Semester

Fall

Date of Graduation

2021

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Division of Plant and Soil Sciences

Committee Chair

Yong-Lak Park

Committee Member

Doanld Brown

Committee Member

Petra Wood

Committee Member

Zachary Freedman

Abstract

The hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), is an invasive insect that is causing mortality of eastern hemlock trees, Tsuga canadensis, and Carolina hemlock trees, Tsuga caroliniana, across the eastern United States. To protect these ecologically important tree species, a neonicotinoid insecticide, imidacloprid, is commonly used. Imidacloprid is an effective treatment and can remain effective against HWA for four to six years but long-term (≥ one year after application) non-target effects of imidacloprid on forest ecosystems are not well-studied. This study examined terrestrial non-target effects of imidacloprid in hemlock stands with different treatment histories to help forest managers better understand the costs and benefits of imidacloprid applications.

To determine the long-term effects of imidacloprid applications on soil collembolan communities, collembolans were systematically sampled, extracted from soil using Berlese funnels, and identified to family, morphospecies, and trophic levels. Overall richness, abundance, and diversity, as well as trophic level (epedaphic, hemiedaphic, and euedaphic) abundance and richness were used to investigate the effects of imidacloprid applications by considering treatment history (control/treatment) and average diameter at breast height (DBH) of treated trees within plots. The collembolan community was not strongly affected by treatment history or DBH of treated trees alone, but epedaphic and hemiedaphic abundance was affected by interactions between the treatment-related variables and soil pH. This study found evidence of long-term effects of imidacloprid applications on the soil collembolan community in hemlock stands when soil pH interactions were considered.

To determine the long-term effects of imidacloprid applications on soil bacteria and fungi, soil samples were systematically taken to investigate the soil microbial community. The community composition was determined through DNA extraction and classification, and the data were analyzed to determine the effects of imidacloprid on the community. Soil pH had an impact on bacterial and fungal species diversity and richness, and richness was positively associated with imidacloprid treatments. Bacterial and fungal groups most correlated with treatment history were identified using species scores from an RDA.

The results of this study suggest that imidacloprid applications applied at the recommended field rate to control HWA have long term effects on the collembolan community when interactions between treatment-related variables and soil pH are considered. Abundance was greater in treated plots when pH was lower and greater in control plots when pH was higher for epedaphic and hemiedaphic communities, and epedaphic abundance decreased as DBH of treated trees increased and pH was lower. This study also showed that soil pH influenced the response of soil microorganisms to imidacloprid. Further studies are needed to understand the effects of other important organisms such as mites and nematodes, and we recommend that before-after control-impact (BACI) designs be used to control for environmental variation among sites that could influence responses.

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