PHEROMONES IN MONITORING SPRUCE BARK BEETLE POPULATIONS: SCENTS AS GUIDES

Semiochemicals in communication

Semiochemicals are chemical substances produced by organisms such as plants and insects for communication. They are divided into interspecies signaling compounds, known as allelochemicals, and intraspecies signaling compounds, known as pheromones. Pheromones play a central role in communication within a species. They are used to transmit information and coordinate complex tasks between individuals. For example, sex pheromones, aggregation pheromones, and alarm pheromones all contribute to intraspecies communication. For example, some pheromones are specifically used to locate mating partners.

The European spruce bark beetle locates a weakened tree based on its scent

A good example is the swarming behavior of the European spruce bark beetle: the beetle locates a weakened tree based on its scent. When the first males become active in spring, they settle on a suitable tree trunk and begin releasing pheromones into their surroundings. Females follow these scent cues. Once egg-laying is underway, individuals start producing pheromones that signal to other individuals that the tree is already sufficiently colonized. As a result, beetles continue to attack other trees.

The “Push and Pull” method

Pheromones are utilized in pest control and population monitoring, often as part of integrated pest management (IPM). In the case of the spruce bark beetle, pheromone traps can be used to monitor population levels and swarming periods, although they do not significantly reduce the overall population. For instance, the Natural Resources Institute Finland conducts annual nationwide monitoring of bark beetle populations using pheromone trapping.

In Central Europe, the damage caused by spruce bark beetles has been very high in recent years. Research has therefore focused on finding practical ways to prevent the spread of damage. One such approach is the “push and pull” method. This involves managing pest insect behavior by manipulating the olfactory environment: making a resource unattractive or unsuitable through repellent odors (“push”) while simultaneously attracting insects to traps using specific scents (“pull”). Repellents may include odors produced by other species that make the host tree less appealing. These so-called “non-host volatiles” include scents from birch or other deciduous trees in the case of the spruce bark beetle. Anti-aggregation pheromone (the opposite of aggregation pheromones) can also be used.

Could this method be applied on a small scale to protect valuable spruce trees or forest edges from bark beetle damage? Results from Central Europe are promising, although the effects are localized and the identification of the most effective scent compounds is still ongoing.

References

Jakuš, R., Trubin, A., Singh, V. V., Zabihi, K., Jirošová, A., Modlinger, R., … & Schlyter, F. (2024). Spruce Protection against Ips typographus with Anti-Attractant Blend of Tree-Based Semiochemicals: From Small Experimental Plots to Stand Scales. Forests, 15(2), 356.

Lindmark, M., Wallin, E. A., & Jonsson, B. G. (2022). Protecting forest edges using trap logs–Limited effects of associated push-pull strategies targeting Ips typographus. Forest Ecology and Management, 505, 119886.

The Natural Resource Data, Map service, Forest Damage. Luonnonvarakeskus. https://luonnonvaratieto.luke.fi/kartat?panel=metsatuhot

Reddy, G. V., & Guerrero, A. (2010). New pheromones and insect control strategies. Vitamins & hormones, 83, 493-519.