OIKOS 103: 3–16, 2003 Effects of host shading on consumption and growth of the geometrid Epirrita autumnata : interactive roles of water, primary and secondary compounds J. Henriksson, E. Haukioja, V. Ossipov, S. Ossipova, S. Sillanpa ¨a ¨, L. Kapari and K. Pihlaja Henriksson, J., Haukioja, E., Ossipov, V., Ossipova, S., Sillanpa ¨a ¨, S., Kapari, L. and Pihlaja, K. 2003. Effects of host shading on consumption and growth of the geometrid Epirrita autumnata : interactive roles of water, primary and secondary compounds. – Oikos 103: 3–16. Shading is assumed to reduce allocation to plant phenolics and to defense in general. We here report the results of experimental shading of individual branches or whole canopies in mountain birch on foliar chemistry and on the growth and consumption of a geometrid, Epirrita autumnata. Branch-wide shading tended to have at least as strong effects on both leaf chemistry and herbivore performance as canopy-wide shading, indicating local responses of the host to shading. Responses to shading varied among the key leaf traits. Leaf water content was higher and toughness lower in shaded than in non-shaded leaves. Leaf sugars were lower and protein-bound and free amino acids higher in shaded than in control leaves. Sucrose and galactose were at high levels in unshaded branches adjacent to shaded ones, suggesting that partial shading enhanced translocation of sugars within canopies. Total phenolics and soluble proanthocyanidins were low in both shading treatments. Of the other phenolic groups, concentrations of gallotannins and cell-wall-bound proan- thocyanidins did not differ between shaded and non-shaded leaves. Epirrita larvae grew better in both types of shading treatments compared to either unshaded control trees or to unshaded branches in the branch-shading trees. By far the most important correlate of larval growth was the amount of water consumed with leaf mass (r =0.94). When variance in water intake was standardized (also largely eliminating parallel variation in proteins), fructose and glucose still had significant positive correlations and proanthocyanidins negative with larval growth on control but not on shade leaves. Concentrations of several phenolic compounds correlated negatively with intake of dry matter and especially water, and different phenolics were impor- tant in shaded (gallotannins) and in control (flavonoids) leaves. Our findings strongly suggest that the effects of putatively defensive leaf traits on insect consumption and growth interact with nutritive leaf traits, particularly with water. J. Henriksson, E. Haukioja, V. Ossipo, S. Ossipoa, S. Sillanpa ¨a ¨ , L. Kapari and K. Pihlaja, Dept of Biology, Uni. of Turku, FIN-20014 Turku, Finland (haukioja@utu.fi). Present address for JH: Orion Corporation, Orion Pharma, Drug Safety, Orionintie 1, PO Box 65, FIN-02101 Espoo, Finland. VO, SO and KP also at: Dept of Chemistry, Uni. of Turku, FIN-20014 Turku, Finland. Shading limits the carbon gain of plants, reducing the carbon/nitrogen ratio of plant tissue. This has been claimed to be the major reason for the reduced concen- trations of defensive secondary compounds and higher nutritive value in shaded plants (Herms and Mattson 1992). There is considerable evidence to support the predicted shading-induced decline in the foliar concen- trations of proanthocyanidins (condensed tannins), less so for hydrolyzable tannins and very little for ter- penoids (Haukioja et al. 1998, Koricheva et al. 1998), indicating that the rate of production of different groups of putatively defensive carbon-rich secondary Accepted 24 February 2003 Copyright © OIKOS 2003 ISSN 0030-1299 OIKOS 103:1 (2003) 3