The hemlock woolly adelgid (HWA) Adelges tsugae (Annand) (Hemiptera: Adelgidae), has spread throughout most of the range of eastern hemlocks, Tsuga canadensis (L.) and the entire range of Carolina hemlocks, Tsuga caroliniana (Engelman) in eastern North America. Forest managers often implement an integrated pest management (IPM) strategy that combines chemical, silvicultural, and biological control tactics to create a more sustainable and effective approach for managing HWA. Laricobius spp. (Coleoptera: Derodontidae) are the primary biological control agents within IPM scenarios for HWA. Imidacloprid (IMI), the most widely used insecticide for HWA management, is typically applied to the soil below HWA infested trees and breaks down into several metabolites, including two with insecticidal properties; 5-hydroxy imidacloprid (5H-IMI) and imidacloprid-olefin (IMI-OLE). Imidacloprid-olefin is known to be up to 14 times as lethal to HWA as IMI and is produced as imidacloprid is first hydroxylated into 5H-IMI and further metabolized into IMI-OLE. Because Laricobius spp. spends half of its life in aestival diapause in soil below HWA infested trees, there is the potential for these biological control agents to encounter toxic residues within an IPM setting. Imidacloprid and its metabolites are known to cause mortality in Laricobius spp. feeding on HWA from previously treated trees, but the impact of soil applied imidacloprid on their subterranean survivorship has not been studied. Furthermore, there is limited knowledge on the subterranean portion of Laricobius spp. life cycle. The first study determined the depth to which Laricobius spp. burrowed during their subterranean life phase using 3D X-ray microscopy. When held in containers with two differing soil compaction intensities, 0.2 kg/cm2 and 0.5 kg/cm2, Laricobius spp. were found to burrow an average (± SE) of 2.7 ± 1.5 cm and 1.4 ± 1.3 cm deep, respectively. The second study assessed the effect of three different imidacloprid soil treatments; soil injection, soil drench and CoreTect tablets, on the subterranean survivorshiop of Laricobius spp. The soil drench application technique applied in fall 2020 consistently resulted in significantly higher IMI, 5H-IMI, and IMI-OLE residues than the other two treatments over both years of this study (2021 and 2022). The soil injection treatments from both 2020 and 2017 resulted in above average field emergence of beetles in 2021 and 2022, with relatively insignificant residue quantities present. This study suggests that the soil injection treatment method provides a lower level risk of metabolite exposure to Laricobius spp. compared to soil drench and CoreTect tablet imidacloprid application methods. / Master of Science in Life Sciences / In eastern North America, there are two species of hemlock trees whose longevity and unique shade tolerant adaptations are important to the ecosystems within which they are found. Eastern hemlocks range from southeastern Canada to the southern Appalachian Mountains, while Carolina hemlocks are restricted to the southern Appalachians from northern Georgia to Virginia. Since its first formal identification in Richmond, VA in 1951, the hemlock woolly adelgid (HWA) has caused widespread damage and mortality throughout much of the range of eastern hemlocks and the entire range of Carolina hemlocks. HWA consumes nutrients and restricts water flow within the tree, eventually leading to tree mortality. Forest managers typically rely on an integrated approach, that combines chemical applications with the simultaneous release of insects that naturally attack HWA on untreated trees within the same forest setting. Imidacloprid is the most widely used insecticide for HWA management and is typically applied to the soil below HWA infested trees. This insecticide is absorbed through the roots and travels through the tree where HWA feed and die as a result. Laricobius spp., the primary insect consuming HWA on infested trees, spends part of their life in a below ground summer dormancy. Imidacloprid and its secondary compounds are known to cause mortality in Laricobius spp. feeding on HWA from treated trees, but the impact of soil applied imidacloprid on their below ground survivorship has not been studied to date. Furthermore, while extensive studies have taken place to understand the biology of Laricobius spp., there is limited knowledge about their dormant state. In one study, the depth to which Laricobius spp. burrowed during their below ground life phase was determined using 3D X-ray microscopy. In a second study, the effect of three common imidacloprid treatments; soil injection, soil drench, and CoreTect tablets, on below ground survivorship was examined. Laricobius spp. were found to burrow an average (± S.E.) of 2.7 ± 1.5 cm and 1.4 ± 1.3 cm deep in soil compacted at 0.2 kg/cm2 and 0.5 kg/cm2, respectively. Chemical residue quantities were greatest in soil treated with soil drench in fall 2020, and were found to significantly impact Laricobius spp. survival. Overall, survival was the lowest from the soil treated with CoreTect tablets in 2020. Surival was the greatest from soil that received soil injection treatments in both 2020 and 2017 and had relatively insignificant imidacloprid residues present where the insects typically occur in the soil. Findings from this study suggest that imidacloprid application using the soil injection method is a less risky technique for chemical control in an IPM setting when compared to soil drench and CoreTect methods.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/114947 |
| Date | 05 May 2023 |
| Creators | Hillen, Ashleigh Paige |
| Contributors | Entomology, Salom, Scott M., Mayfield, Albert, Gross, Aaron Donald, Xia, Kang |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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