What are the Classifications and Functions of Insect Pheromones?

At present, there are two main ways to classify insect pheromones. One is based on the mechanism of action, which is divided into release pheromones and initiation pheromones. The former can quickly regulate insect behavior, with a short response time and rapid reaction, while the latter can regulate insect physiological changes, with a long action period and far-reaching impact; the other is based on the function, which is divided into sex pheromones, aggregation or cluster pheromones, alarm pheromones and marking pheromones.

Classification by Mechanism of Action

Release pheromones: Release pheromones can quickly regulate the behavior of the receiver, and most alarm pheromone substances belong to release pheromones. 2-ethyl-3,6-dimethylpyrazine released by the red fire ant Solenopsis invicta, isoamyl acetate secreted by Italian honey bees and Chinese honey bees, and E-β-afarnesene released by Rhopalosiphum padi and Aphis nerri are all alarm pheromone substances, which can quickly cause alarm responses and behaviors of their groups in dangerous situations, and even have a stimulating effect on insects of other species. The group pheromone phenylacetonitrile released by the desert locust Schistocerca gregaria is also a release pheromone. At low concentrations, it can cause male locusts to gather, and at high concentrations, it can cause male locusts to avoid. (Z,Z)-dodeca-3,6-dien-1-ol released by Ancistrotermes pakis-tanicus is also a release pheromone. It can cause worker ants to release accompanying signals, and at high concentrations, it can also serve as a sexual attraction signal for male ants. After receiving the release pheromone, the individual insect quickly exhibits unique behavior, and when the pheromone signal disappears, the individual insect resumes its original behavior.

Start pheromone: The role of start pheromone is relatively complex. It causes physiological and behavioral changes in the receiver by affecting the receiver's gene expression and material metabolism, and plays an important role in insect physiology and social organization. Because this type of pheromone has a long action cycle and is difficult to study, it has been less studied. The most representative one is the pheromone secreted by social insect queen bees and queen ants. Both Chinese honey bees and Italian honey bees use QMP to maintain the stability of the colony structure. After receiving QMP, the worker bees in the colony have their ovarian development inhibited and lose their normal egg-laying behavior. On the other hand, with the assistance of larval pheromones, the worker bees regulate their feeding behavior and division of labor. The activation pheromone regulates the physiological changes of insects for a long period of time, and this regulation is generally irreversible. After the pheromone disappears, some of the traits that have been formed in the insects will not disappear.

At the same time, it acts as a release pheromone and a starter pheromone; in insects, some substances can act as release pheromones and starter pheromones at the same time. For example, the pheromones released by Italian honey bee larvae include larval ester pheromones and ocimene substances (β-ocimene and allo-ocimene). Both types of substances can act as release pheromones and starter pheromones; the lipid pheromones secreted by the dorsal plate of the Cape honey bee Ap. mellifera capensis worker bees act as release pheromones to stimulate the accompanying behavior of worker bees, and as starter pheromones to inhibit ovarian activity.

Classification by Function

Sex pheromone: Sex pheromone is a chemical substance secreted by male and female insects and received by the opposite sex to induce sexual behavior. It is the earliest pheromone to be studied. Most insect sex pheromones are secreted by female individuals, but some species also secrete males, such as Drosophila melanogaster and Bombus terrestris. Relatively speaking, the amount of pheromones secreted by male individuals is higher than that of females. A male individual can release pheromones at the microgram level, which is conducive to attracting more female individuals. Sex pheromones affect the mating and reproduction of insects, so studying them is of great significance for pest control. Lepidoptera insects are the main pests in agriculture and forestry, so the research on Lepidoptera sex pheromones is relatively extensive. Insect sex pheromones are divided into type I, type II, type III and type O according to their structure. Type I is a straight-chain acetate, alcohol or aldehyde with 10 to 18 carbon atoms. Among the known moth sex pheromones, 75% belong to type I; type II is a straight-chain polyene with 17 to 23 carbon atoms and its epoxy derivatives; type III is an alkane with 17 to 23 carbon atoms containing methyl branches, and type O is a short-chain secondary alcohol or ketone compound. The latter two types of pheromones are relatively rare and are mainly found in Lepidoptera insects.

Aggregation or cluster pheromones: Aggregation or cluster pheromones are a type of chemical substance used by individuals to attract other individuals of the same species to a specific location, which can promote behaviors such as habitat, feeding or mating of the same species. There are two main differences between aggregation pheromones and sex pheromones: First, sex pheromones can only be produced by sexually mature male and female adults, and female individuals secrete relatively more in insects, while aggregation pheromones can be produced in different stages such as male and female adults, nymphs and larvae, and male individuals produce more; second, sex pheromones only attract the opposite sex for mating, while aggregation pheromones can attract both the same sex and the opposite sex at the same time, and in addition to mating, they can also assist the population in completing behaviors such as habitat and feeding. For example, Pityogenes bidentatus and Pi.quadridens are sister species, and the aggregation pheromones secreted by male individuals can not only avoid the interference of interspecific individuals in mating, but also attract their populations to gather to occupy food and space resources.

The species that has studied aggregation pheromones the most are Coleoptera, especially the forestry pests, with as many as 17 genera and 65 species studied. Studies have found that their aggregation pheromones have two characteristics: conservatism and variability, that is, some components are very conservative between species, while some components vary greatly even in closely related species. Another characteristic of aggregation pheromones is the bidirectionality of regulation, that is, the function of high concentration and low impact. For example, the male beetle Den‐droctonus frontalis in the southern United States produces an endogenous beetle protein that can promote the aggregation of individuals of the same species at low concentrations, but avoid the aggregation of individuals of the same species at high concentrations. Some species can also produce anti-aggregation pheromones after the population reaches a certain level to promote individuals within the species to find new hosts or food sources and reduce intraspecific competition. This bidirectional regulatory mechanism can effectively control the population density and maximize the interests of the group.

Alarm pheromone: Alarm pheromone is a key bug protection. Not only can it warn members of the same species about environmental threats, it can train the same species to strike invaders. These days, alarm pheromones of bees, Hymenoptera ants and Hemipteran aphids like Acyrthosiphon pisum have received more attention. Alarm pheromones have two main differences with other pheromones: Alarm pheromones are not very specific compared with other pheromones. For instance, alarm pheromones (isoamyl acetate, octyl acetate, benzyl acetate) in the genus Apis; in most species of the superfamily Aphidoidea, (E)--farnesene is the most prevalent alarm pheromone, and it's sufficient to remind the same species to pause feeding and escape when danger is imminent. Second, alarm pheromones are easy for natural enemies to acquire – ie, insects using alarm pheromones to warn other members of the same species, and they will invite more natural enemies to appear. Thanks to heptanal and limonene, the beehive beetle can pin down the location of the hive. The primary alarm pheromones in insects are made and released by three glands, the maxillary, anal and venom. For instance, when bees sting someone with their maxilla and tail, they release alarm pheromones to signify. Such pheromones may even lead other bees to the suspect.

Marking pheromones: Marking pheromones are also called egg-laying avoidance pheromones. They are chemical substances that insects make marks at the egg-laying site to prevent other individuals of the same species from laying eggs again. If sex pheromones regulate insect mating and egg-laying behavior, then marking pheromones directly affect the nutrition and health level of offspring, especially in closely related or sister species. The ability of insects to recognize marking pheromones directly determines the species' ability to compete for the same host resources. Since marking pheromones take a long time to work, their volatility is lower than that of other types of pheromones, and the receiver can generally only feel them through contact. After laying eggs, insects release marking pheromones in two places: one is to mark directly on the eggs, such as the parasitic wasp Dinar-mus basalis will mark the surface of the eggs with chemicals after laying eggs; the other is to mark on the host plant. Compared with marking on eggs, marking on host plants is safer because marking on eggs may attract predators. At present, only some marking pheromones have been isolated and identified in Tephritidae insects. Compared with other pheromones, less is known about marking pheromones, but they have great potential in fruit protection and other aspects.

Other insect pheromones: In addition to the above pheromones that are commonly found in different insects, there are also some functional pheromones that are only found in a few species. For example, Mastotermes darwiniensis will secrete substances including hydroquinone on the surface of food, which can stimulate other individuals of the same species to gnaw at the same location. Such substances are called attractant pheromones. However, hydroquinone does not have an attractant effect in other termites such as Formosan subterranean, which shows the specificity of its function. When ants go out to forage, in order to help other individuals determine the walking position, they will secrete derivatives of isocoumarin on the route of advancement. This substance is called tracking pheromone.

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