Role of winter temperature and climate change on the survival and future range expansion of the hemlock woolly adelgid (Adelges tsugae) in eastern North America

Global climate change has already affected the abundances, range limits, and interactions of many species. The hemlock woolly adelgid (Adelges tsugae), an invasive insect introduced to eastern North America from Japan, has decimated stands of eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana) from Georgia to Connecticut. However, its spread across central and northern New England has been slowed substantially by its inability to tolerate cold winter temperatures. Using data from previous lab and field studies collected over the past 17 years, including adelgid spread and overwintering mortality, we first characterize the temperature conditions that may limit adelgid spread. We then show how, in the future, rising winter temperatures due to climate change are likely to remove the conditions currently limiting adelgid spread, and facilitate the northward expansion as more suitable habitat becomes available.     

Exploitative competition between invasive herbivores benefits a native host plant

Although biological invasions are of considerable concern to ecologists, relatively little attention has been paid to the potential for and consequences of indirect interactions between invasive species. Such interactions are generally thought to enhance invasives' spread and impact (i.e., the "invasional meltdown" hypothesis); however, exotic species might also act indirectly to slow the spread or blunt the impact of other invasives. On the east coast of the United States, the invasive hemlock woolly adelgid (Adelges tsugae, HWA) and elongate hemlock scale (Fiorinia externa, EHS) both feed on eastern hemlock (Tsuga canadensis). Of the two insects, HWA is considered far more damaging and disproportionately responsible for hemlock mortality. We describe research assessing the interaction between HWA and EHS, and the consequences of this interaction for eastern hemlock. We conducted an experiment in which uninfested hemlock branches were experimentally infested with herbivores in a 2 x 2 factorial design (either, both, or neither herbivore species). Over the 2.5-year course of the experiment, each herbivore's density was approximately 30% lower in mixed- vs. single-species treatments. Intriguingly, however, interspecific competition weakened rather than enhanced plant damage: growth was lower in the HWA-only treatment than in the HWA + EHS, EHS-only, or control treatments. Our results suggest that, for HWA-infested hemlocks, the benefit of co-occurring EHS infestations (reduced HWA density) may outweigh the cost (increased resource depletion).     

Modeling range dynamics in heterogeneous landscapes: invasion of the hemlock woolly adelgid in eastern North America

Range expansion by native and exotic species will continue to be a major component of global change. Anticipating the potential effects of changes in species distributions requires models capable of forecasting population spread across realistic, heterogeneous landscapes and subject to spatiotemporal variability in habitat suitability. Several decades of theory and model development, as well as increased computing power and availability of fine-resolution GIS data, now make such models possible. Still unanswered, however, is the question of how well this new generation of dynamic models will anticipate range expansion. Here we develop a spatially explicit stochastic model that combines dynamic dispersal and population processes with fine-resolution maps characterizing spatiotemporal heterogeneity in climate and habitat to model range expansion of the hemlock woolly adelgid (HWA; Adelges tsugae). We parameterize this model using multiyear data sets describing population and dispersal dynamics of HWA and apply it to eastern North America over a 57-year period (1951-2008). To evaluate the model, the observed pattern of spread of HWA during this same period was compared to model predictions. Our model predicts considerable heterogeneity in the risk of HWA invasion across space and through time, and it suggests that spatiotemporal variation in winter temperature, rather than hemlock abundance, exerts a primary control on the spread of HWA. Although the simulations generally matched the observed current extent of the invasion of HWA and patterns of anisotropic spread, it did not correctly predict when HWA was observed to arrive in different geographic regions. We attribute differences between the modeled and observed dynamics to an inability to capture the timing and direction of long-distance dispersal events that substantially affected the ensuing pattern of spread.     

Ecological boundary detection using Bayesian areal wombling

The study of ecological boundaries and their dynamics is of fundamental importance to much of ecology, biogeography, and evolution. Over the past two decades, boundary analysis (of which wombling is a subfield) has received considerable research attention, resulting in multiple approaches for the quantification of ecological boundaries. Nonetheless, few methods have been developed that can simultaneously (1) analyze spatially homogenized data sets (i.e., areal data in the form of polygons rather than point-reference data); (2) account for spatial structure in these data and uncertainty associated with them; and (3) objectively assign probabilities to boundaries once detected. Here we describe the application of a Bayesian hierarchical framework for boundary detection developed in public health, which addresses these issues but which has seen limited application in ecology. As examples, we analyze simulated spread data and the historic pattern of spread of an invasive species, the hemlock woolly adelgid (Adelges tsugae), using county-level summaries of the year of first reported infestation and several covariates potentially important to influencing the observed spread dynamics. Bayesian areal wombling is a promising approach for analyzing ecological boundaries and dynamics related to changes in the distributions of native and invasive species.     

Effects of a Biological Control Introduction on Three Nontarget Native Species of Saturniid Moths

Abstract: Damage to nontarget (native) invertebrates from biological control introductions is rarely documented. We examined the nontarget effects of a generalist parasitoid fly , Compsilura concinnata ( Diptera: Tachinidae), that has been introduced repeatedly to North America from 1906 to 1986 as a biological control agent against 13 pest species. We tested the effect of previously established populations of this fly on two native, nontarget species of moths ( Lepidoptera: Saturniidae) , Hyalophora cecropia and Callosamia promethea , in Massachusetts forests. We estimated survivorship curves for newly hatched H. cecropia larvae (n = 500), placed five per tree in the field and found no survival beyond the fifth instar. We simultaneously deployed cohorts (n = 100) of each of the first three instars to measure the effect of parasitoids during each stage of development. C. concinnata was responsible for 81% of H. cecropia mortality in the first three instars. We deployed semigregarious C. promethea in aggregations of 1–100 larvae in the field and recorded high rates of parasitism by C. concinnata among C. promethea larvae exposed for 6 days (69.8%) and 8 days (65.6%). We discovered a wild population of a third species of silk moth, the state-listed (threatened) saturniid Hemileuca maia maia, and found that C. concinnata was responsible for 36% (n = 50) mortality in the third instar. Our results suggest that reported declines of silk moth populations in New England may be caused by the importation and introduction of C. concinnata .     

Implicating an introduced generalist parasitoid in the invasive browntail moth's enigmatic demise

Recent attention has focused on the harmful effects of introduced biological control agents on nontarget species. The parasitoid Compsilura concinnata is a notable example of such biological control gone wrong. Introduced in 1906 primarily for control of gypsy moth, Lymantria dispar, this tachinid fly now attacks more than 180 species of native Lepidoptera in North America. While it did not prevent outbreaks or spread of gypsy moth, we present reanalyzed historical data and experimental findings suggesting that parasitism by C. concinnata is the cause of the enigmatic near-extirpation of another of North America's most successful invaders, the browntail moth (Euproctis chrysorrhoea). From a range of approximately 160,000 km2 a century ago, browntail moth (BTM) populations currently exist only in two spatially restricted coastal enclaves, where they have persisted for decades. We experimentally established BTM populations within this area and found that they were largely free of mortality caused by C. concinnata. Experimental populations of BTM at inland sites outside of the currently occupied coastal enclaves were decimated by C. concinnata, a result consistent with our reanalysis of historical data on C. concinnata parasitism of the browntail moth. The role of C. concinnata in the disappearance of browntail moth outside these enclaves has not been reported before. Despite the beneficial role played by C. concinnata in reversing the browntail moth invasion, we do not advocate introduction of generalist biological control agents. Our findings illustrate that the impact of such organisms can be both unpredictable and far-reaching.