The quality of available food may affect insect herbivores directly (via growth and survivorship) and/or indirectly (by modifying insect vulnerability to parasitoids and pathogens). We examined the relationship between different phenolic compounds, belonging to various phenolic groups, in Betula pubescens spp. czerepanovii (mountain birch) foliage and the larval performance of the geometrid Epirrita autumnata (autumnal moth). Direct effects on insect performance were described by pupal weight, developmental rate, and survivorship; indirect effects were described by the encapsulation rate of an implant inserted into the insect hemocoel, a commonly used way to describe insect immune defense. We found profound differences in the effects of different phenolic categories: several individual hydrolyzable tannins were associated positively with larval performance but negatively with level of immune defense, whereas flavonoid glycosides were inversely related to larval survival but showed no association with the larvae immune defense.

Plants have been suggested to have an immunological memory comparable to animals. The evidence for this, however, is scarce. In our study with the mountain birchEpirrita autumnata system, we demonstrated that birches exposed as long as 5 yr to feeding of E. autumnata larvae (delayed induced resistance, DIR), responded more strongly to a new challenge than trees without an herbivory history. Pupal weights remained lower, and the duration of the larval period was prolonged in the DIR trees, although immunity, measured as an encapsulation rate, was not affected. We further demonstrated that the effects of birch phenolics on performance of E. autumnata were different in the exposed (DIR) trees from naive control trees, although we found only one significant change in chemistry. The quercetin:kaemferol ratio was increased in DIR trees, suggesting that herbivory caused oxidative stress in birches. In DIR trees, phenolics, especially hydrolyzable tannins (HTs), affected pupal weights negatively, whereas in control trees, the effects were either nonsignificant or positive. HTs also prolonged the duration of the larval period of females, whereas peroxidase (POD) activity prolonged that of males. We suggest that the causal explanation for the induced resistance was an enhanced oxidation of phenolic compounds from the DIR trees in the larval digestive tract. Phenolic oxidation produces semiquinones, quinones, free radicals, and ROS, which may have toxic, antinutritive, and/or repellent properties against herbivores.

1. Population density of Epirrita autumnata (Lepidoptera: Geometridae) reaches outbreak densities regularly in northernmost Scandinavia. During these outbreak years, the most abundant host species, the mountain birch (Betula pubescens ssp. czerepanovii), is regularly exhausted, although larvae may rescue themselves from starvation by using alternative host species. 2. In this paper, the effects of the shift of host species on the immune defence and other life‐history traits of E. autumnata were investigated, and possible consequences for population dynamics were briefly discussed. Moth larvae were reared on the leaves of the main host, mountain birch, until larvae reached their third instar. After this, larvae were allocated randomly to five treatments: larvae were either allowed to finish larval stage on the mountain birch or were shifted onto four alternative host species that are typical species for the area. 3. As expected, the host species had a major effect on fitness traits: body weight, development, and survival rate of the moths. The pupal weight was lower and development rates slower on the three alternative host species, Salix myrsinifolia Salisb., Vaccinium uliginosum L., and Betula nana L., than on the main host, mountain birch. 4. The immunity was, however, the same or better on the alternative hosts than on the main host. The immunity and pupal weights were negatively related, suggesting a trade‐off between body size and immunocompetence. 5. The decreased body size and fecundity of E. autumnata during outbreak years may be partly due to the shift to alternative host species whereas the host‐plant species probably does not affect markedly the rate of parasitism.

Haviola, S., Saloniemi, I., Ossipov, V. and Haukioja, E. 2006. Additive genetic variation of secondary and primary metabolites in mountain birch. Á/ Oikos 112: 382 Á/ 391.Evolutionary adaptations require genetic variation in the traits concerned. Mountain birch populations suffer from regular autumnal moth defoliations that affect tree survival and growth. Earlier studies have found that birch individuals show clear differences in their leaf chemistry, which may be relevant to herbivore performance. A large part of this variation is assumed to be genetic, since quantitative differences between trees remain the same in different years. We estimated the genetic parameters of several herbivory-related chemical compounds. The material consisted of 30 mature wild trees and their progenies. We treated the data both as half-sib and full-sib families, and calculated parent Á/offspring comparisons as well. The compounds included 12 individual phenolics, 5 carbohydrates and 16 protein-bound amino acids. For comparison, we also analysed the weight and germination percent of seeds and the height of seedlings. Two of the most abundant phenylpropanoids (chlorogenic acid and kaempferol-3-O-a-L-rhamnopyranoside) showed significant genetic variation, but this was not true of all individual phenolic compounds. Amino acids had less genetic variation than phenolic compounds. The high genetic variation of the phenolic compounds indicates that the mountain birch is able to evolve its phenol-based resistance.

Temperature directly affects the growth, survival, and development rates of poikilothermic insect herbivores; it may also have an important indirect impact, via the activities of plant defensive enzymes. The effects of wounding birch leaves and temperature on the growth and development rates of a Lepidopteran moth, Epirrita autumnata, were studied. We also examined the activities of a mountain birch (Betula pubescesns spp. czerepanovii) defensive enzymes, specifically the polyphenoloxidases (PPOs), in relation to temperature and wounding. The optimal temperature for early instars in terms of survival and developmental rates was between +15 and 20 degrees C. Wounding treatment had different effects on birch PPO activity depending on the temperature: at +12 degrees C, wounding decreased the activity, suggesting induced amelioration at that temperature, whereas at +25 degrees C, wounding increased the activity, suggesting induced resistance. However, larval growth was retarded slightly, but significantly, on the leaves of wounded twigs at both temperatures. Both PPO activity and larval growth rates were affected within 12 h, indicating the existence of a transcription- and translation-independent defense system in birch leaves. We suggest that underlying the increase in PPO activity and the decrease in larval growth rate may be H2O2, which has been shown to accumulate in response to wounding. Our results also provide a possible biological mechanism for the hypothesis that low temperatures promote the success of E. autumnata and other Lepidopteran larvae via decreased defensive enzyme activities of host plants at lower temperatures.