CUBE: Class-conditioned Unknown Boundary Evidence for Open-Set Insect Species Recognition

Open-set insect species recognition faces significant challenges when deployed in real-world biodiversity scenarios, particularly when encountering unknown species that are visually close to known training classes. Unlike generic out-of-distribution samples, fine-grained unknown insects are often not visually anomalous. They instead tend to appear near the decision boundary of specific known species, leading to unreliable closed-set predictions. To address these challenges, we propose Class-conditioned Unknown Boundary Evidence (CUBE), a novel evidence-based framework that models unknownness around each known species rather than as a single global unknown class. Our key idea is to evaluate a test sample relative to the known species it most closely resembles, enabling local rejection for visually confusable unknown insects. CUBE learns class-conditioned boundary evidence from auxiliary species, preserves auxiliary species as shadow references during inference, and incorporates taxonomic candidate calibration to verify biological plausibility. When ecological metadata is available, we further introduce CUBE-E, which adds a lightweight location-consistency term without modifying the visual backbone. Experiments on the Open-Insect C-America benchmark show that CUBE achieves superior performance, outperforming state-of-the-art baselines with 95.8%, 98.7%, and 99.1% AUROC on local, non-local, and non-moth unknown splits, respectively, while maintaining 94.9% closed-set accuracy. With ecological metadata, CUBE-E further improves the results to 96.0%, 99.7%, and 99.5% AUROC with 95.2% closed-set accuracy.