Hemlock woolly adelgid is an invasive herbivore causing extensive mortality of eastern hemlock, an important foundation species that provides stable conditions influencing biological communities. Hemlock is often found in riparian areas and following its decline, broadleaved species, including birch, beech, and rhododendron, will replace it. These plants differ from hemlock in patterns of canopy cover and leaf properties, which influence conditions and resources within streams. My goal was to evaluate potential impacts of adelgid-induced alterations to riparian canopies and litter on benthic communities and litter breakdown in streams. I characterized benthic invertebrate communities, litter colonization and litter breakdown in streams with hemlock- or deciduous-dominated riparian canopies. Riparian canopy influenced abundance of some invertebrates, but litter species influences a range of benthic colonizers. Rhododendron and beech litter generally support more invertebrates and decomposes more slowly than birch or hemlock. When invertebrates are excluded, broadleaved litter breakdown is more hindered than hemlock breakdown. My findings suggest that invertebrates may be more affected by future increases in broadleaved litter inputs to streams than by hemlock litter loss. This is significant because benthic invertebrates are important for in-stream litter processing and are linked to aquatic and terrestrial food webs.

To investigate the density-dependent factors that may be partly responsible for the apparent stability of hemlock woolly adelgid (HWA; Adelges tsugae Annand) populations in central New England, we infested 64 eastern hemlock (Tsuga canadensis Carrière) trees with varying densities of HWA sistens ovisacs in a typical eastern hemlock forest in western Massachusetts. We subsequently documented HWA density, fecundity, and the amount of new growth on experimental trees over two consecutive years. We used a 2 x 2 randomized block design using previously and newly infested hemlocks divided into 1 m tall saplings and branches of mature trees. There was a density-dependent decline in the survival and fecundity of HWA in both the spring and winter generations. This response was a function of both previous infestation by HWA and current years crawler density in the spring generation. Additionally, the production of sexuparae in the spring generation played a key role in the overall density-dependent survival of HWA, suggesting that sexuparae production is strongly linked to developing crawler density. In Chapter 2, varying densities of HWA were manipulated on 16 previously uninfested eastern hemlocks in an open field plantation. In contrast to HWA populations in our forest experiment, there was no evidence of density-dependent survival on a tree-wide basis in the plantation in the springtime progrediens generation. However, there were comparable density-dependent survival of settled crawlers and sexuparae production when samples of the population were examined from branches with high density. Plantation hemlocks had 9.3 times more foliage and ten times lower HWA densities per cm than forest hemlocks. Despite the lack of density-dependence in the progrediens generation of this study, HWA populations of the subsequent sistens generation went extinct on 13 out of the 16 trees. These results show that density-dependent processes may only exist when HWA density/cm reaches a certain threshold, and that high mortality may occur during the late-summer aestivation phase at densities below this threshold, perhaps due to high summertime temperatures in HWA’s sistens generation. These results may help explain the slow process of HWA establishment in our region.

Throughout the eastern United States, the spread of the hemlock woolly adelgid, Adelges tsugae Annand, has caused high mortality of eastern hemlocks Tsuga canadensis (L.). We recorded overwintering survival and fecundity of A. tsugae, and tree new growth at sites in the northeastern and southeastern United States and in a common garden experiment in Massachusetts. Overwintering mortality of A. tsugae was much higher in the north (87%) than the south (37%) in 2009, and showed significantly positive density-dependence in the north only. In 2010, overwintering mortality decreased in both regions but remained higher in the north (54%) than the south (34%), and, unlike 2009, density-dependent mortality was strongly negative in the north, and positive in the south. In both years, sistens fecundity was significantly higher in the south than the north, but we observed no density-dependent trends, and fecundity measurements were similar in the two years. The regional discrepancies in fecundity suggested the possibility of an evolutionary trade-off between overwintering mortality and sistens fecundity. However, when we reared samples in a common garden, we found that source region had no effect on either sistens fecundity or overwintering mortality, which suggests observed regional differences can be attributed to environmental factors rather than genetic differences. In our regional studies, branch samples from the north had significantly more new growth than those from the south in both 2009 and 2010, even though A. tsugae densities were comparable. This difference persisted in the common garden wherein branches inoculated with northern-derived A. tsugae had significantly higher new growth than those infested with southern-derived A. tsugae. These findings raise the possibility that A. tsugae may be evolving towards reduced virulence. If true, these findings may help explain why A. tsugae is killing hemlocks much more slowly now in New England than it did when it first invaded this region more than 20 years ago, or as it is doing now in more recently invaded regions in the southern United States.

No description available.

Hemlock woolly adelgid, Adelges tsugae, native to Asia and the Pacific Northwest of North America (Pacific Northwest), has devastated eastern hemlock (Tsuga canadensis) in a major portion of its range in the eastern U.S. After many years and much effort directed towards rearing and releasing biological control agents to manage HWA, one of these agents, Laricobius nigrinus, native to the Pacific Northwest, is now well-established in sites from the southern to the mid-Atlantic states of the eastern U.S. However, there have yet to be studies of its efficacy in lowering A. tsugae densities, and there has been no noticeable drop in A. tsugae densities. Population models for A. tsugae have suggested that even upwards of 90% predation on eggs laid by the overwintering generation will have minimal effect in reducing the population densities of A. tsugae, if A. tsugae are at high density, due to compensatory density-dependent survival in the progrediens generation. Additionally, no studies showing insect predators are indeed what regulate A. tsugae in its native range exist. We established predator exclusion experiments, and recorded A. tsugae densities, mortality factors, and fecundity for multiple generations in both the native and invaded ranges. In the invaded range, we studied A. tsugae populations in sites with well-established populations of L. nigrinus to test its efficacy in reducing A. tsugae and tested model predictions of minimal difference in A. tsugae densities between treatments with and without predators. In the Pacific Northwest we tested the effect of insect predators and tree species, western (Tsuga heterophylla) and eastern hemlock, on populations of A. tsugae. In the invaded range we found that L. nigrinus predation was significantly higher in unbagged branches, however, model predictions were validated, and there was no effect of predation by treatment on the A. tsugae summer generation. In our plots in the Pacific Northwest we found that tree effects were not significant, but that summer-active predators were significantly lowering levels A. tsugae densities on unbagged branches. Our study demonstrates the importance of summer-active predators in reducing A. tsugae and suggest that summer- and winter-active predators are needed to suppress A. tsugae to innocuous densities.

Eastern hemlock hosts the hemlock woolly adelgid, an introduced sap-feeding insect that causes rapid deterioration of the host. Like most conifers, eastern hemlock produces a variety of constitutive and induced defenses, primarily terpenoids. To explore the relationship of terpenoid defenses with adelgid infestations, we artificially infested hemlocks at a forest site and a plantation site, and compared their terpenoid concentrations to those in control trees. Infested trees showed lower terpenoid concentrations than control trees, suggesting that eastern hemlock not only fails to induce production of terpenoids in response to adelgid infestation, but becomes less able to produce carbon-based defenses due to loss of carbon resources to the adelgid. Greater light intensity may account for consistently higher terpenoid concentrations at the plantation site, supporting the explanation that carbon limitation restricts terpenoid production. Recent studies have identified a small number of individual eastern hemlock trees that demonstrate relative resistance to the hemlock woolly adelgid. We compared concentrations of terpenoids in susceptible and relatively resistant trees, both in the forest and in propagated cuttings in a common-garden setting. Terpenoid concentrations were higher in twig tissue of resistant versus susceptible trees, across six sampling dates and at both sites. Because the common-garden cuttings were free of herbivores, the higher terpenoid concentrations are interpreted as a constitutive defense. Increased levels of monoterpenes and sesquiterpenes imply an overall increase in the input of carbon precursors to both terpenoid synthesis pathways. This result suggests either an altered growth-defense balance favoring allocation of carbon resources towards production of defenses, or overall greater carbon availability in growing twig tissue of adelgid-resistant eastern hemlock individuals. We contribute detailed terpenoid data to the study of the eastern hemlock – hemlock woolly adelgid system. Our solvent extraction method permits us to examine needle and twig tissues separately, capture minor components at low concentrations, and focus on stored rather than volatilized terpenoids. By relating terpenoid concentrations to insect densities, we explore the relationships of tentatively defensive chemistry to insect population dynamics. The question remains which terpenoids, if any, directly affect hemlock woolly adelgid and what role phenols may play in the system.

Different Laricobius nigrinus Fender release locations, numbers of predators, and timing of release were evaluated for biological control of the hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae). It established at 59% of the sites and location was the most important factor related with establishment and abundance, HWA density, and hemlock vigor index. Cold locations had poor establishment or low abundance, declines in HWA density, and increases in hemlock vigor over time. Paired release and control sites detected a predator impact on HWA density, but densities remained high and tree vigor declined. The phenology of L. nigrinus, L. rubidus LeConte, and HWA were studied at a field insectary and the species were highly synchronized. A cage exclusion study showed that HWA survival and density were lower and ovisac disturbance was higher when exposed to predation. To improve L. nigrinus monitoring, we compared beat sheets for adults or branch clipping for immatures, and the host searching behavior of L. nigrinus was studied to understand how it locates a tree and HWA. In the Appalachians, beat sheet sampling resulted in false negatives as larvae were collected by branch clipping. Adults orientated to a tree visually, fed when prey were present and flew when absent, and showed different search patterns on infested versus uninfested trees. In Seattle, both sampling methods detected L. nigrinus because the predator was common. Predator : prey ratios were high at heavily infested sites in Seattle and low in the eastern US, where is has been released recently. Partial life tables were constructed for HWA sistentes at four sites for 2 yr in Seattle. Unspecified causes of nymph and adult mortality were high and L. nigrinus was the dominant predator of ovisacs. Adult L. nigrinus abundance was positively related to HWA density and immature abundance was related to ovisac density, indicating an aggregation and numerical response to its prey. Laricobius nigrinus has not demonstrated complete biological control of HWA to date, but it may do so in the future and continued release is justified. / Ph. D.

Hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), is an introduced pest injurious to hemlock trees (Tsuga spp.) in the eastern United States. HWA currently infests hemlock in over 50% of its geographic range and has the potential to spread throughout its entire range. Since HWA populations in the eastern United States are not regulated effectively by natural enemies (Wallace and Hain 2000), classical biological control is the most promising option for controlling this pest in the forest setting. This work evaluates Laricobius nigrinus Fender (Coleoptera: Derodontidae), a predator associated with HWA in western North America, is being evaluated as a potential biological control agent for HWA (Cheah et al. 2004). Observations suggest that natural enemies may help keep HWA populations below injurious levels in the western United States (Mausel, pers. comm.). Laboratory studies have revealed that L. nigrinus feed selectively on HWA and field studies in British Columbia, Canada have shown that its lifecycle is highly synchronous with HWA phenology. This predator is being further evaluated for its suitability as a biological control agent. In order to be a viable candidate, L. nigrinus must survive under natural field conditions in the eastern United States, reduce the density of HWA, and it must be amenable to mass rearing. The survival, reproductive capability and predator efficiency of L. nigrinus were assessed over 3 years in the field. In addition, the environmental factors regulating processes in the life cycle to develop and improve rearing procedures for this insect were identified. In the first year, adults survived from February - May, laid up to 41 eggs/beetle, and consumed approximately 4.3 adelgids per day. In the second season, L. nigrinus adults survived from November - April, laid up to 38 eggs/beetle and consumed approximately 4.5 adelgids per beetle throughout the study. In both seasons, adelgid populations were significantly lower on branches with predators than those without predators. This impact was demonstrated on both the winter and spring generation of HWA. The feasibility of caged field releases of L. nigrinus was determined in a third field study. An estimated 10, 000 L. nigrinus eggs were liberated in field cages in spring 2003. The density of adelgids in the subsequent (spring) generation of the adelgids was significantly lower on branches with larval activity than those with no predators. Despite extensive sampling, no F1 adults were observed, however F2 adults were recovered in the fall of 2004, 20 months after release. The L. nigrinus life stages incurring high mortality during rearing were identified and factors affecting survival in the feeding and non-feeding life stages were investigated. Studies on the survival and feeding of adults, length of ovipositional period, density per cage, and survival of larvae were conducted. The effect of type of pupation medium, moisture level, disturbance, soil sterilization, temperature, and photoperiod on survival of the non-feeding stages and time of emergence from aestivation was examined. The most noteworthy finding, regarding rearing, is that aestivation can be extended by storing adults at high temperatures and long daylength throughout the summer and decreasing the temperature and daylength in the fall. Based on the results of these studies, procedures for rearing L. nigrinus have been developed and it is currently being reared at two other institutions. To date, over 8, 000 adults produced at Virginia Tech have been released in 6 states and both F1 and F2 adults have been recovered from multiple locations. / Ph. D.

The biology and host-specificity of <i>Laricobius kangdingensis</i> sp. n. (Coleoptera: Derodontidae), a new predator of hemlock woolly adelgid (HWA), <i>Adelges tsugae</i> Annand (Homoptera: Adelgidae) were investigated and compared with that of a related predator of HWA, <i>L. nigrinus</i> Fender. Adults became active in June 2003; the males emerged first and in greater numbers than the females. The sex ratio of female to male was 1: 1.1. <i>Laricobius kangdingensis</i> sp. n. has four larval instars. The total number of eggs laid between 7 July 2003 and 7 August 2004 was 1327. Mean lifetime fecundity per female was 196.4 ± 53.4 eggs and 97.9 eggs for the P1 (parental gen.) and F1 generations, respectively. In comparison, lifetime fecundity for F1 <i>L. nigrinus</i> was 100.8 ± 89.6 eggs. Developmental rates were studied in fall 2003 and spring 2004. Combined with data gathered previously at 12 and 15ºC in early 2003, the 6, 9, and 18ºC data were used to determine lower temperature development thresholds for the egg (2.8ºC), larval (1.6ºC), and pre-pupal (5.8ºC) stages. The egg and larval thresholds are lower than those observed for <i>L. nigrinus</i>. <i>Laricobius kangdingensis</i> sp. n. individuals completed development only at 12 and 15ºC; however, these adults did not emerge from aestivation after pupation. <i>Laricobius kangdingensis</i> sp. n. appears to be host-specific. In paired-choice feeding tests, adults consumed significantly more HWA than two other adelgid species [eastern spruce gall adelgid, <i>Adelges abietis</i> (L.), and pine bark adelgid, <i>Pineus strobi</i> (Hartig)], the pine needle scale, <i>Chionaspis pinifoliae</i> (Fitch), and the woolly alder aphid, <i>Paraprociphilus tessellatus</i> (Fitch). In paired-choice oviposition tests, females laid more eggs on HWA-infested eastern hemlock than on any other test prey. In no-choice host suitability experiments larvae fed on HWA; eastern spruce gall adelgid; the larch adelgid, <i>A. laricis</i> Vallot; the balsam woolly adelgid, <i>A. piceae</i> (Ratzeburg); pine bark adelgid; and pine needle scale, but not on woolly alder aphid or elongate hemlock scale, <i>Fiorinia externa</i> Ferris. Although no larvae completed development in any of the bioassays, 6 out of 15 larvae (40%) in the HWA treatment reached the pre-pupal stage. One larva out of 6 reached the pre-pupal stage on pine bark adelgid. / Master of Science

The hemlock woolly adelgid (HWA), Adelges tsugae Annand, is an invasive pest of eastern hemlock, Tsuga canadensis (L.) Carriere and Carolina hemlock Tsuga caroliniana Englem. in the eastern United States. A newly reported beetle predator for HWA, Scymnus (Pullus) coniferarum Crotch (Coleoptera: Cocinellidae) preys on the pest in the western United States, and was approved for release in the eastern United States for the control of HWA. This research investigated the viability of S. coniferarum as a biological control agent of A. tsugae in the eastern United States, as well as the ecological dynamics between S. coniferarum and host prey species in its native range of western North America. In objective one, S. coniferarum predation, reproductive potential, and survival were evaluated in field-cages on adelgid infested T. canadensis in southwestern Virginia. Adult S. coniferarum fed on both generations and all life stages of A. tsugae at rates comparable to other adelgid-specific predators, and survived for extended periods of time in the field. In objective two, host-range tests for S. coniferarum were conducted in a series of no-choice and paired-choice feeding, oviposition, and development studies. Scymnus coniferarum adults fed on all adelgid species, and completed development on HWA and Adelges piceae Ratz. Scymmnus coniferarum oviposition was extremely low. In the final objective, Douglas-fir, Pseudotusga menziesii Mirb., Shore pine, Pinus contorta Dougl., western white pine, Pinus monticola Dougl., and western hemlock, Tsuga heterophylla (Raf.) Sarg. host tree species were sampled in Tacoma, Washington to investigate the life history of S. coniferarum and associated adelgid prey species in the western United States. Scymnus coniferarum adults were found on both pine species, Douglas fir, and western hemlock, and seemed to move between host tree species seasonally. Each host tree supports a different adelgid species, and a limited diet of strictly HWA in host-range tests could have contributed to low oviposition rates. This study suggested that S. coniferarum is a voracious predator of HWA in the field and laboratory. However, S. coniferarum laid very few eggs in laboratory studies, and zero eggs were recovered in field-cage analyses. This suggested that S. coniferarum may rely on multiple adelgid species to reproduce and establish in the eastern United States. / Ph. D. / Biological control is an economically and environmentally logical approach to pest management, through the introduction of a natural enemy or predator. This dissertation investigated the potential of Scymnus (Pullus) coniferarum as a biological control agent of the invasive pest species, hemlock woolly adelgid, otherwise known as HWA. The hemlock woolly adelgid is an invasive pest insect in the eastern United States, responsible for causing widespread death of eastern hemlock trees from southern Maine to Georgia. Scymnus coniferarum is a lady beetle that preys on HWA in the western United States, part of HWA’s native range. In objective one, S. coniferarum predation, reproductive potential, and survival were evaluated in field-cages on HWA-infested eastern hemlock trees in southwestern Virginia. Adult S. coniferarum beetles fed on HWA year-round, at a rate comparable to other successful biological control agents. In objective two, S. coniferarum beetles were confined to a selection of insect species similar to HWA and native to the eastern United States. Scymnus coniferarum predation, oviposition and development was observed in these tests in order to assess the potential impact of S. coniferarum on non-target species in the eastern United States. Scymnus coniferarum beetles fed on all adelgid species, and successfully laid eggs and developed to adults while feeding on the balsam woolly adelgid, another pest species in the eastern United States. Adult beetles laid very few eggs, and were very difficult to rear in a laboratory setting. In the final objective, Douglas-fir, Shore pine, western white pine, and western hemlock trees were sampled in Tacoma, WA to investigate the life history of S. coniferarum and other adelgid prey species in the western United States. Scymnus coniferarum adults were found on every species of sample tree except for Douglas-fir, and seemed to move among host trees seasonally. Host tree species support different adelgid species, and a limited diet of strictly HWA in host-range tests could have contributed to low oviposition rates. The lady beetle S. coniferarum was a voracious predator of HWA in field and laboratory studies. However, S. coniferarum laid very few eggs in laboratory studies, and zero eggs were recovered in field-cage analyses. This suggested that S. coniferarum may rely on multiple adelgid species to reproduce and establish in the eastern United States. This had either positive or negative implications regarding its viability to establish, flourish, and impact HWA in the eastern United States.