**2.2 Allelochemicals**

The second subclass of semiochemicals is allelochemicals which includes substances that transmit chemical messages between different species. Fundamentally, these substances resemble an interspecific communication which are emitted by individuals of one species and are understood by individuals of a different species. Allelochemicals are divided depending on the benefits and costs to the signaler and receiver. They have been divided into five categories according to [1, 20] as follows.

#### *2.2.1 Allomones*

Allomones (from Greek "allos + hormone" = excite others): released from one organism that stimulate a response in an individual of another species. The response is beneficial to the emitter, e.g., poisonous allelochemicals. They can also be seen

as a deterrent emitted by insects against their predators as a defense mechanism. Granular trichomes which cover plant leaves and stems release herbivore-deterring allomones under stress conditions as a defense process. These allomones are toxic for the herbivorous insect pests, e.g., nicotine from a tobacco plant. Moreover, bolas spiders can deceit, lure, and capture male moths by synthesizing and mimicking moth pheromones [14].

### *2.2.2 Kairomones*

Kairomones (from a Greek word "kairos" = opportunistic or exploitative): emitted by one organism that stimulate a response in an individual of another species. The response is beneficial to the recipient, e.g., orientation of predaceous checkered beetles (Coleoptera, Cleridae) toward the aggregation pheromone of their prey and bark beetle (Coleoptera, Curculionidae, Scolytinae) [14, 21]. Kairomones may be allomones or pheromones depending on the circumstances. For example, American bolas spiders attract their prey (male moths) by releasing attractant allomones which serve as sex pheromones emitted by female moths. Also, exudates of warm-blooded animals that pull blood-sucking insects toward their hosts serve as kairomones.

### *2.2.3 Synomones*

Synomones: beneficial to both the releaser and receiver. Examples include scents used by flowers to attract pollinating insects. Moreover, herbivore-induced plant volatiles are considered to be active synomones which recruit natural enemies of insect pests toward the affected plants [22]. Also, synomones play an essential role in mate-finding communication. This role relies on the reduction of competition in the olfaction communication channel between closely related species with overlapping pheromone components. This advisable action is important in preventing exhaustion from the time and energy required for orientation toward heterospecifics [23]. In termites, hydroquinone is a phagostimulant compound secreted by labial glands distinguished as pheromones and synomones when different species are partaking the same foraging territory. It acts as a pheromone when recognized by nest mates of the same species and as a synomone when perceived by another termite species [24].

#### *2.2.4 Antimones*

Antimones: maladaptive for both the releaser and receiver. These substances are produced or acquired by an organism that, when encountered by another individual of a different species in the natural environment, activate in the receiving individual a repellent response to the emitting and receiving individuals [1].

#### *2.2.5 Apneumones*

Apneumones (from a Greek word "a-pneum" = breathless or lifeless): emitted by a non-living source, causing a favorable behavioral or physiological reaction to a receiving organism, but harmful to other species that may be found either in or on the non-living material. Apneumones were suggested by [7]. Rare cases of these allelochemicals have been found later in the literature, e.g., hexanal and 2-methyl-2-butanol released from rabbit stools attract sandfly females for oviposition [25].

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nervous system.

*Pheromones and Chemical Communication in Insects DOI: http://dx.doi.org/10.5772/intechopen.92384*

**4. Chemosensory stimulation in insects**

**3. Mechanisms of chemical communication in insects**

Chemical communication is an essential item for insects' survivals that qualify them to adapt their behavior depending on the surrounding environment [1]. In insects, chemical communication is based on a mixture of one or several semiochemical substances which stimulate various receptor organs. The efficiency of semiochemicals in chemical communication is mainly based on various physical properties such as chemical nature, solubility volatility, and its lifetime in the environment. Also, the stability of such volatiles affects their efficiency in IPM programs [1]. Dispersal is a natural activity of insect where the movement is directed (taxes) or random (kineses) which is motivated by chemical or visual stimuli. There are three mechanisms of insect behavioral responses for finding an odor source. In the first mechanism called true chemotaxis, the insects align their body directly toward the odor source due to sensing the gradient of odor molecules. For the second mechanism, the insect does not discover the odor direction but becomes stimulated either for moving at different rates which is called orthokinesis or turning at various frequencies depending on changes in odor concentration (klinokinesis). The third mechanism depends on the odor of molecules impulse insect toward some other stimulus. Anemotaxis is the most common example for this mechanism where the molecules of an attractive chemical stimulate the receptive insects to fly upwind [26].

In insects, chemosensory stimulation occurs in various receptor organs via constant bombardment of chemical signals which improved the insect's ability to detect, discriminate, and distinguish innumerable different molecules as different odors. The insect receptor organs include antennae, mouthparts, and ovipositors. These receptors are very sensitive even for a few molecules of specific semiochemicals. Attraction (directed movement toward stimuli) and repulsion (directed movement away from stimuli) are the main insect responses to various odors. For field traps, insect catches not only occur via taxes but also via kineses (random movement). The insect can detect any odor by olfactory receptors located in the sensory organs including antennae, mouthparts, and ovipositors [27]. Various types of sensilla are recorded including trichodea, basiconica, styloconica, chaetica, etc. Knowledge of the types of sensilla on the antennae and mouthparts provides a foundation for understanding the olfaction and feeding preferences of herbivorous insect pests and subsequently can be useful for improving new control strategies for the target pests [28, 29]. The basic structure of sensillum is explored by [30] in **Figure 2**. The sensillum formed from the sensory neuron attached to branched cuticular pores (P) which allows odor passage. Sensillum pores act to filter molecules received from the airstream and concentrated it in the lumen of the sensillum and passed to branched neurons which convey impulses from and to the central

In insects, the ability to discriminate different odors depends only on the evolutionary pressures of the molecules which stimulate the development of specific binding proteins (BPs) and specific receptor sites present on individual chemosensory neurons. This selectivity bestowed upon chemosensory neurons by the receptor types expressed represents one level of signal filtering in the insect's olfactory system. The olfaction mechanism in insects is summarized by [14] in **Figure 3**. In brief, a chemical signal crosses the sensillum lymph (SL) through a pore and then binds to highly specific binding proteins: pheromone binding proteins/

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*

moth pheromones [14].

*2.2.2 Kairomones*

as kairomones.

*2.2.3 Synomones*

termite species [24].

*2.2.4 Antimones*

*2.2.5 Apneumones*

oviposition [25].

as a deterrent emitted by insects against their predators as a defense mechanism. Granular trichomes which cover plant leaves and stems release herbivore-deterring allomones under stress conditions as a defense process. These allomones are toxic for the herbivorous insect pests, e.g., nicotine from a tobacco plant. Moreover, bolas spiders can deceit, lure, and capture male moths by synthesizing and mimicking

Kairomones (from a Greek word "kairos" = opportunistic or exploitative): emitted by one organism that stimulate a response in an individual of another species. The response is beneficial to the recipient, e.g., orientation of predaceous checkered beetles (Coleoptera, Cleridae) toward the aggregation pheromone of their prey and bark beetle (Coleoptera, Curculionidae, Scolytinae) [14, 21]. Kairomones may be allomones or pheromones depending on the circumstances. For example, American bolas spiders attract their prey (male moths) by releasing attractant allomones which serve as sex pheromones emitted by female moths. Also, exudates of warm-blooded animals that pull blood-sucking insects toward their hosts serve

Synomones: beneficial to both the releaser and receiver. Examples include scents used by flowers to attract pollinating insects. Moreover, herbivore-induced plant volatiles are considered to be active synomones which recruit natural enemies of insect pests toward the affected plants [22]. Also, synomones play an essential role in mate-finding communication. This role relies on the reduction of competition in the olfaction communication channel between closely related species with overlapping pheromone components. This advisable action is important in preventing exhaustion from the time and energy required for orientation toward heterospecifics [23]. In termites, hydroquinone is a phagostimulant compound secreted by labial glands distinguished as pheromones and synomones when different species are partaking the same foraging territory. It acts as a pheromone when recognized by nest mates of the same species and as a synomone when perceived by another

Antimones: maladaptive for both the releaser and receiver. These substances are produced or acquired by an organism that, when encountered by another individual of a different species in the natural environment, activate in the receiving individual a repellent response to the emitting and receiving individuals [1].

Apneumones (from a Greek word "a-pneum" = breathless or lifeless): emitted by a non-living source, causing a favorable behavioral or physiological reaction to a receiving organism, but harmful to other species that may be found either in or on the non-living material. Apneumones were suggested by [7]. Rare cases of these allelochemicals have been found later in the literature, e.g., hexanal and 2-methyl-2-butanol released from rabbit stools attract sandfly females for

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