The study is the first to capture in situ VOC measurements from mountain birch across a naturally developing outbreak cycle, rather than relying on short-term experiments in which larvae are applied to individual branches. Between 2021 and 2023, the researchers sampled 256 independent trees along a 250-meter transect, enclosing branches in pre-cleaned plastic bags, circulating air through the enclosures with pumps, and trapping the outgoing VOCs on stainless steel adsorbent cartridges for later analysis by gas chromatography-mass spectrometry. Each tree was sampled only once, a strategy designed to maximize spatial coverage and capture the natural variability among individuals and microclimates across the forest.

The herbivore pressure itself told a striking story. In 2021, the average number of geometrid larvae on sampled branches was about two per meter of branch, a background level. By 2022, that figure had risen fivefold to roughly ten larvae per meter, and by 2023, the outbreak year, it had climbed twelvefold to around 24 larvae per meter, with individual branches hosting as many as 173. The winter moth, Operophtera brumata, dominated the larvae on sampled branches during the medium and outbreak years, accounting for roughly 90 percent, while the autumnal moth, Epirrita autumnata, contributed another 6 to 9 percent. Leaf area loss on measured branches averaged about 2.5 times higher in the outbreak year than in the background year, and between 80 and 100 percent of trees showed herbivory damage in every year of the study.

What the researchers found was far from a straightforward dose-response relationship. During peak herbivory, emissions of sesquiterpenes and oxygenated sesquiterpenes, two groups of terpenoid compounds with high potential to form secondary organic aerosols in the atmosphere, increased by roughly 6.8-fold from the medium herbivory year to the outbreak year, while monoterpene emissions rose fourfold. Nitrogen-containing VOCs showed the most dramatic response of all, rising from near-undetectable levels in the background year to a cumulative 67-fold increase by the outbreak year. Homoterpenes, compounds known to attract natural enemies of herbivores, increased 2.4-fold over the same period. Intriguingly, green leaf volatiles, the compounds released immediately upon physical tissue damage, remained similar across all three years during peak larval feeding.

Perhaps the most counterintuitive finding emerged when the researchers compared different phases of the outbreak year itself. Early in the 2023 growing season, before larval feeding reached its peak, emissions of sesquiterpenes were five times higher and oxygenated sesquiterpenes nine times higher than during the height of the outbreak. Nitrogen-containing VOCs and benzenoids showed even steeper declines, dropping 12-fold and 16-fold respectively from the pre-herbivory period to peak herbivory. This pattern suggests that the highest emissions of certain compound groups occurred not when the caterpillars were most voracious, but when leaves were just beginning to expand and larvae were feeding inside the buds.

Disentangling the roles of phenology and herbivory in this early-season surge required careful comparison. When the researchers compared pre-herbivory periods between the background year and the outbreak year, they found that emissions of many VOC groups were similar or even higher in 2021 than in 2023, suggesting that leaf development alone can be a stronger driver of early-season emissions than herbivore presence for most compound classes. However, the outbreak year’s early emissions were not purely a phenological phenomenon. Winter moth larvae feed inside leaf buds in early spring, and this internal feeding, combined with the physiological demands of leaf expansion and the gradual biochemical priming from rising larval abundance, likely shaped the distinctive early-season chemistry observed during the outbreak.

The composition of the volatile blends shifted dramatically through the outbreak season, as revealed by principal component analysis that cleanly separated the pre-, peak-, and post-herbivory periods along the first two principal components, which together explained 23 percent of the total variance. During the pre-herbivory period, the blend was characterized by higher relative proportions of benzyl nitrile, ethyl isocyanide, benzene, toluene, and sesquiterpenes such as beta-caryophyllene and alpha-copaene. By peak herbivory, the profile had diversified into a more complex mixture, with elevated proportions of the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene, indole, and methyl salicylate, all compounds associated with herbivore-induced defense signaling and the attraction of parasitoids and predators that attack the caterpillars.

This temporal restructuring of the volatile profile suggests a flexible and resource-efficient defense strategy on the part of the birch. The trees appear to emphasize certain nitrogen-containing compounds early in the season, possibly for deterrence, and others later, when indirect defense through enemy recruitment becomes most valuable. Indole, in particular, peaked during maximum larval abundance and is thought to play a key role in attracting natural enemies of geometrids in mountain birch systems. Oxygenated monoterpenes such as borneol, camphor, and linalool, normally present in the constitutive blend of unstressed birch, also rose in relative proportion during peak herbivory, blurring the line between baseline and induced emissions.

After the outbreak subsided, the forest told yet another chemical story. Severely defoliated birch trees initiated a second leaf flush, a compensatory response that restored photosynthetic capacity and restructured the emission profile. Twenty-four and thirty-six days after peak herbivory, emissions of monoterpenes had declined 18-fold and nitrogen-containing VOCs roughly 9-fold, indicating a downregulation of stress responses once the immediate threat had passed. Yet benzenoids, compounds involved in defense against pathogens and herbivores, increased fivefold during the post-herbivory period, likely protecting the newly emerged leaves from late-season pathogens such as birch rust. Emissions of sesquiterpenes, oxygenated sesquiterpenes, and green leaf volatiles did not differ significantly between the peak and post-herbivory periods, sustained by the fresh foliage of the second flush, though at levels far below the early-season maximum.

The implications of this work extend well beyond plant-insect ecology. Biogenic VOCs participate in atmospheric ozone chemistry and the formation of secondary organic aerosols, tiny particles that influence cloud formation and radiative forcing in the climate system. Terpenoids, particularly sesquiterpenes and monoterpenes, have higher aerosol-forming potential than constitutively emitted compounds, while green leaf volatiles can suppress secondary organic aerosol formation by depleting atmospheric hydroxyl radicals. Nitrogen-containing VOCs, which responded most dramatically to rising herbivore pressure in this study, are not currently represented in standard VOC emission models, meaning their atmospheric consequences remain entirely unassessed. As climate change is projected to increase the frequency, extent, and severity of insect outbreaks in high-latitude ecosystems, the dynamic interplay between leaf phenology, herbivore intensity, and volatile chemistry documented here provides essential input for models seeking to predict biosphere-atmosphere feedbacks in a rapidly warming Subarctic, where the chemical breath of the forest may shift in ways that ripple upward into the climate itself.

Subject of Research: How a three-year geometrid moth outbreak cycle shapes biogenic volatile organic compound emissions from mountain birch in subarctic Fennoscandia

Article Title: Insect outbreak progression over three years shapes biogenic volatile emissions in a mountain birch forest

Article References: Rieksta, J., Smart, A. S., Davie-Martin, C. L., Engroff, A., Picón García, R. M., Wali, R., Gao, Y., Jiao, Y., Zhang, W., Tang, J., Nyboe Laursen, S., Feng, S., Westergaard-Nielsen, A., Holst, T., Seco, R., Li, T., & Rinnan, R. (2026). Insect outbreak progression over three years shapes biogenic volatile emissions in a mountain birch forest. Communications Earth & Environment, 7(1), Article 816. https://doi.org/10.1038/s43247-026-03920-2

Image Credits: AI Generated

DOI: 10.1038/s43247-026-03920-2

Keywords: biogenic volatile organic compounds, mountain birch, geometrid moth outbreak, herbivory, subarctic forest, sesquiterpenes, monoterpenes, nitrogen-containing VOCs, plant defense, leaf phenology, secondary organic aerosol, climate change