Exogenous application of methyl jasmonate to Ficus hahliana attracts predators of insects along an altitudinal gradient in Papua New Guinea

Simulated Herbivory Affects the Volatile Emissions of Oak Saplings, while Neighbourhood Affects Flavan-3-ols Content of Their Leaves

To what extent particular plant defences against herbivorous insects are constitutive or inducible will depend on the costs and benefits in their neighbourhood. Some defensive chemicals in leaves are thought to be costly and hard to produce rapidly, while others, including volatile organic compounds that attract natural enemies, might be cheaper and can be released rapidly. When surrounding tree species are more closely related, trees can face an increased abundance of both specialist herbivores and their parasitoids, potentially increasing the benefits of constitutive and inducible defences. To test if oaks (Quercus robur) respond more to herbivore attacks with volatile emission than with changes in leaf phenolic chemistry and carbon to nitrogen ratio (C: N), and whether oaks respond to the neighbouring tree species, we performed an experiment in a forest in Poland. Oak saplings were placed in neighbourhoods dominated by oak, beech, or pine trees, and half of them were treated with the phytohormone methyl jasmonate (elicitor of anti-herbivore responses). Oaks responded to the treatment by emitting a different volatile blend within 24 h, while leaf phenolic chemistry and C: N remained largely unaffected after 16 days and multiple treatments. Leaf phenolics were subtly affected by the neighbouring trees with elevated flavan-3-ols concentrations in pine-dominated plots. Our results suggest that these oaks rely on phenols as a constitutive defence and when attacked emit volatiles to attract natural enemies. Further studies might determine if the small effect of the neighbourhood on leaf phenolics is a response to different levels of shading, or if oaks use volatile cues to assess the composition of their neighbourhood.

Exogenous Application of Methyl Jasmonate Increases Emissions of Volatile Organic Compounds in Pyrenean Oak Trees, Quercus pyrenaica

The tri-trophic interactions between plants, insects, and insect predators and parasitoids are often mediated by chemical cues. The attraction to herbivore-induced Plant Volatiles (HIPVs) has been well documented for arthropod predators and parasitoids, and more recently for insectivorous birds. The attraction to plant volatiles induced by the exogenous application of methyl jasmonate (MeJA), a phytohormone typically produced in response to an attack of chewing herbivores, has provided controversial results both in arthropod and avian predators. In this study, we examined whether potential differences in the composition of bouquets of volatiles produced by herbivore-induced and MeJA-treated Pyrenean oak trees (Quercus pyrenaica) were related to differential avian attraction, as results from a previous study suggested. Results showed that the overall emission of volatiles produced by MeJA-treated and herbivore-induced trees did not differ, and were higher than emissions of Control trees, although MeJA treatment showed a more significant reaction and released several specific compounds in contrast to herbivore-induced trees. These slight yet significant differences in the volatile composition may explain why avian predators were not so attracted to MeJA-treated trees, as observed in a previous study in this plant-herbivore system. Unfortunately, the lack of avian visits to the experimental trees in the current study did not allow us to confirm this result and points out the need to perform more robust predator studies.