Chemical communication paves the way for expanding biological control

Chemical compounds released by plants, insects, and microorganisms can guide parasitoids and predators in locating their prey and hosts. The accumulated knowledge about these signals creates prospects for new biological control strategies. However, practical application still faces obstacles. Most studies remain focused on compound identification, laboratory bioassays, and small-scale field experiments.

Chinese researchers analyzed papers published between 1971 and 2025 (doi 10.1127/entomologia/3525). The survey focused on the relationships between plants, herbivorous insects, and natural enemies. It also included studies on receptors involved in the chemical perception of parasitoids, ladybugs, lacewings, and other beneficial arthropods.

Herbivory-induced volatiles

One of the main groups of signals involves herbivory-induced plant volatiles, known by the acronym HIPVs. Insect feeding alters plant metabolism and induces the release of volatile organic compounds. Parasitoids and predators use some of these compounds during their search for a host. Among the substances studied are methyl salicylate, (Z)-3-hexenol, (Z)-3-hexenyl acetate, linalool, octanal, alpha-pinene, and indole.

The response depends on the species involved and the odor composition. In corn attacked by Spodoptera frugiperda, octanal and 2-heptanone attracted the parasitoid Cotesia ruficrus. The combination of the two compounds increased the parasitism of pest larvae in experiments. In another system, cis-jasmone and (Z)-3-hexenyl acetate attracted Campoletis chlorideae, a parasitoid associated with Helicoverpa armigera.

Isolates and mixtures

Mixtures can also produce distinct responses compared to the isolated components. The predatory beetle Stethorus punctum picipes showed a stronger response to a combination of methyl salicylate, (Z)-3-hexenol, and benzaldehyde. Scientists also highlight differences between species. The same compound can attract one natural enemy and repel another. This behavior makes it difficult to develop formulations for broad use.

The plant variety and the herbivore's identity also modify the signals emitted. Microplitis tuberculifer showed a preference for volatiles from Bt cotton attacked by Spodoptera exigua compared to non-Bt cotton. The same parasitoid responded to volatiles induced by Spodoptera exigua, but did not show an equivalent response to signals produced after an attack by Helicoverpa armigera. These results reinforce the need to evaluate each combination of crop, pest, and natural enemy.

Natural enemies

Natural enemies also exploit pheromones produced by the pests themselves. Sexual, alarm, and aggregation pheromones can function as kairomones. (E)-beta-farnesene, known as a component of the alarm pheromone of several aphid species, attracts different parasitoids, hoverflies, ladybugs, and predatory beetles. The response is not uniform. Chrysopa pallens, for example, showed strong repellency to the compound in one of the studies compiled by the researchers.

Feces and cuticular hydrocarbons provide further clues. Compounds present in larval feces can influence host location over short or long distances. The parasitoid Cotesia rubecula showed greater attraction to Pieris rapae feces compared to material from other species evaluated. Larval age also influenced the response. Hydrocarbons present in the cuticle offer additional signals for specific host recognition.

Action of microorganisms

Microorganisms expand this chemical network. Bacteria and yeasts present in nectar alter odors, sugars, amino acids, and other characteristics of the food resource. The parasitoid Aphidius ervi showed an innate preference for nectar fermented by the yeast Metschnikowia reukaufii. Studies have pointed to microbial effects on longevity, survival, and behavior of natural enemies. Researchers consider this component still poorly studied.

Advances in genomics and transcriptomics have also made it possible to map genes linked to chemoreception. These groups include odor receptors, taste receptors, ionotropic receptors, odorant-binding proteins, and chemosensory proteins. The number of these genes varies between species. In hymenopteran parasitoids, some lineages have hundreds of odor receptors.

Functional characterization is beginning to reveal the link between specific molecules and behavior. In Campoletis chlorideae, one receptor responds to cis-jasmone. In Hippodamia variegata, different receptors recognize compounds associated with plants and prey. In Eupeodes corollae, one receptor participates in the perception of (E)-beta-farnesene. Alteration of the Orco coreceptor in this species eliminated the preference for the compound and reduced parameters related to mating, oviposition, and hatching.

Known ligands

Despite molecular advances, many of the identified receptors still lack known ligands. Systematic evidence is also lacking regarding the neural mechanisms involved in encoding the senses of smell and taste in natural enemies.

Scientists have identified another bottleneck for agricultural use. Trials have already proven the attraction of natural enemies by various HIPVs under experimental conditions, and some studies have tested traps in the field. Even so, researchers have not recorded practical application of these volatiles to attract natural enemies for pest control in agricultural production. The field environment contains numerous odor sources. These compounds can compete with the attractant, inhibit the response, or produce synergies.

Researchers advocate for studies on combinations of HIPVs and pheromones. Evaluating mixtures could support strategies such as "push-pull," repelling pests in certain areas and attracting natural enemies in others. They also recommend expanding research on beneficial and harmful microorganisms and identifying the molecular mechanisms involved in the perception of fermentation products.

Identifying the ligands of chemosensory receptors completes this agenda. According to scientists, this knowledge can guide the development of more selective attractants for parasitoids and predators. Before adoption on an agricultural scale, however, it is necessary to understand the interactions within the complete ecological system, considering the plant, herbivore, natural enemy, and microbiota as a whole.