Dr. Michael Andreas Eisenring

• Extreme droughts can have long-lasting effects on forest community dynamics and species interactions. Yet, our understanding of how drought legacy modulates ecological relationships is just unfolding. We tested the hypothesis that leaf chemistry and herbivory show long-term responses to premature defoliation caused by an extreme drought event in European beech (Fagus sylvatica L.).
• For two consecutive years after the extreme European summer drought in 2018, we collected leaves from the upper and lower canopy of adjacently growing drought-stressed and unstressed trees. Leaf chemistry was analyzed and leaf damage by different herbivore-feeding guilds was quantified.
• We found that drought had lasting impacts on leaf nutrients and on specialized metabolomic profiles. However, drought did not affect the primary metabolome. Drought-related phytochemical changes affected damage of leaf-chewing herbivores whereas damage caused by other herbivore-feeding guilds was largely unaffected. Drought legacy effects on phytochemistry and herbivory were often weaker than between-year or between-canopy strata variability.
• Our findings suggest that a single extreme drought event bears the potential to long-lastingly affect tree–herbivore interactions. Drought legacy effects likely become more important in modulating tree–herbivore interactions since drought frequency and severity are projected to globally increase in the coming decades.

Sommerdürren können die Interaktionen zwischen Bäumen und Insekten kurzfristig stark verändern. Wie Dürreeffekte aber auch Jahre später noch auf Baum-Insekten-Interaktionen nachwirken können, zeigen Analysen, die am SwissForestLab Science Day 2023 präsentiert wurden.

The emerald ash borer (EAB), Agrilus planipennis, is a woodboring beetle native to East Asia. It is highly invasive in North America, where it causes large-scale dieback of American ash species, Fraxinus spp. EAB is also invasive in Western Russia and Ukraine, and it continues to spread towards Central and Western Europe where all three native ash species are susceptible. Biological control approaches offer sustainable solutions to control invasive pests. In North America, four Asian parasitoid species that have coevolved with EAB have been introduced to control the pest. In Europe, many species of the genus Agrilus occur, and it is likely that at least some of their parasitoids will adopt EAB as a host as it spreads across the continent. However, parasitoids of Agrilus spp., are very poorly known in Europe because of the difficulty of studying the parasitoid complex of these solitary wood boring insects. In this review, we provide a literature overview of studies and records of European parasitoids of Agrilus spp. to provide a basis for future studies on the emerging parasitoid complex of EAB in Europe. Parasitoid records were found for 24 European species of Agrilus. Sixty-four parasitoid species were recorded, mostly larval parasitoids. However, it is likely that several of them are erroneous host-parasitoid associations or misidentifications, and the biology of most species and their role in the population dynamics of their hosts has been poorly studied. It is recommended to conduct pre-emptive studies on these parasitoids and their suitability as potential biological control organisms with EAB.

European ash, Fraxinus excelsior is facing the double threat of ongoing devastation by the invasive fungal pathogen, Hymenoscyphus fraxineus and the imminent arrival of the non-native emerald ash borer (EAB), Agrilus planipennis. The spread of EAB which is currently moving westwards from European Russia and Ukraine into central Europe, poses an additional substantial threat to European ash, F. excelsior. While the molecular basis for resistance or variation in resistance among European ash genotypes is heavily investigated, comparatively little is known about the molecular ash traits involved in resistance against EAB. In this study we have gathered transcriptomic data from EAB inoculated genotypes of F. excelsior that have previously shown different levels of susceptibility to EAB. Resultant datasets show differential gene expression in susceptible and resistant genotypes in response to EAB infestation. This data will provide important information on the molecular basis of resistance to the EAB and allow the development of management plans to combat a pending threat of a culturally and ecologically important European tree species.

• Background and Aims At the population level, genetic diversity is a key determinant of a tree species’ capacity to cope with stress. However, little is known about the relative importance of the different components of genetic diversity for tree stress responses. We compared how two sources of genetic diversity, genotype and cytotype (i.e. differences in ploidy levels), influence growth, phytochemical and physiological traits of Populus tremuloides in the presence and absence of environmental stress.

• Methods In a series of field studies, we first assessed variation in traits across diploid and triploid aspen genotypes from Utah and Wisconsin under non-stressed conditions. In two follow-up experiments, we exposed diploid and triploid aspen genotypes from Wisconsin to individual and interactive drought stress and defoliation treatments and quantified trait variations under stress.

• Key Results We found that (1) tree growth and associated traits did not differ significantly between ploidy levels under non-stressed conditions. Instead, variation in tree growth and most other traits was driven by genotypic and population differences. (2) Genotypic differences were critical for explaining variation of most functional traits and their responses to stress. (3) Ploidy level played a subtle role in shaping traits and trait stress responses, as its influence was typically obscured by genotypic differences. (4) As an exception to the third conclusion, we showed that triploid trees expressed 17 % higher foliar defence (tremulacin) levels, 11 % higher photosynthesis levels and 23 % higher rubisco activity under well-watered conditions. Moreover, triploid trees displayed greater drought resilience than diploids as they produced 35 % more new tissue than diploids when recovering from drought stress.

• Conclusion Although ploidy level can strongly influence the ecology of tree species, those effects may be relatively small in contrast to the effects of genotypic variation in highly diverse species.

Eine neue Untersuchung an Schweizer und skandinavischen Eschen zeigt, dass Eschen, die eine erhöhte Resistenz gegen das Eschentriebsterben besitzen, besser gegen den Eschenprachtkäfer gewappnet sind. Eine gezielte Förderung könnte die Ausbreitung des Käfers verlangsamen.

Le frêne est menacé par deux espèces invasives différentes; un champignon asiatique, également connu sous le nom de dépérissement des pousses du frêne ou chalarose, ainsi que l'agrile du frêne qui, selon les modèles, devrait atteindre la Suisse dans quelques années.

• Plants rely on cross-resistance traits to defend against multiple, phylogenetically distinct enemies. These traits are often the result of long co-evolutionary histories. Biological invasions can force naïve plants to cope with novel, coincident pests, and pathogens. For example, European ash (Fraxinus excelsior) is substantially threatened by the emerald ash borer (EAB), Agrilus planipennis, a wood-boring beetle, and the ash dieback (ADB) pathogen, Hymenoscyphus fraxineus. Yet, plant cross-resistance traits against novel enemies are poorly explored and it is unknown whether naïve ash trees can defend against novel enemy complexes via cross-resistance mechanisms.
• To gain mechanistic insights, we quantified EAB performance on grafted replicates of ash genotypes varying in ADB resistance and characterized ash phloem chemistry with targeted and untargeted metabolomics.
• Emerald ash borer performed better on ADB-susceptible than on ADB-resistant genotypes. Moreover, changes in EAB performance aligned with differences in phloem chemical profiles between ADB-susceptible and ADB-resistant genotypes.
• We show that intraspecific variation in phloem chemistry in European ash can confer increased cross-resistance to invasive antagonists from different taxonomic kingdoms. Our study suggests that promotion of ADB-resistant ash genotypes may simultaneously help to control the ADB disease and reduce EAB-caused ash losses, which may be critical for the long-term stability of this keystone tree species.

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

Climate change is threatening the persistence of many tree species via independent and interactive effects on abiotic and biotic conditions. In addition, changes in temperature, precipitation, and insect attacks can alter the traits of these trees, disrupting communities and ecosystems. For foundation species such as Populus, phytochemical traits are key mechanisms linking trees with their environment and are likely jointly determined by interactive effects of genetic divergence and variable environments throughout their geographic range. Using reciprocal Fremont cottonwood (Populus fremontii) common gardens along a steep climatic gradient, we explored how environment (garden climate and simulated herbivore damage) and genetics (tree provenance and genotype) affect both foliar chemical traits and the plasticity of these traits. We found that (1) Constitutive and plastic chemical responses to changes in garden climate and damage varied among defense compounds, structural compounds, and leaf nitrogen. (2) For both defense and structural compounds, plastic responses to different garden climates depended on the climate in which a population or genotype originated. Specifically, trees originating from cool provenances showed higher defense plasticity in response to climate changes than trees from warmer provenances. (3) Trees from cool provenances growing in cool garden conditions expressed the lowest constitutive defense levels but the strongest induced (plastic) defenses in response to damage. (4) The combination of hot garden conditions and simulated herbivory switched the strategy used by these genotypes, increasing constitutive defenses but erasing the capacity for induction after damage. Because Fremont cottonwood chemistry plays a major role in shaping riparian communities and ecosystems, the effects of changes in phytochemical traits can be wide reaching. As the southwestern US is confronted with warming temperatures and insect outbreaks, these results improve our capacity to predict ecosystem consequences of climate change and inform selection of tree genotypes for conservation and restoration purposes.

Durch das Eschentriebsterben und den sich vom Einschleppungsort Moskau weiter Richtung Westen ausbreitenden Eschenprachtkäfer ist die Esche als Baumart in Europa stark gefährdet. Die Forschung an Eschen und ihren Interaktionen mit Schadorganismen trägt dazu bei, den Eschenfortbestand in der Schweiz zu sichern.

1. Functional trait variation within individual plants is predicted to have important ecological consequences. However, our understanding of the sources contributing to subindividual trait heterogeneity, and the ramifications thereof, is poor.
2. In a common garden, we sampled multiple genotypes of mature trembling aspen Populus tremuloides at different vertical crown levels and quantified the contributions of genetic, spatial and biotic (herbivory) factors to subindividual morphological and chemical leaf trait variance. Bioassays using gypsy moth Lymantria dispar L. caterpillars were conducted to study the impacts of spatial and genotypic factors on insect performance.
3. Crown position was the primary source of subindividual variation in leaf morphology and explained more of the variance in morphological traits than tree genotype. Effects of spatial, genetic and biotic factors on within-crown chemistry were highly compound specific. For many compounds, changes in within-crown concentrations differed among genotypes, revealing that genotypic variation not only contributes to phytochemical diversity among individuals of the same population, but also affects subindividual chemical trait heterogeneity. For many defence-related compounds, total within-crown variance itself differed among genotypes and was in many cases a heritable trait. Bioassays revealed that effects of crown position on caterpillar performance varied among tree genotypes and caterpillar sexes.
4. We demonstrate that even relatively small trees can express subtle but distinct levels of within-crown leaf trait variation. In aspen, this variation is shaped not only by spatial factors, but also by genetics and to a lesser degree by herbivory. Within-crown variation in defence chemistry might itself provide a defensive phenotype against herbivores and contribute to the biological diversity of canopy insect communities. A free Plain Language Summary can be found within the Supporting Information of this article.

Background: In its invaded regions, Drosophila suzukii (Matsumura) is a novel host for the community of resident parasitoids of Drosophila. To attain a high parasitization rate on the novel host, the parasitoids have to locate it and accept it in the presence of other Drosophila hosts. We conducted a laboratory choice experiment and a semifield trial to investigate host searching and host preference of the three pupal parasitoid species Trichopria drosophilae (Perkins), Pachycrepoideus vindemmiae (Rondani) and Spalangia erythromera Förster.
RESULTS: All three parasitoid species preferred D. suzukii over two common native hosts in the choice experiment. In field cages, most parasitoid offspring emerged from D. suzukii hosts. While P. vindemmiae mainly parasitized hosts in the foliage, most T. drosophilae offspring emerged from pupae presented on the ground.
CONCLUSIONS: Both P. vindemmiae and T. drosophilae have the potential to find and parasitize D. suzukii in the field. If released early in the season, possible nontarget effects on native Drosophila should be minimal.