title: Histopathological and molecular insights into the ovicidal activities of two entomopathogenic fungi against two-spotted spider mite.
abstract: Entomopathogenic fungi can infect and kill spider mite eggs but their ovicidal activities are poorly understood. Here we gain histopathogenical and molecular insights into the ovicidal activities of Beauveria bassiana and Isaria fumosorosea against the two-spotted spider mite, Tetranychus urticae. Scanning electronic microscopy indicated successful adhesion and germination of fungal conidia on egg shell at 24h post-spray (HPS). Germ tubes of both fungi could penetrate into egg shell with penetration pegs at 48 HPS. Interestingly, the germ tubes of B. bassiana may elongate on egg surface to locate appropriate sites for penetration, acting as 'searching' hyphae. Aside from the normal penetration, the germ tubes of I. fumosorosea can be completely or partially embedded into egg shell for a distance of extension, forming shell humps. Light microscopy of ultrathin sections of infected eggs showed shrunken (affected) or disrupted embryos at 48-96 HPS despite little effect on egg cleavage at 24 HPS. However, distinguishable hyphal cells were hardly found inside the embryos lacking oxygen although fungal outgrowths were abundant on unhatched (killed) eggs. In PCR with specific probes, the 18S rDNA signals of B. bassiana (412 bp) and I. fumosorosea (454 bp) in the DNA extracts from surface-cleaned mite eggs increased at 0-96 HPS, confirming fungal colonization in the infected eggs. We consider that the colonization on shell surface and underside could rely upon extending hyphae for uptake of egg nutrition, resulting in embryo disruption. Our observations add knowledge to microbial control of spider mites.
authors.affiliation: School of Biological and Chemical Engineering, Zhejiang University of Science & Technology, Hangzhou 310023, PR China; Institute of Microbiology, College of Life Sciences, Zhejiang University, Hangzhou 310058, PR China.
authors.forename: Lei
authors.initials: L
authors.lastname: Zhang
authors.affiliation: Institute of Microbiology, College of Life Sciences, Zhejiang University, Hangzhou 310058, PR China.
authors.forename: Wei-Bing
authors.initials: WB
authors.lastname: Shi
authors.affiliation: Institute of Microbiology, College of Life Sciences, Zhejiang University, Hangzhou 310058, PR China. Electronic address: mgfeng@zju.edu.cn.
authors.forename: Ming-Guang
authors.initials: MG
authors.lastname: Feng
mesh_terms: D000818:Animals; D052982:Beauveria; D008855:Microscopy, Electron, Scanning; D003904:Mitosporic Fungi; D009181:Mycoses / Q000662:veterinary*; D010063:Ovum / Q000382:microbiology*; D010572:Pest Control, Biological / Q000379:methods*; D016133:Polymerase Chain Reaction; D040102:Tetranychidae / Q000382:microbiology*
keywords: Egg shell penetration; Embryonic disruption; Entomopathogenic fungi; Fungal ovicidal activity; Spider mites