Date of Award

Fall 2025

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

Richard Smith

Second Advisor

David Mortensen

Third Advisor

Rebecca Sideman

Abstract

Arthropods provide essential ecosystem services to our agricultural landscapes. These ecosystem services include biological control of pest organisms, including weed seeds, and plant pollination. Concerningly, however, there has been a global decline in insect populations over the last decade, a major cause of which is agricultural intensification. Previous research has demonstrated that agricultural management practices can strongly influence the species composition and abundance of arthropod communities, and therefore presumably the ecosystem services or disservices these organisms provide; however, our understanding of these interactions remains limited. I conducted three studies to investigate whether agricultural practices common in annual row crop (tillage and the use of pesticide-treated crop seeds) and perennial forage (legume species selection and forage harvest intensity) agroecosystems impact beneficial arthropod communities. In the first study (Chapter 1), I quantified the independent and interactive effects of tillage (full tillage, strip-tillage, and no-tillage) and pesticide seed treatments (seed coatings containing fungicides and neonicotinoid insecticides) on epigeal arthropod communities and weed seed predation in a corn-soybean rotation. I found that both pesticide seed treatments and tillage can influence the communities of epigeal arthropods that inhabit annual row crop agroecosystems, even relatively late in the growing season when most pesticide residues have likely dissipated. Further, I documented that the weed seed predation services provided by members of the epigeal arthropod community can be strongly negatively impacted by intensive tillage. In the second study (Chapter 2), I quantified how floral resources and associated pollinator communities vary among four perennial forage legume species (white clover, red clover, alfalfa, and birdsfoot trefoil), each grown in biculture with orchardgrass, and determined how harvest intensity (harvest treatments varying in cutting frequency and cutting height) affects these relationships. In each of the two years I found differences in floral production between the four legume bicultures, and these were most pronounced in the “early” and “mid-early” periods of the growing season. Floral production in both the alfalfa and the red clover bicultures was higher under the less frequent (3 cuts per season) compared to the more frequent (5 cuts per season) cutting regimes, while the opposite was true in the birdsfoot trefoil and white clover bicultures. Patterns of pollinator abundance and diversity roughly followed patterns of floral production and were strongly influenced by legume species and date of sampling, and to a lesser extent harvest intensity. In the third study (Chapter 3), I quantified and characterized floral resource production and pollinator abundance and diversity over two years in two perennial legume mixtures containing all four legume species (in addition to orchardgrass) and compared these to simple legume-grass bicultures of the constituent species managed under four contrasting levels of harvest intensity. I found extensive evidence of overyielding in mixture floral production, including transgressive overyielding, but not in pollinator abundance. Harvest intensity often played an important role in mediating the apparent legume mixture effects that were observed. Finally, in a one-year companion study (Appendix) I investigated whether the patterns in floral resources and pollinators observed in our previous field studies, where forages were intensively harvested, were broadly representative of an on-farm situation where perennial forages were actively grazed by dairy cows. I found that leguminous forages (primarily white clover) were present in the pasture and these provided floral resources to pollinators, even though the pasture was intensively grazed. Taken together, the results of these studies indicate that agricultural management practices play an important role in mediating the populations of beneficial arthropods and the ecosystem services they provide. These results can be used to help guide agricultural management decisions and practices that promote rather than suppress populations of these essential organisms.

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