Quantity and quality of oaks (Quercus) as suitable habitats for the longhorn beetle species Plagionotus detritus : An inventory study in Stockholm and Uppsala County, Sweden

The longhorn beetle Plagionotus detritus is associated with newly dead oak wood and is declining throughout its distribution range. In Sweden, P.detritus has displayed one of the most dramatic reductions in distribution recorded for insect species and is near regional extinction. As a result of logging, agricultural intensification, fire, land clearing, droughts, competition with invasive plants, edge effects, disease, insect attacks, and air pollution, the availability of substrate as well as the species has declined. Today there are only two current populations in Sweden, a stable one in Stockholm, Stockholm County, and one in Båtfors, Uppsala County (ca 130 km distance) which has been reintroduced after going extinct in the past. Data over available substrate and trapping, and data over landcover type at the two sites, were used to conduct a model selection to find the best-fitted model to predict caught beetles per day. Data over available substrate was taken from this study’s fieldwork and fieldwork conducted by the County Administrative Board. Trapping data were provided through previous pheromone trapping. Data on land cover types were provided by the Swedish Land Survey authority. The results showed that the environment in Stockholm was more suitable than that in Båtfors regarding P.detritus. It also showed that there are, in periods, factors hindering the rejuvenation of oaks due to the deficit of larger rejuvenation trees. Lastly, the results showed that the importance of the surveyed variables (e.g., vitality, sun exposure, basal area) varied between the two sites. This, however, needs to be investigated further with a refined methodological approach. Based on present knowledge, the common underlying factor for the observed differences between the two study sites is divergences in the landscape structure. Stockholm had a higher proportion of land dominated by deciduous species and an overall more open landscape that favored the host tree species Quercus. Although it appears that P. detritus is currently able to reproduce in the Båtfors area, the availability of habitat is lacking in the longer term under the current management regime.