Red alder (alnus rubra) defense mechanisms against western tent caterpillar (malacosoma californicum) defoliation
Date
2019-05-01
Authors
Boateng, Kennedy
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Abstract
Red alder (Alnus rubra) is a tree species with high economic and ecological importance. It is subject to defoliation during unpredictable, episodic outbreaks of tent caterpillars (Malacosoma spp.) that result in reduced growth, decreased wood production, unsightly appearance and mortality in severe cases. Alder trees are weakened by severe and repeated tent caterpillar defoliation, and this can increase the susceptibility of the trees to other pests, diseases and drought. Repeated attack by tent caterpillars can cause decline in red alder populations, which can have potential negative impacts on the ecological and economic benefits of the species. Evidence from other species has shown that plants produce phytochemicals for defense against herbivores at a cost to growth and reproduction, but the relative magnitude of the cost of allocating available resources to defense depends on the level of the resources, and the plant genotype. The quality of a plant as food for herbivores is influenced by leaf physical and biochemical traits, and these traits change during a growing season or upon attack by herbivores. My research aimed to explore the defense mechanisms of red alder against western tent caterpillars (Malacosoma californicum) and determine the resistance variation among and within red alder populations, and to evaluate red alder available resource (nitrogen) allocation to defense and growth. Bioassay feeding trials were conducted in 2014 and 2015 with western tent caterpillars (WTC) (M. californicum) on twenty red alder clones from ten provenances. Phenology and quality of red alder leaves as food for the defoliators were analyzed to determine if budburst, leaf chemical content, water content or physical traits are major determinants of western tent caterpillars preference for red alder leaves. In another experiment, one-year-old seedlings from 100 half-sib red alder families were treated with two levels of ammonium nitrate (NH4NO3) in two growing seasons in a common garden. Growth, herbivore defense-related traits and root nodulation were measured and ranked among the plant genotypes and between the two nitrogen (N) treatments. Leaves from the two N treatments and different alder families were also used for bioassay feeding trials with WTC larvae to determine effects of N and genotype on red alder herbivory resistance. In my final experiments, I harvested and analyzed leaves from three-year-old red alder trees from five different families on eight dates from early April to mid-October 2016 to quantify oregonin and total phenolics concentrations, and wound induction experiments were conducted to determine if the concentrations of the chemicals vary during a growing season and upon attack by insects.
Alder clones and families differed in percentage leaf area eaten by caterpillars and in leaf defense traits. The concentrations of foliar phenolic compounds negatively correlated with the percentage leaf area eaten by the caterpillars, but the results suggest a threshold, above which the concentration of each of the chemicals appeared to reduce WTC feeding, individually. Particularly, foliar oregonin concentration above 20 % leaf dry weight consistently appeared to reduce feeding by caterpillars. N availability had significant effects on red alder seedling total dry biomass and leaf N concentration. There was a clear trade-off between red alder seedling growth, and content of the phenolic compounds and leaf thickness, which supports the growth-differentiation balanced hypothesis in relation to resource availability. The concentration of oregonin varied during the growing season and there were no significant responses of any of the measured compounds to wounding. The results suggest that red alder foliar oregonin, condensed tannin and total phenolics are constitutive defenses and are not wound-induced. The effects of leaf total phenolic and condensed tannin concentrations on insect herbivory have been documented by past studies but the effects of oregonin concentration in red alder leaves on tent caterpillar feeding is a novel finding.
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Keywords
Red alder, defense mechanisms, defoliation, genotype, clones, families, provenances, foliar phenolics, foliar nutrients, leaf phenology, leaf morphology, resource allocation, wounding response, seasonal variation