“Why is the world green?” or “Why is not more of the green biomass eaten by herbivores?” has stimulated a lot of research for more than 50 years. There is still more to learn. Understanding the basic mechanisms involved in interactions between plants, herbivorous insects and their natural enemies are crucial when trying to develop tools for sustainable forest and crop protection. The goal of our research is to provide such knowledge.
Willow (Salix spp.) genotypes vary considerably in resistance against insects, e.g. leaf beetles and gall midges. Our ambition is to find genetic markers for resistance or even the genes behind the resistance. For the gall midge, we are close at reaching this goal. For the leaf beetles, which cause most of the damage in willow plantations, we have recently discovered diploid willow genotypes showing strong resistance against leaf beetles, which has opened up new possibilities. One interesting finding is that there does not seem to be any trade-off between direct and indirect (acting via the natural enemies of the leaf beetles) defences, which seem to be common in other systems.
It is anticipated that climate change will lead to more damage by insects on plants. However, the evidence in support for this prediction is mixed. One reason to why a warmer climate not necessarily will lead to more insect damage is that also the host plants and natural enemies of the insect herbivores are affected by the same change. We are presently testing the hypothesis that different trophic levels differ in their responsiveness to an increase in temperature: plants<herbivores<predators. We use pine sawflies and willow leaf beetles as model systems. The studies are performed both in the laboratory and in the field, e.g. by utilizing latitudinal gradients. One interesting preliminary result is that the response seems to vary with plant genotype.
Plants may respond actively to herbivory by increasing their investment in defences. We have shown that native willows (Salix cinerea) produce new leaves with higher trichome density when fed upon by leaf beetle adults. The response coincides in time with when the offspring, the larvae, starts to feed on the plant. The larvae perform less well on leaves with enhanced trichome density. When there is no more insect feeding, the plants stop to produce leaves with high trichome density. This pattern and other evidence show that the induced defence is costly. An introduced willow (S. viminalis), commonly used in energy forest plantations, show no induced response. Instead these willows seem to invest more in tolerance, i.e. to invest mainly in growth.
Stephan, J., Stenberg, J.A. & Björkman, C. 2015. How far away is the next basket of eggs? Spatial memory and perceived cues shape aggregation patterns in a leaf beetle. Ecology 96: 908-914.
Austel, N., Reinecke, A., Björkman, C., Hilker, M. & Meiners, T. 2015. Phenotypic plasticity in a willow leaf beetle depends on host plant species: Release and recognition of beetle odors. Chemical Senses 40: 109-124.
Austel, N., Björkman, C., Hilker, M. & Meiners, T. 2014. Phenotypic plasticity in host plant preference of the willow leaf beetle Phratora vulgatissima: the impact of experience made by adults. Agricultural and Forest Entomology 16: 417-425
Torp, M., Lehrman, A., Stenberg, J.A., Julkunen-Tiitto, R. & Björkman, C. 2013. Performance of an herbivorous leaf beetle (Phratora vulgatissima) on Salix F2 hybrids: The importance of phenolics. Journal of Chemical Ecology, 39: 516-524.
Kollberg, I., Bylund, H., Schmidt, A., Gershenzon, J. & Björkman, C. 2013. Multiple effects of temperature, photoperiod and food quality on the performance of a pine sawfly. Ecological Entomology 38: 201-208
Lehrman, A., Boddum, T., Stenberg, J.A., Orians, C.M. & Björkman C. 2013. Constitutive and herbivore-induced systemic volatiles differentially attract an omnivorous biocontrol agent to contrasting Salix clones. AoB PLANTS 5: plt005.
Björkman, C., Eklund, K., Lehrman, A. & Stenberg, J.A. 2013. Food conditioning affects expression of insect resistance in diploid willows (Salix spp.). American Journal of Plant Sciences 4: 48-52.
Lehrman, A., Torp, M., Stenberg, J.A., Julkunen-Tiitto, R. & Björkman, C. 2012. Estimating direct resistance in willows against a major insect pest, Phratora vulgatissima, by comparing life history traits. Entomologia Experimentalis et Applicata, 144: 93-100.
Ågren, G.I., Stenberg, J.A. & Björkman, C. 2012. Omnivores as plant bodyguards – A model of the importance of plant quality. Basic and Applied Ecology 13 (2012) 441–448.
Stenberg, J.A., Lehrman, A. & Björkman, C. 2011. Plant defence: feeding your bodyguards can be counter-productive Basic and Applied Ecology 12: 629-633.
Stenberg, J.A., Lehrman, A. & Björkman, C. 2011. Host-plant genotype mediates supply and demand of animal food in an omnivorous insect. Ecological Entomology 36: 442-449.
Björkman, C., Berggren, Å. & Bylund, H. 2011. Causes behind insect folivory patterns in latitudinal gradients. Journal of Ecology 99: 367-369
Orians, C.M. & Björkman, C. 2009. Associational resistance to a tropical leaf-miner: does neighbour identity matter? Journal of Tropical Ecology 25: 551-554.
Björkman, C., Dalin, P. & Ahrné, K. 2008. Leaf trichome responses to herbivory in willows: induction, relaxation and costs. New Phytologist 179: 176-184.