Specificity of Phenolic Glycoside Induction in Willow Seedlings (Salix sericea) in Response to Herbivory

Abstract Salix sericea (Marsh.) (Salicaceae) seedlings were used to investigate phytochemical induction of phenolic glycosides following beetle herbivory. Seven-week-old fullsibling seedlings were subjected to one of three damage treatments: Plagiodera versicolora adults, P. versicolora larvae, or Calligrapha multipunctata bigsbyana adults. Salicylate concentrations were measured locally (within damaged leaves) and systemically (above and below damaged leaves) 4 d later. Herbivory caused differential salicylate induction; 2′-cinnamoylsalicortin was induced, whereas salicortin was not. The induction of 2′-cinnamoylsalicortin was not specific with regard to the species or developmental stage of beetle tested but did vary with leaf age: induction occurred in the younger undamaged leaves but not in the damaged leaves or in the older undamaged leaves. The amount of leaf area consumed had no detectable effect on induction, indicating an "all-or-none" response triggered by even small amounts of herbivory. Locally, herbivory caused a decrease in salicortin concentrations, probably because of degradation within the damaged leaves. These results suggest a specific but generalized induced response to these leaf-feeding beetles

Competitor Avoidance Drives Within-Host Feeding Site Selection in a Passively-Dispersed Herbivore

Evolutionary theory predicts that ovipositing females that can actively disperse should select hosts that increase offspring performance. However, for organisms that are exclusively passively dispersed, feeding site selection is possible only at the within-host level. This is particularly important for their offspring, which have strong temporal and spatial dispersal constraints within a host. Such constraints are probably magnified by tissue quality heterogeneity caused by competing herbivores. We investigated within-host feeding site selection of passively dispersed neonates of a sessile herbivore, the hemlock woolly adelgid (Adelges tsugae Annand), when it shares its eastern hemlock (Tsuga canadensis Carrière) host with another sessile herbivore, the elongate hemlock scale (Fiorinia externa Ferris). Within-host feeding site selection was studied (a) at the shoot level (with or without F. externa) using choice tests, and (b) at the needle level by scoring insect presence in field surveys. Adelges tsugae avoided F. externa-colonised foliage in both the choice tests and field surveys. As A. tsugae has no efficient predators or parasitoids in the study area, we conclude that our results are due to the presence of the competing herbivore. Although A. tsugae cannot actively disperse among hosts, we showed that within-host feeding site selection is an important mechanism to minimise the co-occurrence with a competing herbivore that has known negative impacts on A. tsugae population densities. Studying within-host feeding site selection in a multiple-species context could assist in understanding and predicting the impact of destructive pests such as A. tsugae and the interactions with their novel hosts

Patchy nitrate promotes inter-sector flow and 15 N allocation in Ocimum basilicum: a model and an experiment

Abstract. Root conductance increases under high nitrate conditions. This plasticity might increase water and nutrient transport between parallel xylem pathways, but restrictions to lateral flow -called sectoriality -are expected to limit this crossover. We simulated the effects of a high nitrate patch on root conductance, water uptake and inter-sector water transport, then empirically tested whether a high nitrate patch affects water uptake and nitrogen distribution (applied 15 N as 14 NH 4 15 NO 3 to half the root system) within the crowns of split-root hydroponic basil (Ocimum basilicum L.). Simulations showed that at low sectoriality, the proportion of water taken up in a patch scales with the relative change in root resistance and that this fraction decreases with increasing tangential resistance. The effect of sectoriality decreased when a higher background root resistance was assumed. Empirically, water flow through excised basil roots was 1.4 times higher in the high nitrate than the no nitrate solution. In split-root basil, a nitrate patch resulted in a marginally significant increase in the proportion of water taken up from the patch and water uptake patterns significantly predicted the distribution of 15 N. Our results suggest that root conductance can mediate nitrogen allocation between sectors, a previously unexplored benefit. Additional keywords: hydraulic conductivity, nitrogen transport, nitrogen 15 isotope, water uptake, xylem

Conifer Responses to a Stylet-Feeding Invasive Herbivore and Induction with Methyl Jasmonate: Impact on the Expression of Induced Defenses and a Native Folivore

Trees attacked by multiple herbivores need to defend themselves against dynamic biotic challenges; appropriate responses to one stressor can elicit hormonal responses that are antagonistic to another. Hemlock (Tsuga canadensis) infestation by hemlock woolly adelgid (HWA; Adelges tsugae) results in the accumulation of the defensive hormone salicylic acid. We explored the potential for HWA infestation to interfere with anti‐folivore‐induced defence signalling and its implications for a native folivore (hemlock looper; Lambdina fiscellaria). Hemlocks were infested with HWA and/or sprayed with methyl jasmonate; foliar defences were analyzed and foliage quality for looper larvae was assessed. Both treatments activated foliar defensive traits, including a HWA‐mediated increase in peroxidase activity and an accumulation of cell wall‐bound phenolics and lignin, as well as a methyl jasmonate‐mediated increase in lipoxygenase activity. The two treatments had an additive effect on other defensive traits and both treatments negatively affected looper performance. These results suggest that salicylic acid and jasmonic acid are not strictly antagonistic in conifers and that both have a role in anti‐folivore defence signalling. The present study illustrates the need for a better understanding of hormone signalling, cross‐talk and induced responses in conifers

Tree Responses to an Invasive Sap-Feeding Insect

Although sap-feeding insects are known to negatively affect plant growth and physiology, less is known about sap-feeding insects on woody plants. Adelges tsugae (Annand Hemiptera: Adelgidae), the hemlock woolly adelgid, is an invasive sap-feeding insect in eastern North America that feeds on and kills Tsuga canadensis (L. Carrière), eastern hemlock. Newly hatched adelgid nymphs crawl to young unattacked tissue, settle and immediately enter diapause (aestivation) while attached to hemlock in summer. We assessed the effect of A. tsugae infestation on T. canadensis growth and physiology by analyzing hemlock growth on lateral and terminal branches, water potential, photosynthesis, stomatal conductance, and foliar nitrogen (%N). A. tsugae infestation greatly decreased terminal and lateral growth of eastern hemlock. In addition, A. tsugae presence reduced photosynthesis by 10 % in September and 36 % in October. Adelgid-infested hemlocks also exhibited signs of water stress that included notable reductions in water potential and stomatal conductance. These responses shed light on possible mechanisms of adelgid-induced mortality

Asymmetric Biotic Interactions and Abiotic Niche Differences Revealed by a Dynamic Joint Species Distribution Model

A species’ distribution and abundance are determined by abiotic conditions and biotic interactions with other species in the community. Most species distribution models correlate the occurrence of a single species with environmental variables only, and leave out biotic interactions. To test the importance of biotic interactions on occurrence and abundance, we compared a multivariate spatiotemporal model of the joint abundance of two invasive insects that share a host plant, hemlock woolly adelgid (HWA; Adelges tsugae) and elongate hemlock scale (EHS; Fiorina externa), to independent models that do not account for dependence among co‐occurring species. The joint model revealed that HWA responded more strongly to abiotic conditions than EHS. Additionally, HWA appeared to predispose stands to subsequent increase of EHS, but HWA abundance was not strongly dependent on EHS abundance. This study demonstrates how incorporating spatial and temporal dependence into a species distribution model can reveal the dependence of a species’ abundance on other species in the community. Accounting for dependence among co‐occurring species with a joint distribution model can also improve estimation of the abiotic niche for species affected by interspecific interactions

Failure Under Stress: The Effect of the Exotic Herbivore \u3cem\u3eAdelges tsugae\u3c/em\u3e on Biomechanics of \u3cem\u3eTsuga canadensis\u3c/em\u3e

Background and Aims Exotic herbivores that lack a coevolutionary history with their host plants can benefit from poorly adapted host defences, potentially leading to rapid population growth of the herbivore and severe damage to its plant hosts. The hemlock woolly adelgid (Adelges tsugae) is an exotic hemipteran that feeds on the long-lived conifer eastern hemlock (Tsuga canadensis), causing rapid mortality of infested trees. While the mechanism of this mortality is unknown, evidence indicates that A. tsugae feeding causes a hypersensitive response and alters wood anatomy. This study investigated the effect of A. tsugae feeding on biomechanical properties at different spatial scales: needles, twigs and branches. Methods Uninfested and A. tsugae-infested samples were collected from a common garden experiment as well as from naturally infested urban and rural field sites. Tension and flexure mechanical tests were used to quantify biomechanical properties of the different tissues. In tissues that showed a significant effect of herbivory, the potential contributions of lignin and tissue density on the results were quantified. Key Results Adelges tsugae infestation decreased the abscission strength, but not flexibility, of needles. A. tsugae feeding also decreased mechanical strength and flexibility in currently attacked twigs, but this effect disappeared in older, previously attacked branches. Lignin and twig tissue density contributed to differences in mechanical strength but were not affected by insect treatment. Conclusions Decreased strength and flexibility in twigs, along with decreased needle strength, suggest that infested trees experience resource stress. Altered growth patterns and cell wall chemistry probably contribute to these mechanical effects. Consistent site effects emphasize the role of environmental variation in mechanical traits. The mechanical changes measured here may increase susceptibility to abiotic physical stressors in hemlocks colonized by A. tsugae. Thus, the interaction between herbivore and physical stresses is probably accelerating the decline of eastern hemlock, as HWA continues to expand its range

Chronic impacts of invasive herbivores on a foundational forest species: a whole‐tree perspective

Forests make up a large portion of terrestrial plant biomass, and the long‐lived woody plants that dominate them possess an array of traits that deter consumption by forest pests. Although often extremely effective against native consumers, invasive species that avoid or overcome these defenses can wreak havoc on trees and surrounding ecosystems. This is especially true when multiple invasive species co‐occur, since interactions between invasive herbivores may yield non‐additive effects on the host. While the threat posed by invasive forest pests is well known, long‐term field experiments are necessary to explore these consumer‐host interactions at appropriate spatial and temporal scales. Moreover, it is important to measure multiple variables to get a “whole‐plant” picture of their combined impact. We report the results of a 4‐yr field experiment addressing the individual and combined impacts of two invasive herbivores, the hemlock woolly adelgid (Adelges tsugae) and elongate hemlock scale (Fiorinia externa), on native eastern hemlock (Tsuga canadensis) in southern New England. In 2011, we planted 200 hemlock saplings into a temperate forest understory and experimentally manipulated the presence/absence of both herbivore species; in 2015, we harvested the 88 remaining saplings and assessed plant physiology, growth, and resource allocation. Adelgids strongly affected hemlock growth: infested saplings had lower above/belowground biomass ratios, more needle loss, and produced fewer new needles than control saplings. Hemlock scale did not alter plant biomass allocation or growth, and its co‐occurrence did not alter the impact of adelgid. While both adelgid and scale impacted the concentrations of primary metabolites, adelgid effects were more pronounced. Adelgid feeding simultaneously increased free amino acids local to feeding sites and a ~30% reduction in starch. The cumulative impact of adelgid‐induced needle loss, manipulation of nitrogen pools, and the loss of stored resources likely accelerates host decline through disruption of homeostatic source‐sink dynamics occurring at the whole‐plant level. Our research stresses the importance of considering long‐term impacts to predict how plants will cope with contemporary pressures experienced in disturbed forests

Facilitation between invasive herbivores: hemlock woolly adelgid increases gypsy moth preference for and performance on eastern hemlock

Interactions between invertebrate herbivores with different feeding modes are common on long‐lived woody plants. In cases where one herbivore facilitates the success of another, the consequences for their shared host plant may be severe. Eastern hemlock (Tsuga canadensis), a canopy‐dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet‐feeding hemlock woolly adelgid (Adelges tsugae). The effect of adelgid on invasive hemlock‐feeding folivores remains unknown. This study evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, 245 field‐grown hemlocks were surveyed for gypsy moth herbivory damage and laboratory paired‐choice bioassays were conducted. To assess performance, gypsy moth larvae were reared to pupation on adelgid‐infested or uninfested hemlock foliage, and pupal weight, proportional weight gain, and larval period were analysed. Adelgid‐infested hemlocks experienced more gypsy moth herbivory than did uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid‐infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid‐infested foliage gained more weight than larvae reared on uninfested control foliage. These results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range

Seasonal changes in eastern hemlock (\u3cem\u3eTsuga canadensis\u3c/em\u3e) foliar chemistry

Eastern hemlock (Tsuga canadensis (L.) Carriére; hemlock) is an eastern North American conifer threatened by the invasive hemlock woolly adelgid (Adelges tsugae Annand). Changes in foliar terpenes and phenolics were evaluated in new (current year growth) and mature (1-year old growth) hemlock needles during the growing season and into plant dormancy. From April through September, foliar concentrations of non-volatile soluble phenolics, condensed tannins, lignin, mono- and sesquiterpenes α-pinene, camphene, isobornyl acetate, and diterpene resin were quantified. After September, additional analyses of metabolites that continued to differ significantly in new versus mature foliage were carried out. Total soluble phenolic concentration and condensed tannin concentration in new foliage remained low relative to mature foliage throughout the growing season and converged in December. Lignin concentration in new foliage converged with that of mature foliage by July. Concentrations of α-pinene, camphene, isobornyl acetate, and diterpene resin in new foliage converged with mature foliage within one month of budbreak. The convergence of terpene concentrations in new and mature foliage suggests that these metabolites may play a role in herbivore defense during the peak growing season. Conversely, soluble phenolics, including condensed tannins, may defend foliage from herbivory outside of the spring growth period