From the largest nesting aggregation ever recorded for the genus Melissodes, we took diverse bionomic measurements of Melissodes bimaculatus (Two-spotted Longhorn Bee). Our results show a protandrous reproductive strategy occurring from July through August in New York. We observed parasitism by the kleptoparasitic bee Triepeolus simplex as well as nest-architecture modifications to ease this burden that support the selfish-herd hypothesis. In this population, we also found a proclivity for grass (Poaceae) pollen, a previously undocumented diet preference for Two-spotted Longhorn Bees. We further showed that this bee species has widespread climatically suitable habitat, with expected range expansion under future climate conditions. Altogether, our results offer novel insights into the ecology of theTwo-spotted Longhorn Bee and its gregarious nesting behavior.