Semiochemical repellents as a post-felling management strategy for Rapid 'Ōhi'a Death

The fungal disease Rapid 'Ōhi'a Death (ROD) has caused extensive mortality in 'ōhi'a (Metrosideros polymorpha Gaudich) forests on Hawai'i Island since the mid-2010s. As the keystone species in native Hawaiian wet forests, the decimation of 'ōhi'a threatens the stability of Hawaiian forest communities. Invasive ambrosia beetles (Coleoptera: Curculionidae: Scolytinae) are the primary agents producing ROD inoculum and can directly transmit the pathogens as well. The most common management recommendation for ROD-infected trees is to fell and tarp. However, this is often logistically impractical and does not proactively suppress inoculum production. Alternatively, the semiochemical verbenone has proven effective in repelling ROD-associated beetles from standing 'ōhi'a and may have application in post-felling management. Across two 10-wk field and lab trials, we evaluated two semiochemical formulations, SPLAT Verb (10% verbenone) and SPLAT Beetle Guard (10% verbenone + 10% methyl salicylate), in reducing ambrosia beetle attack, emergence, and frass production when applied to felled ROD-Ceratocystis--infected 'ōhi'a. Verbenone alone significantly reduced beetle attacks and frass production, while verbenone + methyl salicylate also reduced attacks, outperforming verbenone alone by 37% in one trial, and reduced frass production and emergence. Beetle attacks subsided to near zero by week 10, presumably due to bolt desiccation, suggesting a single application may provide sufficient protection for the extent of a felled tree's host-suitability period. Semiochemical repellents may play a pivotal role in the multifaceted management approach needed to control ROD, and these findings validate and expand the emerging body of evidence establishing their efficacy in repelling beetles within the ROD pathosystem.