Phenotypic variation in host quality of pines for the European pine sawfly (Neodiprion sertifer)

Phenotypic variation in host quality can affect ecological and evolutionary interactions between plants and herbivores. Effects of phenotypic variation in host quality of woody-plants for leaf feeding insects were investigated in three studies: (1) phenological variation of Scots pine (Pinus sylvestris L.) needle quality for the European pine sawfly ( Neodiprion sertifer (Geoffroy)) as a test of the "phenological window of host susceptibility hypothesis", (2) effects of fertilization on the expression of defoliation-induced resistance of Austrian pine ( P. nigra Arnold) to European pine sawfly, and (3) manipulation of growth/defense trade-offs in paper birch (Betula papyrifera Marsh.) and Austrian pine through application of the plant growth regulator paclobutrazol and fertilization, separately and in combination, as well as resulting effects on resistance to gypsy moth (Lymantria dispar L.), forest tent caterpillar (Malacosoma disstria Hubner), whitemarked tussock moth (Orgyia leucostigma (J. E. Smith)), and European pine sawfly.Consistent with the predictions of the phenological window hypothesis, larval growth and survival decreased as host-insect phenological synchronicity was modified. As current-year shoots and needles elongated, total nitrogen, free amino acids, and soluble carbohydrates concentrations decreased, while mono-, sesqui- and ditepenoid concentrations increased in previous-year needles. Strong correlations between the concentrations of nutrients, and terpenoids and larval growth suggest that this insect can tolerate plant defenses only if nutrients are relatively high with respect to chemical defenses. As foliar quality declined, insects ate more in an attempt to compensate but were less efficient at converting consumed foliage to body mass. A slow increase in host quality occurred late in season as mature foliage once again became suitable. The survival of an experimentally-generated second generation of larvae manipulated to emerge in September was only 20% on current- or previous-year foliage however survival was 80% the following spring. Hence, even though Scots pine foliage is present throughout the year, phenological variation in foliar nutrients and defensive chemistry restricts the life-cycle of European pine sawfly to a narrow window of host susceptibility on previous-year needles in early to mid spring. This phenological window of host susceptibility appears to constrain the evolution of a second generation.The expression of defoliation-induced responses of Austrian pine to European pine sawfly larvae varied with level of fertilization. In the nutrient-rich environment, previous defoliation improved host quality for European pine sawfly larvae, which may promote insect outbreaks through positive density dependent (Allee) effects on population growth. However in the low nutrient treatment, host quality rapidly decreased following defoliation, which is thought to generate negative density dependent effects on population growth thus stabilizing population density. Austrian pines tolerated defoliation through compensatory growth responses, an effect that was independent of the fertilization level and that was evident two growing seasons after the defoliation. The concentration of monoterpenes strongly increased one year after the defoliation and across fertility levels. These results are inconsistent with predictions of the carbon nutrient balance hypothesis, which proposes that physiological constraints prevent defoliation-induced accumulation of carbon-based secondary metabolites in evergreens. These phenotypic responses of Austrian pine may represent adaptive plasticity to defoliation that enhances fitness. In nutrient-poor soils Austrian pines would be expected to express rapid induced resistance that would help limit outbreaks and thus defoliation, while compensatory growth responses would enhance competitive ability.Applications of the plant growth regulator paclobutrazol slowed the growth of paper birch and Austrian pine with no effect on photosynthesis. Consistent with the predictions of the growth-differentiation balance hypothesis, application of paclobutrazol increased the concentration of defensive chemicals (tannins) and resistance of paper birch to gypsy moth and whitemarked tussock moth. However, this effect was only observed in the second growing season following application of paclobutrazol. Conversely, paclobutrazol had no effect on foliar defensive chemistry (tannins, phenolics and monoterpenes) of Austrian pine or its resistance to European pine sawfly larvae. Although it has been suggested that paclobutrazol generally enhances tree resistance to insects, it was found here that its effect on tree resistance to leaf-feeding insects was species-specific and time-sensitive.