Screening and evaluation of acaropathogenic fungi against the bulb mite Rhizoglyphus robini

Abstract

• Fungal isolates effective in controlling bulb mites ( Rhizoglyphus robini ) were screened. • The selected fungal isolates have high virulence and thermotolerance. • Three Metarhizium anisopliae were determined to be the most suitable for bulb mite control. Bulb mites ( Rhizoglyphus robini ) damage the bulbs, corns, and tubers of garlic, shallots, and onions. Bulb mites have recently become a serious problem because of the continuous use of acaricides, which has resulted in resistance among bulb mite populations. Thus, there is a need to find alternative control methods to suppress bulb mite populations. Here, we report the results of screening pathogenic fungi for the control of R. robini . The initial screen was performed using 342 isolates of entomopathogenic fungi from soils from South Korea. As a result, 57 isolates of putative acaropathogenic fungi were selected from cadavers of bulb mites supporting fungal sporulation. However, 11 isolates were finally selected for further study through a re-evaluation of the pathogenicity of the isolates. These isolates were identified as two isolates of Metarhizium pemphigi , two isolates of Metarhizium pingshaense , and seven isolates of Metarhizium anisopliae by a microscopic examination and sequence analysis of the ITS region and EF-1α gene. The virulence, thermotolerance and UV-B tolerance of the 11 isolates were further evaluated and compared. Eight isolates showed more than 80% mortality and three isolates showed 100% mortality at 7 days after treatment. The thermotolerance of conidia showed a large difference depending on the fungal isolate, and five isolates showed a conidial germination rate of approximately 60% or more even after 2 h of heat treatment. On the other hand, the UV-B tolerance was very low in all the isolates, and only one isolate showed more than approximately 80% tolerance to 0.1 J cm −2 , but the other isolates showed a conidial germination rate of less than 30%. Based on all of the above results, three isolates, M. anisopliae 4–3-2, 4–8-1, and 4–31-2, were the most effective isolates in controlling bulb mites and could be considered promising biological control agents against bulb mites.