The New Nematicide Cyclobutrifluram Targets the Mitochondrial Succinate Dehydrogenase Complex in Bursaphelenchus xylophilus

Bursaphelenchus xylophilus is a dangerous quarantine pest that causes extensive damage to pine ecosystems worldwide. Cyclobutrifluram, a succinate dehydrogenase inhibitor (SDHI), is a novel nematicide introduced by Syngenta in 2013. However, the nematocidal effect of cyclobutrifluram against plant-parasitic nematodes remains underexplored. Therefore, here, we aim to address this knowledge gap by evaluating the toxicity, effects, and mode of action of cyclobutrifluram on B. xylophilus. The result shows that cyclobutrifluram is the most effective agent, with an LC50 value of 0.1078 mg·L−1. At an LC20 dose, it significantly reduced the population size to 10.40 × 103 ± 737.56—approximately 1/23 that of the control group. This notable impact may stem from the agent’s ability to diminish egg-laying and hatching rates, as well as to impede the nematodes’ development. In addition, it has also performed well in the prevention of pine wilt disease, significantly reducing the incidence in greenhouses and in the field. SDH consists of a transmembrane assembly composed of four protein subunits (SDHA to SDHD). Four sdh genes were characterized and proved by RNAi to regulate the spawning capacity, locomotion ability, and body size of B. xylophilus. The mortality of nematodes treated with sdhc-dsRNA significantly decreased upon cyclobutrifluram application. Molecular docking further confirmed that SDHC, a cytochrome-binding protein, is the target. In conclusion, cyclobutrifluram has a good potential for trunk injection against B. xylophilus. This study provides valuable information for the screening and application of effective agents in controlling and preventing PWD in forests.