Solar energy facilities are rapidly expanding in their land use footprint worldwide, with significant implications for biodiversity. Although impacts are often negative, it is possible for some species to take advantage of the novel anthropogenic structures and microhabitats provided by solar facilities.

We describe the frequent nesting of non-native European paper wasps (Polistes dominula) at two solar facilities in the Central Valley of California (USA), conducting nest censuses to further investigate population density and nest siting behaviour.

Active nests were found to occur at a density of 10-23 per hectare of solar facility, and paper wasps had a preference for nesting in sheltered metal torque tubes compared with the more exposed undersides of photovoltaic panels.

Our study shows that P. dominula might benefit from the construction of solar energy facilities, which could have a variety of impacts on native species and surrounding agriculture that warrant further study. The European paper wasp therefore provides an example to illustrate the potential for a varied and relatively unpredictable set of ecological outcomes to follow land-use change resulting from solar energy development.

To estimate wasp nest density at two GPV facilities, we conducted censuses by counting all nests of P. dominula along a randomly selected set of PV panel strings (rows of panels) in summer 2024. At the UC Davis facility 47 panel strings (5% of the total) were counted along their entire length between 09:30 and 14:30 on June 7^th, July 9^th and August 1^st. At the SacSewer facility, 35 panel strings (10% of the total) were counted along their entire length between 08:40 and 11:30 on July 16^th and August 21^st.

Nests were categorized as active if at least one live adult wasp was present and inactive if no live adult wasps were present at the time of counting. PV panel strings were visually inspected for nests, with their location on the structure also recorded (torque tube or underside of panel). Reported values are likely an underestimate given that we could not fully examine hollow elements of the PV string infrastructure (e.g., metal torque tubes within the racking infrastructure that support the panels). During the July and August censuses, we also recorded the number of live wasps per active nest at both facilities by visually counting wasps on each active nest we encountered, where visibility permitted complete counting.