Poor infectivity of Beauveria bassiana to eggs and immatures causes the failure of suppression on Tetranychus urticae population

Abstract

The entomopathogenic fungus, Beauveria bassiana, is capable of infecting pest mites, while the effect on different mite stages has rarely been reported. The present study evaluates the effect of several B. bassiana isolates (GZGY-1-3, LNSZ-26, SDDZ-9, XJWLMQ-32, SCWJ-2 and JXJGS-1) on Tetranychus urticae by a series of assays and observations. A potted bean plant assay indicated the fungal sprays resulted in greater reduction of adult T. urticae populations, but poor suppression in eggs and immature stages. During the one-month experiment, the different fungal isolates reduced the numbers of T. urticae eggs, immatures and adults by 38.7–55.2, 3.7–18.7 and 61.0–72.1%, respectively. Laboratory bioassays showed their corrected mortalities were 2.7–3.8, 17.5–25.8 and 63.2–71.2%, respectively, at seven days days post-fungal treatment. Fecundity of female mites was significantly reduced after fungal spray due to the lethal effect on females, while egg hatchability was not affected. Light microscopy observations indicated fungal outgrowths were evident in mite cadavers, but were not visible on eggs. Scanning electronic microscopy observations demonstrated fungal mycelia grew prolifically from the adult mite 60 h following fungal spray, although no symptoms of fungal infection were exhibited in most immature cadavers. Despite the fact that fungal conidia were able to adhere to and germinate on eggshells, and the germ tubes elongated on the shell surface, they were never observed to penetrate the eggs. Our results demonstrating that the failure to control T. urticae on bean plants using B. bassiana, which we attributed to its poor infectivity to mite eggs and immature stages, may provide useful information in future attempts to develop effective mite control strategies.