Article Filamentous Fungi and Yeasts Associated with Mites Phoretic on Ips typographus in Eastern Finland Riikka Linnakoski 1, * , Ilmeini Lasarov 2 , Pyry Veteli 1 , Olli-Pekka Tikkanen 2 , Heli Viiri 3 , Tuula Jyske 1 , Risto Kasanen 4 , Tuan A. Duong 5 and Michael J. Wingfield 5   Citation: Linnakoski, R.; Lasarov, I.; Veteli,P.; Tikkanen, O.-P.; Viiri, H.; Jyske, T.; Kasanen, R.; Duong, T.A.; Wingfield, M.J. Filamentous Fungi and Yeasts Associated with Mites Phoretic on Ips typographus in Eastern Finland. Forests 2021, 12, 743. https://doi.org/10.3390/ f12060743 Academic Editor: Nicola Luchi Received: 1 May 2021 Accepted: 12 May 2021 Published: 4 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Natural Resources Institute Finland (Luke), FI-00790 Helsinki, Finland; pyry.veteli@gmail.com (P.V.); tuula.jyske@luke.fi (T.J.) 2 School of Forest Sciences, University of Eastern Finland, FI-80100 Joensuu, Finland; ilmeini.lasarov@gmail.com (I.L.); olli-pekka.tikkanen@uef.fi (O.-P.T.) 3 UPM Kymmene Forestry and Wood Processing, FI-33101 Tampere, Finland; heli.viiri@upm.com 4 Department of Forest Sciences, University of Helsinki, FI-00014 Helsinki, Finland; risto.kasanen@helsinki.fi 5 Department of Biochemistry, Genetics and Microbiology, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria 0083, South Africa; tuan.duong@up.ac.za (T.A.D.); mike.wingfield@fabi.up.ac.za (M.J.W.) * Correspondence: riikka.linnakoski@luke.fi; Tel.: +358-29-532-2289 Abstract: The European spruce bark beetle (Ips typographus) has become a major forest pest in Finland in recent years. The beetle is a well-known vector of mainly ophiostomatoid fungi causing blue- stain of timber and pathogens that have the ability to amplify the insect damage. It also vectors other associated organisms, such as phoretic mites. The ecology of these mites remains poorly understood, including their associations with fungi. In this study, we considered filamentous fungi and yeasts associated with mites phoretic on I. typographus. Fungal identifications were based on DNA sequences and phylogenetic analyses of the ITS and/or partial β-tubulin gene regions. Fifteen fungal species were detected, including eight yeasts and seven filamentous fungi. Eleven percent of the beetles carried mites and of these 74% carried at least one fungal species. An average of two fungal species were carried per mite. The most commonly found filamentous fungi were Grosmannia penicillata (25%), Ophiostoma bicolor (19%), O. ainoae (12%) and O. brunneolum (12%). Of the yeast species, the most commonly found was Wickerhamomyces bisporus (47%). This study is the first to report yeasts associated with I. typographus and its phoretic mites in Finland. Majority of the filamentous fungal species found are those previously reported in association with I. typographus. The results also confirmed that many of the fungal species commonly found on I. typographus are also associated with its phoretic mites. However, the nature of the symbiosis between the mites, beetles and fungal associates remains to be understood. Keywords: boreal forests; fungal diversity; insect-fungus symbiosis; ophiostomatoid fungi; spruce bark beetle; yeasts 1. Introduction The European spruce bark beetle (Ips typographus L.) is a major forest pest in Europe. In recent years, its importance has increased also in the Northern Europe due to changing climate [1,2]. The beetle is a vector of fungi and other microorganisms, such as phoretic mites and nematodes [3–5]. Particularly common fungal associates are ophiostomatoid fungi (Ophiostomatales and Microascales), a polyphyletic group of fungi having morpho- logical and chemical co-adaptations to arthropod dispersal [6,7]. The majority of these fungi are saprophytic species, but some are tree-killing pathogens having the ability to amplify the insect damage. Fungi associated with I. typographus have been relatively well studied in Europe. Certain species such as Endoconidiophora polonica (Siemaszko) Z.W. de Beer, T.A. Duong & M.J. Wing- field, Grosmannia penicillata (Grosmann) Goid., Grosmannia europhioides (E.F. Wright & Cain) Forests 2021, 12, 743. https://doi.org/10.3390/f12060743 https://www.mdpi.com/journal/forests