How do Insect Pheromones Spread?

Insects release pheromones into the environment, and individuals can only respond after receiving them through their own sensory systems. For non-volatile pheromones, they can only be perceived through contact, and they can only function with the assistance of volatile pheromones. The transmission and perception process of volatile pheromones is extremely complex, requiring not only suitable external conditions, but also the participation of multiple proteins in the insect's own chemical sensory system. The specific process is as follows: the odor or pheromone molecules in the environment diffuse into the specialized receptors of insects (antennae, feet, etc.), and with the assistance of the receptor olfactory binding proteins, they pass through the hemolymph to the olfactory receptors on the membrane of peripheral nerve dendrites. The signal molecules interact with the olfactory receptors, and the chemical signals are converted into electrical signals to stimulate the excitation of dendritic nerves; the electrical signals of dendritic nerves are processed by the olfactory nerve lobes and transmitted to the central nervous system.

The key part of pheromone recognition is the transmembrane receptors on the olfactory receptor neurons. For example, the receptor Bm OR-1 specifically expressed in the antennae of male silkworms can sense the female adult sex pheromone Bombyx mori; the aggregation pheromone component 11-cis-vaccenyl acetate of Drosophila melanogaster is specifically recognized by Or67d; the receptor for the queen pheromone 9-ODA in Italian honey bees is Or11; the Or41 gene highly expressed in the antennae of Plutella xylostella responds to both pheromones Z9-14∶AC and Z9-14∶OH. In noctuids, male individuals have evolved some special receptors to sense female pheromones, and these receptors are closely related in evolution. For example, the male adult receptor Slit OR5 of Spodoptera littoralis can specifically recognize the sex pheromone (Z,E)-9,11-tetradecadien-1-ol acetate.

After the pheromone activates the receptor, the chemical signal is converted into an electrical signal and transmitted sequentially in the insect olfactory system. This reaction is very fast, and the whole process can be completed within milliseconds. The specific process is as follows: the electrical signal of OSN will be directly transmitted to the primary center of insect olfaction - the antennal (olfactory) nerve lobe. The antennal nerve lobe contains a large number of olfactory glomeruli. In addition to the co-receptors, the neurons in an olfactory glomerulus generally only express one olfactory receptor, so that the same signal can be processed. For example, there are 57 olfactory receptor genes in the Drosophila melanogaster genome, of which 32 are only expressed in the antennae. In addition to the co-receptor Or83b, which is expressed in all neurons, other receptors are only expressed in a single neuron. The signal processed by the antennal nerve lobe is then transmitted to the central nervous system - the mushroom body and the lateral horn. The lateral horn area has an area dedicated to processing pheromone stimulation. After receiving the pheromone, the insect will respond behaviorally and physiologically.

The above are all mechanisms for insects to recognize exogenous signals under the stimulation of a single pheromone substance, which is relatively easy to achieve. However, the natural environment is very complex, and there are various odors and pheromone molecules around insects. How do insects recognize specific pheromones in a complex environment? The theory of rapid stimulus dynamics can explain how insects can quickly and accurately identify different signals. In the study of Drosophila melanogaster, it was found that the concentration range of neuronal receptors that sense chemical stimulation is very wide, but different receptors tend to receive stimulation from specific substances, and the pulse time can reach 0.1 ms, so that different receptors can be used to quickly alternately receive different exogenous signals. Although there are relatively few related studies and many theories are still imperfect, it is certain that different insects are very sensitive to pheromone signals and the signal transmission speed is also very fast.

Insects are not only capable of identifying specific pheromone components in complex environments, but even some chemical substances from plants play an important role in insect pheromone perception. First, after insects receive and capture chemical substances from plants, they can use them as pheromones or pheromone precursors. For example, the larvae of the beautiful star moth Utetheisa ornatrix feed on the host plant Crotalaria spectabilis, accumulating a large amount of toxins-pyrrolizidine alkaloids in their bodies. When the larvae develop into adults, the male adults pass the pyrrolizidine alkaloids to the female adults through mating and leave them on the eggs, so that the eggs can be protected from being eaten by natural enemies under the protection of the toxins. Second, some plant-derived chemicals can stimulate insects to produce and release pheromones, and synergistically enhance the function of insect pheromones. For example, when linalool, a volatile substance of Cucurbita pepo pepe, is mixed into the aggregation pheromone of the male striped melon leaf beetle Acalymma vittatum, it can enhance the aggregation behavior of the group; Grapholita molesta, a pear borer, mainly harms Rosaceae plants. When pheromone traps are used for prevention and control, the volatile substances (E)-β-ocimene and (E)-β-farnesene of Rosaceae plants can significantly enhance the attraction effect of pheromones. Third, some plant-derived chemicals can inhibit insects from recognizing and responding to pheromones. For example, myrcene can reduce the attraction of Thanasimus dubius fronta-lis to sex pheromones. Under the influence of long-term evolution and ecological environment, insects have not only formed a complete perception mechanism for population pheromones, but also a complete adaptation mechanism.

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