Search

In the eastern United States, hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae) is considered an invasive pest of eastern hemlocks; an ecologically foundational tree species. Current management of HWA focuses on chemical and biological controls, with recent research suggesting that these two tactics could be integrated successfully. The approach is to protect a subset of hemlocks with systemic insecticides while releasing predatory insects onto adjacent, unprotected trees. The goal of this study was to assess the effects of chemical and biological control tactics, alone and in combination, on hemlock health and HWA densities at three southern Appalachian sites (KY, WV, and TN) from 2010 to 2016. Although insecticide applications were effective at protecting individual trees, none of the overall treatments (chemical, biological, or combined) had a significant effect on tree health or HWA population index values relative to untreated plots. Tree health generally declined at all sites over time. HWA populations were highly variable over time and were likely more strongly influenced by extremely low, winter temperatures than by the treatments. Cross-correlation analysis of tree health and HWA population indicated a time-lag effect. At two of the three sites, recovery of tree health lagged 0 – 3 years behind decline in HWA population, and decline in HWA populations lagged approximately 0 – 1 years behind decline in tree health. The predatory beetle, Laricobius nigrinus , was recovered two-years, post-release at the KY and WV sites in 2012 and 2013, but was not recovered from the TN site. The lack of sustained recovery of L. nigrinus may be attributable to the occurrence of extremely low, winter temperatures in 2014 and 2015, which produced subsequent crashes in the HWA populations. In TN, the L. nigrinus population may have been unrecoverable due to a decline in the HWA population shortly after initial release. [ABSTRACT FROM AUTHOR]

Thousand cankers disease (TCD), caused by the walnut twig beetle (WTB), Pityophthorus juglandis , and its associated fungal symbiont, Geosmithia morbida , is a disease of economic and ecological concern on eastern black walnut, Juglans nigra . Numerous attacks and gallery formation by the WTB and subsequent development of cankers caused by the fungus result in progressive crown dieback. The disease can kill affected trees in a few to several years following initial infestation and very few management options are available for preventing or reducing impact of TCD on black walnut trees. Since advanced development of TCD requires multiple WTB generations and numerous beetle attacks, control strategies that reduce beetle attacks and brood production, without completely eliminating infestation, could still significantly benefit tree health and survival. We evaluated the use of entomopathogenic fungi Beauveria bassiana and Metarhizium brunneum against the WTB. Laboratory and field studies showed that WTB adults are susceptible to commercial strains B. bassiana GHA and M. brunneum F52. Exposure of beetles to sprayed walnut bolts resulted in reduced brood production, primarily due to the death of parental adults from fungal infection prior to egg laying. Spraying walnut bolts with B. bassiana GHA and M. brunneum F52 prior to field exposure to natural WTB populations reduced emergence of next generation adults by up to 98% and 96%, respectively, due to a combination of fewer beetle attacks and mortality among those that tunneled. These results demonstrate the potential use of entomopathogenic fungi in the integrated management of TCD in walnut trees. [ABSTRACT FROM AUTHOR]

The rapid loss of eastern hemlock ( Tsuga canadensis ) due to infestation with hemlock woolly adelgid ( Adelges tsugae , HWA) has greatly altered structure and function of eastern forests. Numerous control strategies including local pesticide use and biocontrol with predator beetles have been implemented with considerable cost and varying success. Silviculture treatments that increase incident light on surviving hemlock trees to reduce infestation and ameliorate carbon starvation have been proposed as another possible conservation strategy, yet no controlled studies have evaluated the coupled plant-insect responses to increased light. We conducted a nursery experiment on artificially infested eastern hemlock seedlings under varying levels of ambient light, ranging from 0 to 90% shade. We measured HWA infestation (ovisac density), short- and long-term indicators of carbon balance (leaf chlorophyll fluorescence, net photosynthesis, total nonstructural carbohydrate content, and shoot growth), and nutrition (leaf N content). We hypothesized that higher light would result in reduced HWA densities, higher C assimilation rates, and improved tissue non-structural carbohydrate balance; and these effects would ameliorate the effects of infestation on C balance and lead to improved seedling growth. HWA density decreased with increasing light, and was highest in the 90% shade treatments. However, photosystem II efficiency and net assimilation were also lower under the higher light treatment. Despite tradeoffs between reduced infestation and reduced leaf function from higher light, and little variation in sugar content among treatments, both leaf and root starch content and seedling growth were higher under the higher light treatment. Increasing light levels improves long-term carbon balance for hemlock seedlings in the presence of HWA. Although hemlock typically occurs in deeply shaded forests, our results suggest that silvicultural treatments such as forest thinning that increase light exposure may reduce HWA abundance and lead to better tree C balance, and may be an effective component of large-scale conservation and restoration strategies. [ABSTRACT FROM AUTHOR]

An integrated management approach is needed to maintain eastern hemlock ( Tsuga canadensis (L.) Carrière) in eastern North America and to minimize tree damage and mortality caused by the invasive hemlock woolly adelgid ( Adelges tsugae Annand). This study examined the hypothesis that chemical control with low rates of insecticide and biological control can be combined in the same stand to impact adelgid populations, prolong crown health, and allow predator proliferation. Sixty T . canadensis trees in northern Georgia were individually treated via soil injection with 0%, 10%, or 25% of the label rate of imidacloprid insecticide, and the biological control predator Laricobius nigrinus Fender was released in the stand, two and four years later. By year seven, hemlocks treated with the 25% imidacloprid rate lost their insecticide protection, had significantly better crown health and higher adelgid densities than untreated trees, and supported as many Laricobius predator larvae as untreated trees. In year seven, no residues of imidacloprid were detected in Laricobius larvae feeding on previously-treated hemlocks. Most (77%) of the predators collected on study trees were identified as L . nigrinus , 12% were the native congener Laricobius rubidus LeConte, and 11% were hybrids between the introduced and native species. The hybridization rate remained stable over time. The density of undisturbed A . tsugae ovisacs was twice as high on branches protected from predators as compared with branches exposed to predators. Results suggest that chemical and biological control of A . tsugae can be successfully integrated to help prolong hemlock health, although additional predators may be necessary to protect hemlock trees in the southern Appalachians. [ABSTRACT FROM AUTHOR]