES: It is a white amorphous powder with a mild, but not unpleasant smell, insoluble in water but dissolves in most organic solvents.
ACTION: Primarily a contact poison. It acts on the nervous system of insects. It permeates into the insect body through the cuticle, after dissolving in the waxy covering of the feet, and causes paralysis of legs and wings, convulsions and finally death. It does not cause immediate death, but it takes several hours to kill.
The residual action of DDT may last as long as 18 months depending upon the treated surface & has no repellent action on insects.
APPLICATION: As a residual spray, DDT is applied at a dosage of 100-200 mg. per square foot area.
The formulation of choice is a 5 per cent suspension of DDT, which when sprayed at a rate of 1 gallon over an area of 1,000 sq. feet, gives a dosage of 200 mg. per sq. ft.
As dust, DDT is used in 5 to 10 per cent strength for the control of lice, fleas, ticks and bugs.
In aerosol or space sprays, DDT is one of the main constituents.
DDT is still one of the most widely used insecticides. Its persistence in the living organisms and plants and also its adverse effects on certain species of wild animals have led to controversy on its use.
Many developed countries (e.g., U.K., Sweden, Norway) have restricted the use of DDT because of its potential danger to ecosystems of wildlife in the environment.
2. BHC (Benzene hexachloride) or HCH (hexachlorocyclo-hexane) or gammexane or hexidol:
It was synthesized by Michael Faraday in 1825 & its insecticidal activity was discovered in the USA in 1933.
Important points:
PROPERTIES: It is a white or chocolate coloured powder with a musty smell, irritating to the eyes, nose and skin & is slightly volatile. It is more insecticidal than DDT.
ACTION: It kills insects by direct contact, but its residual action is of a shorter duration, up to 3 months. It kills insects by its vapour effect also.
APPLICATION: It is used like DDT. A dose of 25 to 50 mg of gamma-HCH per sq. ft. is recommended for residual treatment.
3. Paris green (copper aceto-arsenite):
Important points:
PROPERTIES: It is an emerald green microcrystalline powder, practically insoluble in water but soluble in ammonia and concentrated acids.
A good sample of Paris green contains over 50 per cent of arsenious oxide.
ACTION: Paris green is a stomach poison.
APPLICATION: Till the advent of DDT, Paris green was extensively used in the control of anopheline larvae, by spraying as a 2 per cent dust over breeding places once a week.
The use of Paris green was largely responsible for the eradication of A gambiae from Brazil in 1940, and the near-eradication of the same species in Egypt in 1944.
4. Temephos (Abate):
It is an organophosphorus compound.
Important points:
PROPERTIES: It is a brown viscous liquid, soluble in petroleum solvents.
ACTION: Because of its low toxicity, it has been extensively used in India for the control of A.stephensi in wells and in domestic water containers with good results at a dosage not greater than 1.0 ppm.
Abate is less effective as an adulticide
5. Pyrethrum:
It is an insecticide of vegetable ongm. It is extracted from the flowers of Chrysanthemum cinerariafolium, a plant that is cultivated in Kashmir, Simla and Nilgiris in India.
Important points:
PROPERTIES: The flowers contain 4 active principles: Pyrethrins I and II, cinerins I and II - all nerve poisons.
ACTION: Pyrethrum extract is prepared by soaking 1/2 to 1 lb of dried, powdered flowers in 1 gallon of kerosene oil for 72 hours when the active principles are extracted.
The ready-to-spray solution contains up to 0.1 per cent of pyrethrins.
Pyrethrum extract is an excellent space spray. It has been used extensively in the past for killing adult mosquitoes, and other insects. It kills instantly on mere contact.
It is sprayed at the rate of 1/2 to 1 oz. of the spray solution per 1,000 cu. ft. of space. As a space spray, fine atomization of the spray solution is necessary and the doors and windows should be kept closed for 1/2 hour after spraying.
Pyrethrum suffers from the disadvantage that it does not possess the residual action of DDT and other synthetic insecticides.
References:
- Park, K. (2019). Park's textbook of preventive and social medicine. 25th. Jabalpur, India: M/S Banarsidas Bhanot.
- Kadri AM. (2019). IAPSM's Textbook of Community Medicine. 1st. New Delhi, India: Jaypee Brothers Medical Publishers (P) Ltd.
- https://www.sciencephoto.com/media/445216/view/bottle-of-ddt-pesticide photo credit
- https://www.beeculture.com/politics-killing-bees-pesticides/ photo credit
- https://www.indiamart.com/proddetail/gamma-vet-bhc-hemisulphate-veterinary-spray-21566098362.html photo credit
- https://www.adplindia.in/product/gamma-benzene-hexachloride-application-nfi79/ photo credit
- http://nhathuoconline247.com/san-pham/thuoc-tri-ghe-lindane-shampoo-usp-1/ photo credit
- https://pyrodata.com/chemicals/Paris-Green photo credit
- https://www.wetcanvas.com/forums/topic/emerald-green-genuine-poison-do-not-use/ photo credit
- https://www.interagro.co.uk/product/abate/ photo credit
- https://www.indiamart.com/proddetail/abate-1sg-12943344391.html photo credit
- https://shopee.com.my/One-Box-BASF-Abate-1.1G-Aedes-Mosquito-Larvae-Killer-10s-Box-i.10113942.256088710 photo credit
- https://www.gardenersdream.co.uk/pyrethrum-5ec-natural-organic-insect-pest-spray-p193 photo credit
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