The fragmentation of urban green spaces in megacities such as Istanbul has raised concerns about the biodiversity of insects, particularly solitary hymenopterans. This study reports the first occurrence of Psenulus fuscipennis (Dahlbom, 1843) in Istanbul, Türkiye, and evaluates its nesting behaviour across an urbanisation gradient using artificial nest blocks. Twenty-five nest blocks, with entrance cavity diameters of 1.0, 0.7, and 0.4 cm, were monitored from March to September 2020. Nesting occurred exclusively in 0.4 cm cavities (n = 325 emergence events). The adults were identified morphologically, while the immature stages were confirmed by COI DNA barcoding. Importantly, the relationship between abundance and urbanisation depended on the analytical framework used. A non-parametric Kruskal-Wallis test comparing abundance among three urbanisation categories (low, semi, high) found no significant differences. However, a Generalised Linear Model (Poisson, log-link), which treated urbanisation as an ordinal predictor and controlled for habitat type (garden, park or rural), revealed a significant positive association between higher urbanisation intensity and abundance. This model also indicated higher abundance in rural locations than in gardens and lower abundance in parks. Thus, while categorical comparisons alone did not detect differences, model-based inference revealed a positive relationship between urbanisation and abundance once habitat variation was accounted for. Our results demonstrate that P. fuscipennis can utilise fragmented green spaces under urban pressure, with microhabitat (cavity size) remaining the primary constraint and the urbanisation signal likely reflecting unmeasured covariates rather than a general increase.
İnci et al. (Mon,) studied this question.