Lethality of Essential Oils to the Pests Arion circumscriptus (Gastropoda) and Noctua pronuba (Insecta) in Laboratory and Greenhouse Trials.

Abstract

The sustainable management of invertebrate pests in grass seed crops in Oregon's Willamette Valley necessitates the development of novel approaches due to environmental concerns and inconsistent results associated with traditional methods. Under the Federal Insecticide, Fungicide, and Rodenticide Act in the United States, essential oils are exempt from pesticide registration and residue tolerance requirements, and this, combined with their known biocidal activity, suggests that they maybe a compelling alternative to conventional synthetic pesticides. Thus, this study explored the potential of using these oils as efficacious alternatives for pest control. Laboratory Petri dish bioassays revealed garlic oil as significantly more lethal to the white-soled slug (Arion circumscriptus) and the large yellow underwing moth (Noctua pronuba) compared to many other oils, with lethal concentration 50s (LC50) of 0.31% v/v and 0.19% v/v, respectively. Further microcosm experiments in annual ryegrass (Lolium multiflorum) confirmed the molluscicidal and insecticidal efficacy of this oil, where it caused 100% mortality of A. circumscriptus and N. pronuba at concentrations of 1.24% v/v and 0.76% v/v, respectively. In comparison to commercial standards, garlic oil

outperformed Baythroid® XL (a synthetic pesticide used to control N. pronuba) which achieved 81% mortality. Slug-Fest® which is used by growers to control pest slugs also caused 100% mortality, albeit more quickly (1 day) than garlic oil (11 days). One of the limitations of using essential oils in pest management is that the concentration of oils that are lethal to pests is often also phytotoxic. However, in the microcosm studies with A. circumscriptus and N. pronuba, there were no significant differences in phytotoxicity between the garlic treatments and the water control. Another limitation to using essential oils to control pests is that their high volatility tends to cause a short field life. As a result, this study also explored various garlic oil formulations, including STOMP, a commercially available pesticide with garlic oil as the active ingredient (2%), and garlic oil encapsulated with β-cyclodextrins with the goal of enhancing its stability and efficacy. Encapsulated garlic oil showed promising initial results with slugs (89% mortality); however, its effectiveness decreased with age (67% mortality after 21 days), illustrating a decline in pest control efficacy over time compared to other commercial products such as SlugFest® and Baythroid® XL which caused 100% mortality. STOMP was more effective at controlling A. circumscriptus than N. pronuba larvae but its efficacy also decreased with time (100% to 67% mortality for slugs and 48% to 24% for cutworms, after 21 days). Continued research across all aspects of essential oil application, including field trials and formulation stability, will be crucial to fully establish their role as economically viable and environmentally friendly solutions in diverse agricultural settings.