Weak population genetic structure in Eurasian spruce bark beetle over large regional scales in Sweden

Abstract

The Eurasian spruce bark beetle, Ips typographus , is a major pest, capable of killing spruce forests during large population outbreaks. Recorded dispersal distances of individual beetles are typically within hundreds of meters or a few kilometers. However, the connectivity between populations at larger distances and longer time spans and how this is affected by the habitat is less studied, despite its importance for understanding at which distances local outbreaks may spread. Previous population genetic studies in I. typographus typically used low resolution markers. Here, we use genome‐wide data to assess population structure and connectivity of I. typographus in Sweden. We used 152 individuals from 19 population samples, distributed over 830 km from Strömsund (63° 46′ 8″ N) in the north to Nyteboda (56° 8′ 50″ N) in the south, to capture processes at a large regional scale, and a transect sampling design adjacent to a recent outbreak to capture processes at a smaller scale (76 km). Using restriction site‐associated DNA sequencing (RADseq) markers capturing 1409–1997 SNPs throughout the genome, we document a weak genetic structure over the large scale, potentially indicative of high connectivity with extensive gene flow. No differentiation was detected at the smaller scale. We find indications of isolation‐by‐distance both for relative ( F ST ) and absolute divergence (Dxy). The two northernmost populations are most differentiated from the remaining populations, and diverge in parallel to the southern populations for a set of outlier loci. In conclusion, the population structure of I. typographus in Sweden is weak, suggesting a high capacity to disperse and establish outbreak populations in new territories.

Complex genomic landscape of inversion polymorphism in Europe’s most destructive forest pest

“…The data suggests high levels of connectivity among populations and a very recent differentiation into two genetic clusters (Stauffer et al 1999; Sallé et al 2007; Bertheau et al 2013; Mayer et al 2015). More recent RADseq data confirms a very weak genetic structure in the spruce bark beetle across much of Sweden (Ellerstrand et al 2022), as is expected in a species with high dispersal (Nilssen 1984) and/or recent divergence. Tajima’s D values indicate different demographic history of southern and northern group (e.g., recent expansion of southern populations and bottleneck in northern populations, Figure S7).…”

“…The data suggests high levels of connectivity among spruce bark beetle populations and a very recent differentiation into two genetic clusters [38][39][40][41] . More recent RADseq data confirms a very weak genetic structure in the spruce bark beetle across much of Sweden 42 , as is expected in a species with high dispersal 43 and recent divergence.…”

Spatial and temporal patterns in the population genomics of the European cockchafer Melolontha melolontha in the Alpine region

“…A pattern of IBD was also observed among the collections of the northwestern alpine genetic cluster when the South Tyrolean cluster was excluded for analysis (Figure S4b ). Studies on Coleoptera species with great dispersal capacity have shown a weak pattern of isolation by distance, such as in Ips typographus along a transect with a diameter of 300 km in Sweden (Ellerstrand et al, 2022 ), in Ootheca mutabilis along a transect of 300 kilometres in Uganda (Kanyesigye et al, 2022 ), or in Cerotoma trifurcata along a transect of 1545 km in the USA (Tiroesele et al, 2014 ). However, the results of the present study indicate, even though genetic distance values across collections were low, that geographic distance had an effect on the genomic structure of M. melolontha , along a transect of 398 km.…”

Do pheromone traps help to reduce new attacks ofIps typographusat the local scale after a sanitary cut?

“…These beetles may originate from the clear-cut (overwintering beetles), but also from other stands further away (dispersing beetles flying over the clear-cut by chance). Ips typographus has good dispersal capacities ( Valeria et al, 2016 ; Ellerstrand et al, 2022 ; Müller et al, 2022 ) and many specimens from foreign forest stands are likely to be attracted by pheromone traps when transiting by the experimental patches ( Weslien & Lindelöw, 1989 ). It is also possible that sanitation felling is so effective to reduce the new attacks that adding traps makes little difference.…”