**2. Insect-insect and plant-insect interactions**

In insects, the interaction of chemical signals can either stimulate or inhibit the behavior of the pest and so change its response. The response of insects to plant volatiles differs and is either attractive (adapted herbivore) or repellent (non-adapted herbivore). The classification of plant volatiles as attractants and repellents is not standardized due to fluctuation of insect behavior responses to such volatiles depending on their concentration. Herbivorous insects develop host plant compounds and use them as sex pheromone precursors or sex pheromones [9]. For example, male orchid bees assemble terpenoid mixtures from orchids and transfer them as an aggregation pheromone to stimulate leakage in mating [10]. Furthermore, moths, butterflies, grasshoppers, beetles, and aphids utilize pyrrolizidine alkaloids as feeding deterrents against their parasites and/or predators [11]. The interactions which occur between different organisms are divided into two main categories, intraspecific and interspecific, depending on how the interactions occur. An intraspecific communication passes between individuals of the same species, while an interspecific communication involves an interaction between members of different species. Based on the communication signal and subsequently the relation between the receiver and the emitter, semiochemicals are classified into two main functional groups: pheromones and allelochemicals [1].

#### **2.1 Pheromones**

Pheromones are defined as species-specific chemical signals which enable communication between life-forms of the same species. Pheromones are secreted by insects which caused a specific reaction, for example, either a definite behavior (immediate effect on the behavior of the receiver) which is called a releaser pheromone or a developmental process (physiological effects on the receiver) which is called a primer pheromone [12]. Pheromones have been classified into eight various types: aggregation pheromones, alarm pheromones, oviposition-deterrent pheromones, home recognition pheromones, sex pheromones, trail pheromones, recruitment pheromones, and royal pheromones. Primer pheromones stimulate the olfactory sensory neurons that emit signals to the insect's brain which stimulate hormones released by the endocrine system [13]. Caste determination in social insects (bees, wasps, ants, termites, locusts) resembles the most famous example for primer pheromone in **Figure 1** [14]. Releaser pheromones are divided by function into sex pheromones, trail pheromones, alarm pheromones, etc. Sex pheromone is the most commonly known which species specific that attract opposite sexes for mating is highly. Concerning trail pheromones, these are commonly known in social insects for orientation and also for recruit nest mates toward a suitable food source. For example, ants and termites deposit these pheromones as they navigate their territory, thus promoting extensive nets of chemical routes [15].

**19**

killing of the host tree [14, 19].

**2.2 Allelochemicals**

**Figure 1.**

*lettering) [14].*

to [1, 20] as follows.

*2.2.1 Allomones*

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

On the other hand, bees release airborne orientation pheromones including forage marking, nest entrance finding, and swarming from the Nasonov gland. These pheromones are composed of mixtures of geraniol, farnesol, citral, and other minor compounds [16]. Alarm pheromones are well-developed pheromones in social insects for defensive function and are composed of multicomponent volatiles as mono- and sesquiterpenes and acetates [16–18]. The aggregation pheromones attract conspecifics of both sexes, e.g., bark beetles. The beetles start digging up into the bark of the host tree, thus releasing a mixture of terpenoids which are long-range aggregation pheromones that synthesized de novo, and others produced terpenoids via gut symbiotic bacteria or sequestered from the host tree. Depending on evoke aggregation pheromones, a great number of beetles attack, leading to

*Schematic profile drawings for exocrine glands of some social insects with a pheromonal function (capital* 

The second subclass of semiochemicals is allelochemicals which includes

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

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

substances that transmit chemical messages between different species.

**Figure 1.**

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

push-pull strategies [1].

**2. Insect-insect and plant-insect interactions**

groups: pheromones and allelochemicals [1].

**2.1 Pheromones**

or physiological response between members of the same or different species. They are mainly used in plant-insect or insect-insect interactions as alternative or complementary components to insecticide approaches in different integrated pest management (IPM) strategies. Such compounds are mainly affecting the behavior of various insect pests via chemical signals which occur between insect and insect or plant and insect. Semiochemicals considered a promising component in IPM programs for controlling insect pests. They are involved in different control strategies such as monitoring, mass trapping, mating disruption, and attract-and-kill and

In insects, the interaction of chemical signals can either stimulate or inhibit the behavior of the pest and so change its response. The response of insects to plant volatiles differs and is either attractive (adapted herbivore) or repellent (non-adapted herbivore). The classification of plant volatiles as attractants and repellents is not standardized due to fluctuation of insect behavior responses to such volatiles depending on their concentration. Herbivorous insects develop host plant compounds and use them as sex pheromone precursors or sex pheromones [9]. For example, male orchid bees assemble terpenoid mixtures from orchids and transfer them as an aggregation pheromone to stimulate leakage in mating [10]. Furthermore, moths, butterflies, grasshoppers, beetles, and aphids utilize pyrrolizidine alkaloids as feeding deterrents against their parasites and/or predators [11]. The interactions which occur between different organisms are divided into two main categories, intraspecific and interspecific, depending on how the interactions occur. An intraspecific communication passes between individuals of the same species, while an interspecific communication involves an interaction between members of different species. Based on the communication signal and subsequently the relation between the receiver and the emitter, semiochemicals are classified into two main functional

Pheromones are defined as species-specific chemical signals which enable communication between life-forms of the same species. Pheromones are secreted by insects which caused a specific reaction, for example, either a definite behavior (immediate effect on the behavior of the receiver) which is called a releaser pheromone or a developmental process (physiological effects on the receiver) which is called a primer pheromone [12]. Pheromones have been classified into eight various types: aggregation pheromones, alarm pheromones, oviposition-deterrent pheromones, home recognition pheromones, sex pheromones, trail pheromones, recruitment pheromones, and royal pheromones. Primer pheromones stimulate the olfactory sensory neurons that emit signals to the insect's brain which stimulate hormones released by the endocrine system [13]. Caste determination in social insects (bees, wasps, ants, termites, locusts) resembles the most famous example for primer pheromone in **Figure 1** [14]. Releaser pheromones are divided by function into sex pheromones, trail pheromones, alarm pheromones, etc. Sex pheromone is the most commonly known which species specific that attract opposite sexes for mating is highly. Concerning trail pheromones, these are commonly known in social insects for orientation and also for recruit nest mates toward a suitable food source. For example, ants and termites deposit these pheromones as they navigate their territory, thus promoting extensive nets of chemical routes [15].

**18**

*Schematic profile drawings for exocrine glands of some social insects with a pheromonal function (capital lettering) [14].*

On the other hand, bees release airborne orientation pheromones including forage marking, nest entrance finding, and swarming from the Nasonov gland. These pheromones are composed of mixtures of geraniol, farnesol, citral, and other minor compounds [16]. Alarm pheromones are well-developed pheromones in social insects for defensive function and are composed of multicomponent volatiles as mono- and sesquiterpenes and acetates [16–18]. The aggregation pheromones attract conspecifics of both sexes, e.g., bark beetles. The beetles start digging up into the bark of the host tree, thus releasing a mixture of terpenoids which are long-range aggregation pheromones that synthesized de novo, and others produced terpenoids via gut symbiotic bacteria or sequestered from the host tree. Depending on evoke aggregation pheromones, a great number of beetles attack, leading to killing of the host tree [14, 19].
