Safe, environmentally friendly solutions for pest management
Semiochemicals
The term semiochemical was first proposed by Law & Regnier in 1971 (simeon-marker or signal). Chemical substances that mediate communication between organisms. Semiochemicals maybe classified into Pheromones (intraspecific semiochemicals) and Allelochemics (interspecific semiochemicals).
Classification
Pheromones
In 1959, Karlson & Lusher proposed the term pheromone (phrein-to transfer, hormone-to excite).Chemicals secreted into the external environment by an animal which elicit a specific reaction in a receiving individual of the same species. The earliest record on the probable role of pheromones dates back to 1870 by French naturalist, Jean Henri Fabre who observed that the female peacock moth, Saturnia pyri attracted male moths from miles away and later on by Joseph Linter again in 1870 noticed that females of spice bush silk moth attracted male moths. As pheromones are exocrine in origin (i.e. secreted outside the body), they were earlier called as ectohormones (Bethe ,1932). Pheromones are divided into two categories, i.e. releasers, which induce an immediate behavioural change & primers, which initiate changes in development such as sexual maturation & don’t result in sudden behavioural changes. Primer pheromones are not of much practical value in Integrated Pest Management. Releaser pheromones act through olfactory (smell) sensilla and directly act on the central nervous system of the recipient and modify their behaviour. They can be successfully used in pest management programmes.
Sex pheromones
First discovered by A.A. Butenandt in 1959 from silkworm moth, Bombyx mori (Bombykol, chemical name is 10,12 – hexadeca dienol). Released by one sex only and trigger behaviour patterns in the other sex that facilitate in mating, commonly released by females but may be released by males also. In over 150 species of insects, females have been found to release sex pheromones and about 50 species males produce.
The following are some of the female sex pheromones identified in insects
Examples of male sex pheromones
Cotton boll weevil, Anthonomas grandis, Coleoptera
Cabbage looper, Trichoplusia ni, Lepidoptera
Mediterranean fruitfly, Ceratitis capitata, Diptera.
Insect orders producing sex pheromones
Lepidoptera, Orthoptera, Dictyoptera, Diptera, Coleoptera, Hymenoptera, Hemiptera, Neuroptera and mecoptera. In Lepidoptera, sex pheromonal system is highly evolved.
Pheromone producing glands
In Lepidoptera they are produced by eversible glands at the tip of the abdomen of the females. The posture shown during pheromone release is called ‘calling position’. Aphrodisiac glands of male insects are present as scent brushes (or hair pencils) at the tip of the abdomen (eg. Male butterfly of Danaus sp.). Andraconia are glandular scales on wings of male moths producing aphrodisiacs.
Pheromone reception
Female sex pheromones are usually received by olfactory sensillae on male antennae and males search upwind, following the odour corridor of the females. In pheromone perceiving insects, the antennae of male moths are larger and greatly branched than female moths to accommodate numerous olfactory sensilla.
Chemical nature of sex pheromones
In general pheromones have a large number of carbon atoms (10-20) and high molecular weight (180 – 300 daltons). Narrow specificity and high potency are two attributes which depend on long chain carbon atoms and high molecular weight. But since pheromones are volatile their molecular weights cannot be very high as they cannot be carried by wind.
Pest Management With Sex Pheromones
Sex pheromones are being used in pest management in three different ways.
In sampling and detection (Monitoring): Developed trap baited with sex pheromones on a large scale can do the monitoring of insect pest.
To attract and kill (Mass trapping): Sufficient traps are to be used to capture as many adult insects as possible over a large area to avoid any immigration of insects from adjoining areas. The tarps can be impregnated with insecticides to kill insects falling in traps.
Mating disruption by Confusion method : Synthetic pheromones are permeated into the environment to mask the natural pheromones and thus disrupt the normal pheromonal communication among insects. Such disruption will cause failure of insects to locate their mates thereby prevent mating. Pink bollworm, Pectinophora gossypiella was among the first insect species for which this strategy was commercialized using parapheromone (hexalure, 7-hexadecanol acetate).
Alarm pheromones : Substance produced by an insect to repel & disperse other insects in the area. Organ producing alarm pheromones – Cornicles(aphid), Abdominal terga (Dysdergus sp.),Sting glands(Worker bee) etc.
Aggregation pheromone : An insect pheromone which attracts other members of the same species to a location, especially as a defence against predators or to attract potential mates. Example :Frontalin (Dendroctonus frontalis), Ipsenol (Ips confuses), Periplanone (Perplanata americana).
Trial pheromones : It helps in finding the mates & food source. In social insects it helps to maintain cohesion & social integration. Example : Dufour’s gland, Tibial gland secretion in ants.
Allelochemicals
This interspecific semiochemicals may be classified into – Allomones, Kairomones, Synamones, Apneumones.
Allomones : Chemical released by one organism which is advantageous to the releaser. Example : Reflex bleeding in aphid, Secration of osmatiria in Papilio demoleus repel the attack of predator.
Kairomones : Chemicals released by one organism that indues a response in another organism which is advantageous to the recipient. Example : Heptanoic acid released by larva of potato tuber moth (Phthorimoya operculella) increases searchinh by its parasitoid.
Synamones : Chemical released by one organism that induces a response in another organism which is advantageous to both releaser & recipient. Example :Termites & protozoa.
Apneumones : Any substance produced by nonliving material that benefits a recipient species but is detrimental to a different species associated with the nonliving material. Venturia canescns is attracted by smell of oatmeal which is the food od its host. Here it is advantageous to the recipient which is the parasitoid but detrimental to host insect living on the oat meal(non-living material)
They can be used effectively for sustainable agriculture as because of the following reason
Effective : They are a natural part of an insect’s makeup, are highly effective at controlling pest behavior.
Resistance-proof : Insects do not develop resistance to semiochemicals as they do with conventional pesticides.
Non-toxic : Semiochemicals contain no poisonous substances and pose no threat to humans, non-target pests, or wildlife.
Environmentally-friendly Semiochemicals are only needed in low amounts, and their non-toxic nature make them better for the planet.
Conclusion
There is an urgent need to develop sustainable pest management systems to protect field crops in order to replace the current over-reliance on synthetic insecticides. Integrated pest management tools are currently in development that utilize semiochemicals to manipulate the behaviour of pest insects and their natural enemies to provide effective control of pests within the crop. These innovative tools usually require fewer inputs and can involve multiple elements, therefore reducing the likelihood of resistance developing compared with use of synthetic toxicants.
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