Phenolic and phenolic-related factors as determinants of suitability of mountain birch leaves to an herbivorous insect.

We investigated the role of phenolic and phenolic-related traits of the leaves of mountain birch (Betula pubescens ssp. czerepanovii) as determinants of their suitability for the growth of larvae of the geometrid Epirrita autumnata. As parameters of leaf suitability, we determined the contents of total phenolics, gallotannins, soluble and cell-wall-bound proanthocyanidins (PAS and PAB, respectively), lignin, protein precipitation capacity of tannins (PPC), and leaf toughness. In addition, we examined concentrations of soluble carbohydrates and protein-bound amino acids as background variables describing the nutritive value of leaves. The correlation of the leaf traits of our 40 study trees with the tree-specific relative growth rate (RGR) of E. autumnata showed that the only significant correlation with RGR was that of PAS - the largest fraction of total phenolics - and even that explained only 15% of the variation in E. autumnata growth. The nonlinear estimation of the relationship between RGR and PAS by piecewise linear regression divided the 40 study trees into two groups: (i) 19 trees with good leaves for E. autumnata (RGR ranging from 0.301 to 0.390), and (ii) 21 trees with poor leaves (RGR ranging from 0.196 to 0.296). The suitability of leaves within these two groups of trees was determined by different phenolic traits. Within the good group, the suitability of leaves for larvae was determined by the PPC of extracts, which strongly correlated with gallotannins, and by the total content of gallotannins. In contrast, the leaves of poor trees had significantly higher contents of both PAS and PAB, but leaf toughness correlated only negatively with the RGR of E. autumnata larvae. We also discuss the causes of variation in the phenolic and phenolic-related factors that determine the suitability of leaves for E. autumnata larvae in different groups of trees.

Seasonally varying diet quality and the quantitative genetics of development time and body size in birch feeding insects.

Genetic variance-covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life-history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch-insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet-imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early-summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild-wide patterns in the genetic architectures brought about by seasonal variation in resource quality.

Sources of variation in rapidly inducible responses to leaf damage in the mountain birch-insect herbivore system.

Studies on rapidly inducible resistance in trees against insect herbivores show substantial variation in the strength of responses. Here we report the results of a study which examined causes of this variation. We bioassayed the quality of leaves of two developmental phases (young vs. mature) of the mountain birch Betula pubescens ssp. tortuosa by measuring the growth of two instars of Epirrita autumnata larvae. We used only short shoot leaves from trees of a natural stand, uniform in size and age. Damage was caused by larvae and artificial tearing of leaf lamina, varying the scale and time. We separated seasonal changes in plants from instar-dependent effects of the animals by testing experimental larvae in two subsequent growth trials. We found that only larval-made damage induced responses in leaves that made the leaves significantly poorer quality for the test larvae. Artificial damage induced only weak responses, and artificial canopy-wide damage even caused slight improvement of leaf quality. Cumulative leaf damage did not strengthen birch responses. Leaves that were in the expansion phase responded to damage while fully-expanded, mature leaves showed no response. The pattern of responses indicated that there might be physiological constraints: small-scale damage induced resistance against the larvae but largescale damage did not. Prevalent weather conditions might have modified these responses. Larvae of two instars and sexes, of low- and high-density populations responded to leaf damage similarly. However, prior experience of larvae with the host plant may have affected subsequent larval performance. Variation in rapidly inducible responses in birches was caused by plant characters rather than by test animals.