The fungal community associated with the ambrosia beetle Xylosandrus compactus invading the mediterranean maquis in central Italy reveals high biodiversity and suggests environmental acquisitions

Abstract

In summer 2016 a severe infestation of the alien ambrosia beetle Xylosandrus compactus was recorded from the Mediterranean maquis in the Circeo National Park in Central Italy. Trees and shrubs were infested and displayed wilting and necrosis of terminal branches caused by the combined impact of the insect and associated pathogenic fungi. A preliminary screening carried out on captured adults resulted in the isolation of a discrete number of fungal taxa with different life strategies, ranging from true mutualist (e.g. Ambrosiella xylebori) to plant pathogens (Fusarium spp.). In the present study, high-throughput sequencing was applied to determine the total diversity and functionality of the fungal community associated with X. compactus adults collected in the galleries of three Mediterranean woody hosts, Quercus ilex, Laurus nobilis, and Ceratonia siliqua. The effect of season and host in determining the composition of the associated fungal community was investigated. A total of 206 OTUs composed the fungal community associated with X. compactus. Eighteen OTUs were shared among the three hosts, including A. xylebori and members of the Fusarium solani complex. All but two were previously associated with beetles.

Sixty-nine out of 206 OTUs were resolved to species level, identifying 60 different fungal species, 22 of which already reported in the literature as associated with beetles or other insects. Functional guild assigned most of the fungal species to saprotrophs and plant pathogens. Effects of seasonality and host on fungal community assemblage were highlighted suggesting the acquisition by the insect of new fungal taxa during the invasion process. The consequences of enriched fungal community on the risk of the insurgence of novel threatful insect–fungus association are discussed considering direct and indirect effects on the invaded habitat.

Introduction

The ambrosia beetle Xylosandrus compactus (Eichhoff) (Coleoptera: Curculionidae, Scolytinae; black twig borer) is an important pest of a wide range of woody hosts. It is widespread in Africa, the Americas, and Southeast Asia. It has recently invaded Europe and is spreading in natural ecosystems and parks and gardens in Italy and Southern France (Delgado and Couturier, 2010; Rabaglia et al., 2006; Vannini et al., 2017). The native range of X. compactus is probably East Asia (Chong et al., 2009; Wood, 1982). X. compactus is a primary pest of over 200 hosts worldwide with a high impact in cultivated and natural environments. Healthy twigs are attacked by females that bore into the living tissues (Ngoan et al., 1976). X. compactus is currently included in the European Plant Protection Organization (EPPO) alert list, in the A1 list of the Caribbean Agricultural Health and Food Safety Agency (CAHFSA), and the Organismo Internacional Regionale de Sanidad Agropecuaria and considered a quarantine pest in Israel (OIRSA – Central America); It is considered a quarantine pest in Israel and Morocco. Due to its capacity for inbreeding, X. compactus establishes active populations very rapidly especially in the presence of suitable environmental conditions, killing live shoots in a short period. Symptoms include small entrance holes (0.8 mm in diameter), shoots wilt, and wood necrosis often extending distally and proximally alongside the branches (Dixon et al., 2003; Vannini et al., 2017). X. compactus is commonly associated with fungi. Some of these fungi, such as Ambrosiella xylebori, are considered true mutualists and supply the diet for the larvae in the galleries (Bateman et al., 2016). True mutualists are typically carried in the mycangia, and provide the diet for larvae through so-called fungus-farming (Hulcr and Stelinski, 2017). Others may be pathogenic and contribute to symptom developments on host plants. Among these many Fusarium spp., often members of the Fusarium solani complex (FSSC), considered stable associates and that are involved in necrosis development on infested twigs/branches (Bateman et al., 2016; Bosso et al., 2012; Vannini et al., 2017). Beside establishing a stable association, Fusarium spp. are not harboured in X. compactus mycangia; Bateman et al. (2016) demonstrated their localization mostly in the abdomen and on external surfaces of the insect.

In summer 2016 a severe infestation of the alien ambrosia beetle X. compactus was recorded from the Mediterranean maquis in the Circeo National Park in Central Italy. Trees and shrubs were infested and displayed wilting and necrosis of terminal branches caused by the combined impact of the insect and associated pathogenic fungi. A multimember community of fungal species, many originating in the invaded environment, are found associated with X. compactus in Central Italy, with life strategies and functionality ranging from parasitism to saprotrophic. Vannini et al. (2017) isolated 18 different taxa from X. compactus bodies and galleries in Italy, including A. xylebori, FSSC members, and many other taxa; the native Biscogniauxia mediterranea, and the cosmopolitan Botryosphaeria stevensii were also present. Other fungi present included Geosmithia pallida, a recently described pathogen of live oak in the USA (Lynch et al., 2014), often associated with a range of wood–boring insects (Kolařík et al., 2004); Epicoccum nigrum reported associated with other species of Xylosandrus (Kostovcik et al., 2015); and Pestalotiopsis sp. previously recorded from X. compactus in Florida (Bateman et al., 2016).

The ongoing EU LIFE project SAMFIX was launched having among the objectives to monitor X. compactus invasion of the Mediterranean basin and to predict the risks for natural areas. The introduction and diffusion of plant pathogenic fungi associated with the insect are among the potential risks to monitor. As recently reported by Rassati et al. (2019), the mycobiome of ambrosia beetles is enriched with fungi found in the invaded areas that can establish new associations with the insect. The authors demonstrated such a process for Xylosandrus germanus invading forest areas in northern Italy. Thus, the present study aims to describe and analyze the total fungal community associated with the alien ambrosia beetle X. compactus invading the Mediterranean maquis; to determine their functional guild, ecology, and taxonomic position; to analyze ‘new’ and already recorded associations using taxonomy and life-style as references; finally to discuss of the possible risk for an insurgence of invasive insect–fungus interactions.