No abstract

In Portugal, fungal symbionts of the ambrosia beetle Platypus cylindrus affect tree vigor of cork oak (Quercus suber) and are linked with the cork oak decline process. Fungal symbionts play crucial roles in bark and ambrosia beetles life history and recent work indicates complex interactions on the fungal and plant metabolic level. Colonized trees may respond with an array of currently unknown volatile metabolites being indicative of such interactions, acting as infochemicals with their environment. In this study, we examined volatile organic compounds (VOCs) of cork oak seedlings wound-inoculated with strains of three fungal associates of P. cylindrus (Raffaelea montetyi, R. quercina and Ceratocystiopsis sp. nov.) over a 45-day period by means of thermodesorption GC/MS techniques. Fungal strains induced largely quantitative but species-specific changes among the 58 VOCs characterized. Overall, monoterpenes – the major volatiles of cork oak foliage, were significantly reduced, possibly a result of fungal biotransformation. Acetophenone, sulcatone, nonanal – volatiles known for mediating ambrosia beetle behavior, increased in response to fungal inoculation. Qualitative VOC profiles of excised tissue of wood lesions (21 VOCs) and pure fungal cultures (60 VOC) showed little overlap with seedling VOCs, indicating their plant derived but fungal induced origin. This chemoecological study expands on the limited knowledge of VOCs as infochemicals emitted from oak trees threatened by oak decline in relation to beetle-vectored ophiostomatoid fungi. It opens new avenues of research to clarify mutualistic or pathogenic aspects of these complex symbiotic interactions and develop new control strategies for P. cylindrus including its mycobiota.

In recent decades, oak forests in Europe (Fagaceae family) have faced a severe multifactorial process of decline, characterized by a fast drying of the canopy and loss of tree vigour. Climate change and increased susceptibility to pests and diseases (e.g., the fungal pathogens Diplodia corticola and Phytophthora cinnamomi; Moricca et al., 2016) probably have a major role in this process (Sallé et al., 2014). These changes are registered worldwide, often without any precedents and with alarming outcomes (Hulcr & Dunn, 2011). In the Mediterranean area and mainly in Portugal, the evergreen Quercus suber (cork oak) woodlands have been showing this decline process since the 1980s, highly affecting the cork production (half of the world cork production is Portuguese; APCOR, 2020) and the biodiversity of the ecosystem. The symptoms include leaf discolouration, defoliation, small branches and sometimes exudates (Tiberi et al., 2016).

The ambrosia beetle Platypus cylindrus Fab. (Coleoptera: Curculionidae) is a major cork oak pest in Portugal. Female and male beetles have different roles in host tree colonization and are both equipped with prothoracic mycangia for fungal transportation. Despite a known beneficial role of bacteria in ambrosia beetles, information on bacterial composition associated with prothoracic mycangia structures is scarce. Bacterial community from mycangia of P. cylindrus male and female beetles collected from cork oak galleries was investigated by means of 16S metagenomics. Mycangia anatomical structure was also explored with histological techniques and X-ray computed microtomography to highlight evidence supporting biological sexual dimorphism. A bacterial community with highly diverse bacterial taxa with low abundances at the genus level was revealed. Lactobacillales, Leptotrichia, Neisseria, Rothia, and Sphingomonadaceae were significantly more abundant in males, while Acinetobacter, Chitinophagaceae, Enterobacteriaceae, Erwiniaceae, Microbacteriaceae, and Pseudoclavibacter were more abundant in females. Additionally, a core bacteriome of five genera was shared by both sexes. Histological examination revealed visible connections linking external and internal tissues in females, but none in males. Overall, these results provide the first insights into sexual differentiation for bacteria in a Platypodinae beetle species, identifying key patterns of bacteria distribution in the context of beetle ecology and functional behavior.

The European spruce bark beetle Ips typographus (L.) is a pest causing severe damage to Norway spruce-dominated forests in Europe. Microorganisms play an essential role in the species life history, including nutrition, fitness as well as in overcoming host defenses. Here, we performed high-throughput 16S rRNA metabarcoding of I. typographus across different populations in Europe. We investigated four postglacial refugial areas in Europe and focused specifically on a current bark beetle hot spot in the Dolomites where we compared populations with different epidemiological phases (outbreaking vs. non-outbreaking) and across different seasons (pre-overwintering vs. overwintering). Our results show that the bacterial community structure varied among populations from the refugial areas and geographic regions within the Dolomites. We found a significant difference in the bacterial community between pre-overwintering and overwintering individuals, highlighting a potential role of the microbiome in I. typographus overwintering but we did not find differences between epidemic and endemic populations. The genera Erwinia and Pseudoxanthomonas - previously reported for their role in nutrition and protection from conifer defense compounds - were present in every individual across all populations, suggesting that these taxa form the bacterial core community of I. typographus. Furthermore, several additional bacterial taxa occurred in all populations, but with variable frequencies within and between individuals. This study highlights a complex interaction of various bacterial taxa across different regions and ecological phases of I. typographus populations and provides new insights into the role of microorganisms in the biology of this important pest species.