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  Alternative Control Strategies-SemioChemicals, Biology tutorial

Introduction:

Primary metabolic procedures in plants generate substances to catalyze reactions, build up tissues and supply energy. The plant needs inorganic ions and generates enzymes, carbohydrates, hormones, lipids, proteins and phosphorus compounds for energy transfer. Altogether, such primary metabolites encourage growth and reproduction of the plant. For insects, a few of these primary metabolites are feeding stimulants, nutrients and toxicants. The other primary metabolites are inert as far as an insect is concerned.

Secondary metabolic procedures in plants seem to be coincidental to the primary metabolism. The chemicals generated, secondary metabolites differ broadly among plants and are assumed nonessential in the primary metabolism. A few of these secondary metabolites are assumed to have occurred as methods of chemical defense against plant eating. They might be stored in any suitable place in the plant structure and frequently are exuded from the outer layers of plant tissues.

Here, they might be sensed by insects and function as token stimuli. A token stimulus draws response initially and afterward has no effect.

This relationship among plant chemical stimuli and insect response is a form of chemical communication among such organisms. These chemicals are termed as Semio-chemicals. They are chemical substances which mediate communication among organisms. 

The goals of employing semio-chemicals in pest management are as follows: 

1) To monitor pest populations to find out when control is warranted.

2) To transform the behavior of the pest or its enemies to the detriment of the pest. 

In general, the benefits of employing semio-chemicals are as follows: 

1) They encompass poor consequences merely on target pests.

2) They are comparatively nontoxic and needed in low amounts.

3) They are non-persistent and environmentally safe.

4) They appear hard for insects to build up resistance against. The monitoring of pest populations having pheromones is frequently integrated in the management programs.

Classification of Semio-Chemicals:

Semio-chemicals perhaps categorized into Pheromones (that is, Intraspecific Semio-chemicals) and Allelochemics (that is, Interspecific Semio-chemicals).

I) Pheromones:

Pheromones are the chemicals secreted to the external environment through an animal that draw out a particular reaction in a receiving individual of the similar species. They promote communication among the members of similar species. Pheromones are volatile in nature and they help in communication among the insects.  

Pheromones are basically exocrine in origin (that is, secreted outside the body). Therefore they were earlier termed as ectohormones. In the year 1959, German chemists, Karlson and Butenandt isolated and recognized the first pheromone, a sex attractant from the silkworm moths. They introduced the word pheromone. As this first report, hundreds of pheromones have been recognized in most of the organisms. The improvement made in analytical chemistry aided pheromone research. 

Based on the responses draw out, pheromones can be categorized into two groups:

1) Primer pheromones: 

They basically trigger off a chain of physiological modifications in the recipient devoid of any immediate change in the behavior. They act via gustatory (or taste) sensilla. Example: Caste determination and reproduction in the social insects such as bees, ants, wasps and termites are mediated through primer pheromones. Such pheromones are not of much practical value in the IPM. 

2) Releaser pheromones:  

Such pheromones generate an immediate change in the behavior of the recipient. They act via olfactory (or smell) sensilla and directly act on the central nervous system of the recipient and alter their behavior. They can be successfully employed in the pest management programs. 

-  Sex pheromones: They are discharged by one sex only and trigger behavior patterns in the other sex which facilitate mating. Such substances are frequently generated through females to fascinate or attract males for mating, however they might as well be produced through males to attract females.

- Alarm Pheromones: Such pheromones are very common in social insects like bees and ants. They draw out attack or retreat behavior.

- Trail-marking Pheromones: Such chemicals are generated by foraging ants and termites to point out courses of requisites to the other members of the colony. 

- Aggregation Pheromones: Such pheromones are prominent in certain species of beetles and cause insects to aggregate or congregate at food sites, reproductive habits, hibernation sites and the like.

- Epideictic Pheromones: It is as well termed as Spacing Pheromones. They draw dispersal away from potentially crowded food sources, thus reducing the numbers. Therefore, they are one of the few pheromones which serve to repel instead of attract.

II) Allelochemics:

Such promote communication among members of various species. They can be categorized further into Allomones and Kairomones.

1) Allomones: These are the chemical substances generated and discharged through an individual of one species which influences the behaviour of a member of the other species to the profit of the originator however not the receiver. They are mainly defensive chemicals producing the negative responses in insects and reducing chances of contact and utilization. The production of allomones is a general form of defense, specifically through plant species against insect herbivores. They comprise repellents, oviposition and feeding toxicants and deterrents.

2)  Kairomones:

They are semio-chemicals emitted through an organism that mediates interspecific interactions in a manner that benefits an individual of the other species that receives it, devoid of benefitting the emitter. They are beneficial to an insect promoting host finding, oviposition and feeding. They comprise: arrestants, attractants, excitants and stimulants. The two major ecological cues are given by kairomones; they usually either point out a food source for the receiver or provide warning of the presence of the predator.

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