False Ring Formation in Eastern Hemlock Branches: Impacts of Hemlock Woolly Adelgid and Elongate Hemlock Scale

Herbivores can alter plant physiology through the induction of abnormal wood formation. Feeding by some insects induces the formation of false rings, a band of thick-walled latewood cells within the earlywood portion of the tree ring that reduces water transport. Hemlock woolly adelgid (Adelges tsugae Annand) and elongate hemlock scale (Fiorinia externa Ferris) are invasive insects that both feed on eastern hemlock [Tsuga canadensis (L.) CarriÈre]. Adelges tsugae has a greater effect on tree health than F. externa, but the mechanism underlying their differential effect is unknown. We explored the effects of these herbivores by assessing growth ring formation in branches of trees that had been experimentally infested for 4 yr with A. tsugae, F. externa, or neither insect. We measured false ring density, ring growth, and earlywood: latewood ratios in the two most recently deposited growth rings. Branches from A. tsugae-infested trees had 30% more false rings than branches from F. externa-infested trees and 50% more than branches from uninfested trees. In contrast, branches from F. externa-infested trees and control trees did not differ in false ring formation. Radial growth and earlywood: latewood ratios did not differ among treatments. Our results show that two invasive herbivores with piercing-sucking mouth parts have differing effects on false ring formation in eastern hemlock. These false rings may be the product of a systemic plant hypersensitive response to feeding by A. tsugae on hemlock stems. If false rings are responsible for or symptomatic of hemlock water stress, this may provide a potential explanation for the relatively large effect of A. tsugae infestations on tree health

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

Are Exotic Herbivores Better Competitors? A Meta-Analysis

Competition plays an important role in structuring the community dynamics of phytophagous insects. As the number and impact of biological invasions increase, it has become increasingly important to determine whether competitive differences exist between native and exotic insects. We conducted a meta-analysis to test the hypothesis that native/exotic status affects the outcome of herbivore competition. Specifically, we used data from 160 published studies to assess plant-mediated competition in phytophagous insects. For each pair of competing herbivores, we determined the native range and coevolutionary history of each herbivore and host plant. Plant-mediated competition occurred frequently, but neither native nor exotic insects were consistently better competitors. Spatial separation reduced competition in native insects but showed little effect on exotics. Temporal separation negatively impacted native insects but did not affect competition in exotics. Insects that coevolved with their host plant were more affected by interspecific competition than herbivores that lacked a coevolutionary history. Insects that have not coevolved with their host plant may be at a competitive advantage if they overcome plant defenses. As native/exotic status does not consistently predict outcomes of competitive interactions, plant–insect coevolutionary history should be considered in studies of competition

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

Pretty Picky for a Generalist: Impacts of Toxicity and Nutritional Quality on Mantid Prey Processing

Prey have evolved a number of defenses against predation, and predators have developed means of countering these protective measures. Although caterpillars of the monarch butterfly, Danaus plexippus L., are defended by cardenolides sequestered from their host plants, the Chinese mantid Tenodera sinensis Saussure guts the caterpillar before consuming the rest of the body. We hypothesized that this gutting behavior might be driven by the heterogeneous quality of prey tissue with respect to toxicity and/or nutrients. We conducted behavioral trials in which mantids were offered cardenolide-containing and cardenolide-free D. plexippus caterpillars and butterflies. In addition, we fed mantids starved and unstarved D. plexippus caterpillars from each cardenolide treatment and nontoxic Ostrinia nubilalis Hübner caterpillars. These trials were coupled with elemental analysis of the gut and body tissues of both D. plexippus caterpillars and corn borers. Cardenolides did not affect mantid behavior: mantids gutted both cardenolide-containing and cardenolide-free caterpillars. In contrast, mantids consumed both O. nubilalis and starved D. plexippus caterpillars entirely. Danaus plexippus body tissue has a lower C:N ratio than their gut contents, while O. nubilalis have similar ratios; gutting may reflect the mantid’s ability to regulate nutrient uptake. Our results suggest that post-capture prey processing by mantids is likely driven by a sophisticated assessment of resource quality

Impact of Consuming ‘Toxic’ Monarch Caterpillars on Adult Chinese Mantid Mass Gain and Fecundity

Predators that feed on chemically-defended prey often experience non-lethal effects that result in learned avoidance of the prey species. Some predators are able to consume toxic prey without ill-effect. The Chinese mantid is able to consume cardenolide-containing monarch caterpillars without immediate adverse effects. Although they discard the caterpillars’ gut contents, mantids consume sequestered cardenolides. Although consumption of these cardenolides does not elicit an acute response, there may be long-term costs associated with cardenolide consumption. We tested the hypothesis that consumption of monarch caterpillars will adversely affect adult mantid biomass gain and reproductive condition. We reared mantids from egg to adult and assigned them to one of four toxicity groups that differed in the number of monarch caterpillars offered over a 15-day period. Mantids consumed similar amounts of prey biomass during the experiment. Yet, mantids in the high-toxicity group had a higher conversion efficiency and gained more biomass than mantids in other groups. Mantids in all treatment groups produced similar numbers of eggs. However, mantids in the high-toxicity group produced heavier eggs and devoted a greater portion of their biomass toward egg production than those in the control group. This increase in reproductive condition is probably driven by variation in prey nutritional value and/or the nutritional advantages inherent in eating multiple food types. Our results demonstrate the mantid is able to incorporate ‘toxic’ monarch prey into its diet without acute or chronic ill-effects

Interactions between invasive herbivores and their long-term impact on New England hemlock forests

The introduction of the hemlock woolly adelgid (Adelges tsugae) in the eastern United States has caused extensive damage to eastern hemlock (Tsuga canadensis) stands. The presence of other herbivores can affect adelgid density and hemlock health. Here, we report the results of long-term work monitoring hemlock forests in a 7,500 km2 latitudinal transect of southern New England. In biannual surveys between 1997 and 2011, we assessed change in hemlock basal area, stand-level hemlock defoliation, the density of four exotic herbivores (A. tsugae, Fiorinia externa, Nuculaspis tsugae and an unidentified Nuculaspis sp.) and explored relationships between biotic and abiotic variables. Hemlock basal area decreased by approximately 60 % over the 14 years of the study. Hemlock stand defoliation increased locally in some stands but on average stand defoliation did not change throughout the study period/area. Local increases in defoliation were probably driven by the northward expansion by A. tsugae, and to a certain extent by the co-occurring presence of F. externa and Nuculaspis scales. Average F. externa density increased during the study and also expanded dramatically northward, although its density did not impact hemlock stand defoliation and only reduced hemlock basal area in some stands. Stand-level variation in F. externa densities was most strongly affected by the presence of Nuculaspis scales, which were present in about 80 % of the stands. While A. tsugae continues to be the primary threat to eastern hemlock, its impact needs to be studied in the context of a broader herbivore community