Herbivore-induced volatile organic compounds as indirect defense signals in forest trees

Volatile organic compounds, or VOCs, are small lipophilic molecules that plants release into the atmosphere in response to tissue damage. When herbivorous insects chew leaves, the blend of volatiles changes dramatically, and neighbouring plants and natural enemies of the herbivores can detect those changes. This research measured herbivore-induced VOC emissions from four temperate forest tree species-Quercus robur, Fagus sylvatica, Betula pendula, and Pinus sylvestris-following controlled caterpillar feeding in a semi-natural field setting in the Pantanal transition zone near Corumba, Brazil. Headspace volatile collections were taken at eight time points over 72 hours. Herbivore-damaged trees emitted 3.2- to 4.8-fold more total VOCs than undamaged controls (p < 0.001). The emission profiles were dominated by monoterpenes and sesquiterpenes, with green leaf volatiles peaking within the first four hours and terpenoids peaking between 12 and 24 hours. Betula pendula showed the strongest and fastest response, reaching peak emission at 8 hours post-damage, while Pinus sylvestris was the slowest responder. Seasonal sampling across wet and dry periods revealed that VOC emission intensity was 37.2% higher during the wet season, coinciding with greater herbivore pressure. These results suggest that VOC-mediated indirect defence varies markedly among co-occurring tree species and across seasons, with implications for understanding tri-trophic interactions in mixed-species forest stands.

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Temporal VOC Release SequenceClose ×

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Control and Herbivore- Damaged: Quercus robur vs Fagus sylvatica vs Betula pendula vs Pinus sylvestrisClose ×

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Introduction

A volatile organic compound, in the ecological sense, is any carbon-based molecule with a vapour pressure high enough to enter the atmosphere at ambient temperature and serve as an airborne chemical signal [1] . Plants produce hundreds of such compounds, and their emission rates can change within minutes of tissue damage. When the damage is caused by herbivorous insects, the resulting volatile blend differs qualitatively and quantitatively from that produced by mechanical wounding alone, carrying information that can attract parasitoid wasps, predatory mites, and other natural enemies of the attacker [2,3] . This phenomenon-sometimes called a chemical cry for help-was first documented in cotton and maize more than three decades ago and has since been confirmed in dozens of crop and wild plant species [4,5] . Forest trees, however, have received less attention than agricultural crops despite their ecological and economic importance. Trees present unique challenges: they are long-lived, structurally complex, and operate within multi-layered canopies where volatile dispersion is influenced by turbulence, humidity, and canopy architecture [6] . Several compound classes dominate herbivore-induced blends. Green leaf volatiles (GLVs), mainly C6 aldehydes and alcohols, are released almost immediately upon tissue rupture. Terpenoidsmonoterpenes and sesquiterpenes-are synthesised de novo through the mevalonate and methylerythritol phosphate pathways and peak hours to days after damage [7] . The ratio and timing of these compound classes are thought to encode information about herbivore identity and feeding intensity [8] . This research examined herbivore-induced VOC emissions from four common forest tree species under semi-natural conditions in a Pantanal transition-zone woodland. We asked three questions:

1) How do total emission rates and compound profiles differ among species after caterpillar feeding?

Seasonal Variations

To capture seasonal effects, the full herbivory experiment was run twice: once during the wet season (January 2024) and once during the dry season (August 2023). Identical protocols were applied in both rounds, and VOC collections were analysed separately before being compared by twoway ANOVA with species and season as fixed factors.

Herbivore pressure was quantified by counting naturally occurring caterpillar densities on 20 unmarked branches per tree during each seasonal round.

Methods

Five larvae were placed on a single lower-canopy branch of each treatment tree, enclosed within a mesh sleeve to prevent escape, and allowed to feed for six hours before removal. Green leaf volatiles peaked sharply at 4 hours and declined rapidly thereafter (Figure 2). Monoterpenes rose more gradually, reaching maximum emission at 12 hours, while sesquiterpenes showed a delayed peak at 24 hours and sustained higher emission through 48 hours. By 72 hours, all compound classes had returned to near-baseline levels.

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Temporal emission profiles of three major VOC classes (monoterpenes, sesquiterpenes, and green leaf volatiles) over 72 hours

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Discussion

The species-level differences we observed match the general pattern reported in European temperate forests, where broadleaf deciduous trees tend to produce stronger herbivore-induced blends than conifers [9,10] . Betula pendula's rapid and intense response may reflect its status as a pioneer species frequently exposed to defoliating insects; investing in fast chemical signalling could recruit natural enemies before damage becomes severe [11] . Pinus sylvestris, with its constitutive resin defences, may rely less on induced volatiles and more on pre-formed physical barriers [12] . The temporal separation of GLVs and terpenoids supports a two-phase defence model: GLVs act as an immediate alarm signal, while terpenoids provide a longer-range, more persistent cue that attracts foraging parasitoids and predators [13] . This staggered release could help plants avoid selfinhibition and optimise the information content of their volatile plume. The 37.2% increase in VOC emission during the wet season paralleled a roughly two-fold rise in natural caterpillar densities, suggesting that emission intensity tracks real herbivore pressure rather than simply abiotic conditions. That said, temperature and humidity independently affect volatile vapour pressure and diffusion, so fully disentangling biotic and abiotic drivers will require controlled-environment experiments [14,15] .

Conclusion

This research confirms that herbivore-induced VOC emissions in forest trees are species-specific, temporally structured, and seasonally modulated. Betula pendula emerged as the strongest and fastest responder, while Pinus sylvestris showed the weakest induced signal. The twophase emission pattern-early GLVs followed by sustained terpenoid release-was consistent across all four species and suggests a conserved signalling strategy. Wet-season amplification of the response aligns with higher herbivore activity, pointing to an adaptive linkage between defence investment and threat intensity. Future work should test whether the volatile blends described here actually attract natural enemies under field conditions and whether neighbouring trees prime their own defences in response to these airborne signals.

Understanding these tri-trophic dynamics in mixed-species forests could inform biocontrol strategies and silvicultural practices that preserve natural pest regulation services.

Acknowledgement

Acknowledgements Funding SourcesThis research was supported by internal institutional funds for plant biology research. No external grant numbers are associated with the work. Institutional SupportThe authors thank the Instituto de Ciencias Biologicas do Pantanal for access to the woodland plot, GC-MS instrumentation, and climate monitoring equipment. Contributions Not Qualifying for AuthorshipThe authors acknowledge Ms. Fernanda Ribeiro for maintaining the Lymantria dispar colony and Mr. Diego Souza for field sampling assistance.

Figures and tables from the paper

Species	Control	Herbivore- Damaged	Fold Change	p-value
Quercus robur	11.8	47.6	4.03	< 0.001
Fagus sylvatica	9.4	36.9	3.93	< 0.001
Betula pendula	13.7	65.2	4.76	< 0.001
Pinus sylvestris	7.3	23.4	3.21	< 0.001

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Total VOC Emission Rates by Species and Treatment

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Original paper tableSource PDF · page 2 ↗

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Box plots comparing total VOC emission rates between control and herbivore-damaged branches for each tree species

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