Gregory J. Wiggins

Associations Between Causal Agents of the Beech Bark Disease Complex [Cryptococcus fagisuga (Homoptera: Cryptococcidae) and Nectria spp.] in the Great Smoky Mountains National Park

Abstract American beech, Fagus grandifolia Ehrhart, is currently threatened by the insect-mediated disease complex known as beech bark disease. The organisms (beech scale, Cryptococcus fagisuga Lindinger, and two pathogenic fungi, Nectria galligena Bresadola and Nectria coccinea variety faginata Lohman, Watson, and Ayers) associated with beech bark disease were assessed using a qualitative rating system and correlated with other biotic and abiotic factors in 10 permanent plots in the Great Smoky Mountains National Park from spring 1994 through spring 1997. Tree mortality, as well as the presence of another scale species [Xylococculus betulae (Pergande)], also were documented and analyzed. During this study, incidence of C. fagisuga and overall tree mortality increased (55.6–87.9 and 16.0–26.8%, respectively). A forward stepwise logistic regression model selected average overall ratings of C. fagisuga, sum of presence of X. betulae, average presence of. X. betulae, average overall ratings of Nectria spp., maximum presence of X. betulae, sum of south ratings of C. fagisuga, average south ratings of C. fagisuga, and diameter at breast height (dbh) of the tree as significant variables that best explained mortality of American beech. Pearson correlation analysis showed significant associations between ratings of Nectria spp. and C. fagisuga from both the previous season and previous year during fall 1996 and spring 1997. Moderate correlations between elevation, as well as aspect, and north, south, and overall ratings of C. fagisuga from fall 1995 to spring 1997 were documented. The significant association between presence of C. fagisuga and infection by Nectria spp. suggests that control of C. fagisuga could decrease occurrence and slow the spread of beech bark disease. Unless appropriate control and/or management of the disease complex is identified and implemented, mortality of American beech is expected to increase throughout most areas in the Great Smoky Mountains.

Establishment and coexistence of two predators, Laricobius nigrinus and Sasajiscymnus tsugae, introduced against hemlock woolly adelgid on eastern hemlock

Abstract The coexistence of two introduced predatory species, Laricobius nigrinus Fender and Sasajiscymnus tsugae (Sasaji and McClure), and a native predator, L. rubidus LeConte, on eastern hemlock was documented for the first time. Details of their coexistence and implications to management of hemlock woolly adelgid, Adelges tsugae Annand, are discussed.

Recovery of Sasajiscymnus tsugae, released against hemlock woolly adelgid, Adelges tsugae, in the southern Appalachians

Abstract Eastern hemlock in the Great Smoky Mountains National Park is currently threatened by the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae). As part of a management plan against this invasive insect pest, about 350,000 adults of the predatory beetle Sasajiscymnus tsugae (Sasaji and McClure) (Coleoptera: Coccinellidae) were released at ca. 150 sites in the Park from 2002 to 2007. Of these adult release sites, 33 were sampled in 2008 and 2009 using beat-sheet sampling for 4 man-hours. Sasajiscymnus tsugae adults (n=78) and/or larvae (n=145) were recovered from seven sites (21.2% of the release sites sampled). Recovery of S. tsugae was significantly associated with older release sites, with the most beetles recovered from 2002 release sites. These results indicate that S. tsugae may require more time (i.e., 5–7 years) than anticipated for population densities to reach readily detectable levels in some areas.

Host utilization of field-caged native and introduced thistle species by Rhinocyllus conicus.

ABSTRACT Rhinocyllus conicus Fröelich was introduced from Europe into North America as a biological control agent of the exotic weed Carduus nutans L. Concern exists over the feeding of this weevil on at least 25 species of native Cirsium thistles. Beginning in 2008, cage studies isolating adults of R. conicus on buds and flower heads of all eight thistle species (native and introduced) recorded from Tennessee were conducted to test if R. conicus could use these species for reproduction and what impacts larval feeding of R. conicus may have on seed production. Larvae of R. conicus completed development in heads of the native species C. carolinianum (Walter) Fernald and Schubert, and C. horridulum Michaux, and significant reductions in seed numbers of both species occurred during 2008. Rhinocyllus conicus oviposited on both C. carolinianum and C. horridulum at significantly greater levels than the introduced species C. arvense (L.) Scopoli and C. vulgare (Savi) Tenore. Infested heads of C. carolinianum contained numbers of R. conicus per centimeter of plant head width similar to Ca. nutans in 2008, and both native species contained numbers of R. conicus per centimeter of plant head width similar to C. arvense and C. vulgare in 2009. Body length was similar between R. conicus reared on native thistles and its target host Ca. nutans. This report is the first documentation of R. conicus feeding and reproducing on C. carolinianum and C. horridulum. Although R. conicus has been observed only on introduced thistles in naturally occurring populations in this region, the utilization of C. carolinianum and C. horridulum as host species in controlled conditions warrants continued monitoring of field populations and further investigation into factors that may influence nontarget feeding in the future.

Native parasitoids and recovery of Spathius agrili from areas of release against emerald ash borer in eastern Tennessee, USA

The parasitoid Spathius agrili Yang, introduced in the USA to suppress populations of the emerald ash borer (EAB), Agrilus planipennis Fairmaire, has been recovered at a release site for the first time in eastern Tennessee after a single year of releases. Other native parasitoids, including Spathius floridanus Ashmead, undetermined species of Spathius (possibly Spathius elegans Matthews and Spathius parvulus Matthews) and Atanycolus cappaerti Marsh & Strazanac, also known to be associated with EAB, were recovered. These recoveries represent the first documentation of these four species, including the introduced S. agrili, associated with EAB in the southern USA. Implications for biological control efforts against EAB are discussed.

Allothrombium mitchelli (Acari: Trombidiidae) in the Great Smoky Mountains National Park: Incidence, seasonality, and predation on beech scale (Homoptera : Eriococcidae)

Abstract American beech gaps in the southern Appalachian mountains are currently threatened by an insect-mediated disease complex known as beech bark disease. Cryptococcus fagisuga Lindinger, or beech scale, a major component of beech bark disease, wounds trees through feeding on vascular tissue through the outer cambium. This feeding method leaves numerous wounds and provides entryways for infection by fungal pathogens. Allothrombium mitchelli Davis, a large red velvet mite, was found in the Great Smoky Mountains National Park, where it was observed to feed on beech scale. First-instar larvae of A. mitchelli are quiescent, and deutonymphs are the only mobile immature stage. A. mitchelli probably has one generation per year with adult population peaks in late spring and early fall; deutonymph populations also peaked in early fall within the areas studied. Both adults and deutonymphs feed on beech scale. In 1996, mites were found on 22.1% of beech trees (n = 453) in selected plots; incidence was greatest at Sweat Heifer on 9 May (81%, n = 29). Mite incidence was significantly influenced by categorized aspect (χ2 = 57.92, P < 0.0001, n = 453) and categorized elevation (χ2 = 101.33, P < 0.0001, n = 453). A strong significant (0.4528, P < 0.0001) correlation was observed between mite incidence and elevation. A weak but significant negative correlation (−0.1341, P = 0.0001) was found between numbers of A. mitchelli on individual trees and beech scale ratings at Sweat Heifer and Indian Gap plots. Although their status as a biological control agent of beech scale is uncertain, A. mitchelli is one of only a few species that has been observed to feed consistently on beech scale.

Spatial Prediction of Habitat Overlap of Introduced and Native Thistles to Identify Potential Areas of Nontarget Activity of Biological Control Agents

ABSTRACT Nontarget feeding of Rhinocyllus conicus Fröelich and Trichosirocalus horridus (Panzer) on native North American thistles in the genus Cirsium has been documented. Some species of these native thistles have shown greater infestation levels of R. conicus in populations that are in close proximity to the target plant species, Carduus nutans L. In 2005 a study was initiated to identify areas of potential nontarget feeding by R. conicus and T. horridus on thistle species by predicting habitats of two known introduced hosts | C. nutans and Cirsium vulgare (Savi) Tenore | and two native species [Cirsium carolinianum (Walter) Fernald and Schubert and C. discolor (Muhlenberg ex Willdenow) Sprengel] using Mahalanobis distance (D2). Cumulative frequency graphs showed that the D2 models for all four plant species effectively identified site conditions that contribute to the presence of the respective species. Poisson regression showed an association between D2 values and plant counts at field-test sites for C. nutans and C. carolinianum. However, negative binomial regression detected no association between D2 values and plant counts for C. discolor or C. vulgare. Chi-square analysis indicated associations between both weevil species and sites where C. vulgare and Carduus nutans were found, but not between the weevil and native thistle species. Habitats of C. nutans and Cirsium carolinianum overlapped in ≈12% of the study area. Data-based habitat models may provide a powerful tool for land managers and scientists to monitor native plant populations for nontarget feeding by introduced biological control agents.

First documentation of adult Trichosirocalus horridus on several non-target native Cirsium species in Tennessee

Abstract Releases of Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), native to Europe, began in Tennessee in 1989 as part of a biological control program against musk thistle (Carduus nutans L.). In surveys conducted to investigate non-target feeding of T. horridus on native Cirsium thistle species from 2005 to 2008, adults of T. horridus were observed on all five native Cirsium thistles. These adult occurrences are the first documentation of T. horridus occurring on three of these native species [C. carolinianum (Walt.) Fern & Schub., C. horridulum Michx. and C. muticum Michx.], and the first record of T. horridus occurring in the Great Smoky Mountains National Park. While C. carolinianum and C. horridulum did not show symptoms of larval feeding in the meristematic tissues, C. altissimum, C. discolor, and C. muticum all had damaged meristems and possible oviposition scars on the midribs of the leaves. However, the impact of feeding by larvae of T. horridus on the reproductive potential of native plants is uncertain, because even in the target species (musk thistle and other introduced Cirsium species) plant death only sometimes occurs, and seed production continues.

Emergence, seasonality, and hybridization of Laricobius nigrinus (Coleoptera: Derodontidae), an introduced predator of hemlock woolly adelgid (Hemiptera: Adelgidae), in the Tennessee Appalachians

Abstract From 2010 through 2013, adult emergence and seasonality of Laricobius nigrinus Fender, an introduced predatory species native to western North America, as well as hybridization with the native species Laricobius rubidus (LeConte), were evaluated using emergence traps and beat-sheet sampling in areas of previous release against hemlock woolly adelgid, Adelges tsugae Annand. The shortest emergence period of adult L. nigrinus was 7 wk beginning 22 October 2010, and the longest emergence was 15 wk beginning 17 October 2012. Native L. rubidus also were collected from emergence traps placed on the ground surface and beat-sheet samples all 3 yr, with emergence of L. rubidus initiating later than L. nigrinus each season. Seasonality of both Laricobius species was similar across a 44-mo study period. Adult L. nigrinus were present from October through April, and larvae of Laricobius spp. were collected from February to May. The average number of L. nigrinus from emergence traps was significantly greater than the average number of beetles collected from beat-sheet samples in 2010, while the converse was observed during 2012. Hybridization between L. nigrinus and L. rubidus was documented from 10.75% of specimens collected during 2010 and 2011, indicating periodic interbreeding between the introduced and native species. These findings suggest emergence trapping may be a useful method to assess establishment, population densities, and seasonality of Laricobius species in areas of release to enhance their use in management of A. tsuage.

Development of microsatellite loci in Pityophthorus juglandis, a vector of thousand cankers disease in Juglans spp.

Using next-generation sequencing, 18 microsatellite loci were developed and characterized for walnut twig beetle, Pityophthorus juglandis, a vector of thousand cankers disease (TCD) affecting Juglans spp. Although all Juglans species are susceptible to TCD infection, native populations of J. nigra and J. cinerea, which is endangered in Canada, are most susceptible and threatened by habitat loss. Novel primers amplified di-, tri-, and tetra nucleotide repeats and detected 4–14 alleles per locus. Averaged observed and expected heterozygosity was 0.22 and 0.67, respectively. Our results indicate that P. juglandis microsatellite loci can be used to investigate genetic diversity and population structure of this vector across a widespread geography. These markers will be useful tools for evaluating genetic structure of P. juglandis population outbreaks and developing appropriate conservation strategies. Microsatellite loci obtained in this study can also be utilized to determine relationships of P. juglandis to other closely related Pityophthorus spp.