Date of Award

6-2026

Degree Name

MS in Agriculture - Plant Protection Science

Department/Program

Horticulture and Crop Science

College

College of Agriculture, Food, and Environmental Sciences

Advisor

Gerald Holmes

Advisor Department

Horticulture and Crop Science

Advisor College

College of Agriculture, Food, and Environmental Sciences

Abstract

California strawberry production relies heavily on chemical pesticides to manage arthropod pests and diseases, but pesticide resistance, regulatory pressure, and the need to conserve biological control agents have increased interest in alternative pest management tools. This thesis evaluated ultraviolet-C (UV-C) light as a nonchemical management tool for key strawberry arthropod pests and assessed the compatibility of common strawberry fungicides and adjuvants with the predatory mite Phytoseiulus persimilis Athias-Henriot. The first objective was to determine the ovicidal and lethal effects of UV-C light on Lygus hesperus Knight eggs and nymphs. Laboratory and field ovicide assays were conducted using UV-C doses relevant to commercial strawberry applications, and laboratory dose–response assays were used to estimate LD₅₀ and LD₉₀ values for 1st, 3rd, and 5th instars. UV-C significantly reduced L. hesperus egg hatch in the laboratory at 350 J/m², but field results were inconsistent. In one field assay, UV-C did not significantly affect egg hatch, whereas in a second field assay, 1015 J/m² significantly reduced egg hatch compared with the untreated control. Dose–response assays showed that L. hesperus nymphs required much higher doses than those used commercially, with LD₅₀ values of 19,527, 23,879, and 25,591 J/m² for 1st, 3rd, and 5th instars, respectively. The second objective was to evaluate UV-C efficacy against the two-spotted spider mite, Tetranychus urticae Koch, and Lewis mite, Eotetranychus lewisi McGregor. Laboratory and field ovicide assays showed that UV-C strongly reduced egg hatch of both mite species, whereas laboratory dose–response assays demonstrated limited practical efficacy against motile stages at field-applied doses. For T. urticae, UV-C reduced egg hatch under laboratory and field conditions, but adult and larval LD estimates were substantially higher than commercial field doses. Similar patterns were observed for E. lewisi, with strong ovicidal activity but limited expected mortality of adults at practical field doses. These results indicate that UV-C is best suited as an ovicidal tool for spider mite management rather than as a direct control measure for motile stages. The third objective was to quantify the lethal effects of six commonly used strawberry fungicides, five adjuvants, and their combinations on P. persimilis nymph mortality under laboratory direct exposure. Bias-reduced logistic regression showed significant effects of fungicide, adjuvant, and their interaction on P. persimilis mortality. Fungicides applied alone generally caused less than 25% mortality, whereas adjuvants were the primary drivers of mortality. Silwet L-77 consistently caused high mortality alone and in combination with most fungicides, while Broadspred was generally compatible with P. persimilis. These results demonstrate that adjuvant selection can strongly influence the compatibility of fungicide programs with biological control. Overall, this thesis shows that UV-C has strong potential as an ovicidal tool for mite management in strawberries but is unlikely to provide direct control of L. hesperus nymphs or motile mite stages at commercial field doses. The findings also highlight that successful integrated pest management requires evaluating not only pest suppression tools, but also their compatibility with beneficial arthropods. Integrating UV-C applications, selective pesticide programs, and biologically compatible adjuvants may help reduce reliance on conventional pesticides while improving the sustainability of strawberry pest management.

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