The enigmatic historical record of the leaf beetle <i>Acentroptera norrisii</i> in Spain: failed biological introduction or curatorial artefact?

Background Natural History Collections (NHC) often contain exceptional specimens documenting unusual historical events, and these records can be valuable for detecting overlooked biological introductions. Here, we examine a striking case: a Neotropical leaf beetle, Acentroptera norrisii , labelled as collected in central Spain in 1998 by Dr J. Modolell. Given the species’ native distribution in the Neotropics, this record raises the question of whether it represents a failed historical introduction or a curatorial artefact. Methods We reconstructed the circumstances of the collection event using the collector’s personal archives. We conducted scanning electron microscopy (SEM) to document microscopic particles on the specimen’s cuticle. We also analysed the content of the original specimen box to evaluate the likelihood of pollen cross-contamination, considering the geographical origin and family-level identity of all co-stored specimens. Results Archival evidence confirms the collector’s presence at the labelled locality and date. SEM revealed several pollen grains consistent with plant taxa occurring at the collection site. However, the original box also contained numerous Spanish specimens belonging to beetle families with known anthophilous habits, meaning that cross-contamination during storage cannot be fully excluded. As a result, the palynological evidence remains inconclusive. Conclusions Together, the available evidence makes a failed historical introduction a plausible scenario, yet the inconclusive particle analysis prevents any confident confirmation. This case highlights both the potential and the limitations of NHC-derived data in invasion biology: while museum collections can preserve traces of otherwise “invisible” introductions, interpreting isolated and context-poor specimens remains inherently uncertain.