Shiyong Yang

Effects of host-plant shift on immune and other key life-history traits of an eruptive Geometrid, Epirrita autumnata (Borkhausen)

Abstract 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.

Foliar oxidases as mediators of the rapidly induced resistance of mountain birch against Epirrita autumnata

Induced resistance of the mountain birch against its main defoliator Epirrita autumnata is a well-characterized phenomenon. The causal mechanism for this induced deterioration, however, has not been unequivocally explained, and no individual compound or group of traditional defensive compounds has been shown to explain the phenomenon. Phenolic compounds are the main secondary metabolites in mountain birch leaves, and the biological activity of phenolics usually depends on their oxidation. In this study, we found that the activity of polyphenoloxidases (PPOs), enzymes that oxidize o-diphenols to o-diquinones, was induced in trees with introduced larvae, and bioassays showed that both growth and consumption rates of larvae were reduced in damaged trees. PPO activity was negatively associated with both larval growth and consumption rates in trees with bagged larvae, but not in control trees. Our results suggest that the oxidation of phenolics by PPOs may be a causal explanation for the rapidly induced resistance of mountain birch against E. autumnata. This finding also helps to explain why correlations between insect performance and phenolics (without measuring indices explaining their oxidation) may not produce consistent results.

Effects of elevated ultraviolet-B radiation on a plant–herbivore interaction

Enhanced ultraviolet-B (UV-B) radiation may have multiple effects on both plants and animals and affect plant–herbivore interactions directly and indirectly by inducing changes in host plant quality. In this study, we examined combined effects of UV-B and herbivory on the defence of the mountain birch (Betula pubescens ssp. czerepanovii) and also the effects of enhanced UV-B radiation on a geometrid with an outbreak cycle: the autumnal moth (Epirrita autumnata). We established an experiment mimicking ozone depletion of 30% (a relevant level when simulating ozone depletion above Northern Lapland). Both arctic species responded only slightly to the enhanced level of UV-B radiation, which may indicate that these species are already adapted to a broader range of UV-B radiation. UV-B exposure slightly induced the accumulation of myricetin glycosides but had no significant effect on the contents of quercetin or kaempferol derivatives. Mountain birch seedlings responded more efficiently to herbivory wounding than to enhanced UV-B exposure. Herbivory induced the activities of foliar oxidases that had earlier been shown to impair both feeding and growth of moth larvae. In contrast, the contents of foliar phenolics did not show the same response in different clones, except for a decrease in the contents of tannin precursors. The induction of foliar phenoloxidase activities is a specific defence response of mountain birches against insect herbivory. To conclude, our results do not support the hypothesis that the outbreak cycle of the autumnal moth can be explained by the cycles of solar activity and UV-B.

Increase in the substrate availability decreases phenoloxidase activity in the autumnal moth, Epirrita autumnata

Both the activity of phenoloxidases and the availability of phenoloxidase substrates are said to regulate melanotic encapsulation in invertebrates. The delayed induced resistance of mountain birches has been shown to increase the immunity of Epirrita autumnata (Geometridae: Lepidoptera), but the mechanism behind the increased immunity is unknown. The increase in the contents of aromatic amino acids as a response to herbivory in plants can in theory be one explanation for this carry-over effect. To test this hypothesis that an increase in the substrate availability increases also the melanotic encapsulation rate and the immunity of E. autumnata, we supplied excess levels of two aromatic amino acids, tyrosine (l-Tyr) and phenylalanine (l-Phe), to the diet of moth larvae. Surprisingly, the supply of both aromatic amino acids down-regulated PO activity, whereas the melanotic encapsulation rate was maintained at an unchanged level. PO activity was traded-off with body size and thus the cost of resistance is a probable explanation for the down-regulation of PO activity in the presence of surplus substrates. Our results suggest that both aromatic amino acids act as substrates of melanin, and an increase in substrate contents allows insects to decrease their PO activity without affecting the encapsulation rate.

Genetic and Environmental Factors Behind Foliar Chemistry of the Mature Mountain Birch

Previous studies of mountain birch (Betula pubescens spp. czerepanovii) repeatedly have found differences between individual trees in herbivory-related traits, but rarely have yielded estimates of the additive genetic variation of these traits or of their relationship to habitat. We used thirty-year-old birch half-sibs in a northern common garden to estimate the effect of genetics and local microhabitat on resistance-related traits. Genetic estimates of foliar chemistry have been studied only rarely with trees as old as these. Moth performance (Epirrita autumnata), rust (Melampsoridium betulinum) incidence levels, and the general level of natural herbivory damage to individual trees were used as direct measures of birch resistance. Chemical resistance-related traits in plant chemistry included 15 individual phenolics, 16 amino acids, and phenoloxidase activities in the foliage. We also followed birch phenology and growth. Our results show that the genotype of the birch was the most important determinant of phenolic composition and phenoloxidase activity, but that amino acid levels were best explained by the microhabitat of the birch. We also found that the phenology of the birch had a high heritability, although its variation was low. Our results reveal rich genetic variation in birch chemistry.

Depressed performance and detoxification enzyme activities of Helicoverpa armigera fed with conventional cotton foliage subjected to methyl jasmonate exposure

Methyl jasmonate (MeJA)‐mediated defense in conventional cotton, Gossypium hirsutum L. (Malvaceae), against cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), was investigated with respect to the activities of the detoxification enzymes acetylcholinesterase (AChE), carboxylesterase (CarE), and glutathione S‐transferases (GST) in pupae as well as the performance of larvae. The results suggested that exogenous application of MeJA to cotton leaves depressed the activities of AChE, CarE, and GST of cotton bollworm pupae. Both the absolute and protein‐specific AChE activities of pupae were depressed at all three MeJA concentrations applied as compared with a control, and the effects of 0.4 mM MeJA were significantly higher than those of 0.1 and 0.2 mM. A marked reduction in absolute CarE activity was observed at the 0.4 mM MeJA treatment, whereas the protein‐specific activity was increased by 0.2 and 0.4 mM. Absolute GST activity was significantly depressed only by the 0.4 mM MeJA treatment, whereas protein‐specific GST activity was not markedly affected by MeJA. Protein content of pupae was reduced by 0.4 mM MeJA‐induced defense in cotton leaves. The development time of larvae was protracted and pupal weight was reduced by 0.1 and 0.4 mM MeJA‐treated cotton leaves. Larval weight gain was inhibited significantly on 0.2 and 0.4 mM MeJA‐treated cotton leaves. The results suggested that MeJA‐induced plant defense may have adverse effects on H. armigera. In addition to the inhibition of growth and development, induced defense may also impair the insects ability to detoxify toxic plant secondary metabolites.

Temporal and spatial variation in mountain birch foliar enzyme activities during the larval period of Epirrita autumnata

Summary.Mountain birches are deciduous trees consisting of several clonal and partly autonomous ramets. Autumnal moth, Epirrita autumnata, is an extremely harmful pest of mountain birches that during outbreak years may cause the death of trees over large areas. During the larval development of E. autumnata, leaf characters and nutritional suitability vary considerably both temporally and spatially among and within mountain birches. Compared to variation between trees, however, the extent of within-tree variation is known for only a limited number of potentially defensive traits. Plant oxidases, polyphenoloxidases (PPOs) and peroxidases (PODs) may play an important part in dictating the suitability of a tree for plant-eating animals, including E. autumnata. In this study, we observed changes in the activities of oxidative and antioxidative enzymes along leaf development during the larval period of E. autumnata. We also estimated the relative amounts of spatial variation among and within trees. Our results show that POD activities were remarkably high during the rapid elongation growth phase of leaves but decreased with leaf maturation. Thus PODs are suggested to take part in leaf elongation growth. Time and within tree variation accounted for the most of variation in POD activities whereas there was no among tree variation. In contrast, the activities of antioxidative CATs, which act as antagonists to PODs, were low in young leaves and increased towards leaf maturation, reflecting an increase in the oxidative status of source leaves. Within tree variation accounted for the most of the variation in CAT activities. The absolute PPO activities decreased along leaf growth due to the dilution effect whereas the specific activity of PPOs, which has been shown to be defensive against E. autumnata larvae, did not vary temporally that might reflect the importance of these enzymes in the defence of birches. Acidic PPOs showed marked within- and among-tree variation, which may impact herbivores performance on certain genotypes and increase larval mobility within the tree canopy.

Effects of Cd2+ exposure on key life history traits and activities of four metabolic enzymes in Helicoverpa armigera (Lepidopteran: Noctuidae)

ABSTRACT Cadmium (Cd) is a toxic non-essential element accumulated easily in living organisms due to its non-biodegradable nature. A slight increase in Cd levels will threaten the health of terrestrial and aquatic organisms. Therefore, Cd contamination and its effects on the ecosystems and organisms have become a major topic of eco-toxicological research. The present study investigated the eco-toxicological effects of Cd-supplemented diet on some life history traits and specific enzyme activities of carboxylesterase (CarE), glutathione S-transferase (GST), acetylcholinesterase (AChE) and cytochrome P450 monooxygenases (P450s) in the cotton bollworm, Helicoverpa armigera. The results showed that Cd significantly decreased larval survival, pupation and emergence, and reduced body mass of the sixth instar larvae, pupae, adult moths and female fecundity. Cd-supplemented diet also delayed the development of larvae and pupae and shortened the life span of the cotton bollworm moth. In addition, dietary Cd inhibited the activities of GST, CarE and P450, but increased AChE activity of the sixth instar larvae. We concluded that life history traits and metabolic enzymes are sensitive to Cd stress in the cotton bollworm. These parameters are potential biomarkers for monitoring environmental stress in Lepidopteran insect pests.