Chemical signals broaden options for biological control

The combined use of chemical signals from plants and insects can increase the efficiency of biological control in crops, orchards, vegetable gardens, and protected cultivation. A study by Chinese researchers brings together results on volatile compounds, pheromones, kairomones, push-pull systems, habitat management, and plant breeding. However, the scientists warn of a key difference: attracting predators and parasitoids does not guarantee pest reduction, less damage, or increased productivity (doi 10.3390/insects17080808).

Plants attacked by herbivorous insects release volatile organic compounds. This group includes herbivory-induced volatiles, known by the acronym HIPVs. Terpenoids, green leaf volatiles, and methyl salicylate are part of this group. Predators and parasitoids use these signals to locate infested plants.

Agricultural application

The agricultural application of these compounds can promote conservative biological control. Methyl salicylate is among the most studied attractants. Methyl jasmonate can also induce the release of defensive volatiles by plants. The effect varies according to the compound, dose, formulation, release rate, crop, and environment.

The integration of long- and short-range signals represents one of the research fronts. HIPVs can guide parasitoids to the infested area. Cuticular hydrocarbons left by insects on leaves help in locating the host nearby. Female Trissolcus japonicus, for example, intensified foraging on surfaces with chemical traces associated with Halyomorpha halys.

Mixtures with pheromones

Mixtures with pheromones also showed results. In wheat, the combination of the alarm pheromone (E)-beta-farnesene with methyl salicylate increased parasitism and reduced the presence of the aphid Sitobion miscanthi. In tomato, the combined use of methyl salicylate and the aggregation pheromone of the predatory stink bug Podisus maculiventris increased the persistence of the natural enemy and reduced the infestation of Manduca sexta.

The selection of mixtures requires caution. Natural enemies may respond to specific proportions of compounds. The indiscriminate addition of volatiles can mask signals, reduce attraction, or cause repellency. Some attractants also recruit the pest itself. Trials with bell peppers showed greater thrips capture in traps with methyl salicylate. The result reinforces the need to evaluate the balance between recruitment of natural enemies and the risk of increased crop colonization.

The "attract and reward" strategy links volatile dispensers to food resources and shelter. Flowering strips, nectar, pollen, alternative prey, and oviposition sites can prolong the presence of predators and parasitoids. Studies with buckwheat, *Fagopyrum esculentum*, have recorded an increase in parasitoids in brassicas, sweet corn, grapevines, and broccoli. In apple orchards, the combination of methyl salicylate with *Calendula officinalis* increased the presence of *Propylea japonica* and favored the suppression of *Aphis citricola*.

Agent ecology

The type of resource needs to match the ecology of the control agent. Nectar and pollen benefit adult parasitoids and omnivorous predators. Strictly carnivorous predators depend more on alternative prey, shelter, and suitable micro-habitats. Guttation can also provide food. In blueberries, droplets remained available throughout the day and during the growing season. Pesticide residues or toxic metabolites, however, can turn this resource into a risk.

Push-pull systems are another application. The technique uses repellent stimuli to drive the pest away from the main crop and attractive stimuli to concentrate it on trap plants or baited traps. In corn, elephant grass, Pennisetum purpureum, acts as an attractant. Desmodium, Desmodium uncinatum, exerts a repellent effect on borers such as Eldana saccharina, Chilo partellus, Sesamia calamistis, and Busseola fusca. Compounds released by companion plants can also attract parasitoids.

The environment modifies the emission and stability of signals. Drought, increased temperature, elevated carbon dioxide, ozone, nutrient deficiency, and air pollution can alter the composition of volatiles. Ozone can degrade terpenoids used by parasitoids. Water deficit can also reduce the attractiveness of infested plants.

The study proposes four priorities: improving the induction of plant defenses; combining HIPVs with pheromones and other signals from natural enemies; integrating habitat management with food resources; and incorporating defensive traits through breeding or genetic engineering. The researchers recommend field assessments with a complete sequence of indicators: arrival and persistence of natural enemies, predation or parasitism, pest density, damage, productivity, pesticide use, and economic return.