Fine-grained cotton detection is challenging because targets at different developmental stages exhibit substantial variations in scale and morphology, while visually similar categories, partial occlusion, and complex field backgrounds further increase recognition difficulty. To address these issues, this study proposes WINGS-Det, a lightweight object detector that coordinates spatial, scale, and frequency-aware feature modeling within a unified architecture. Built on YOLO26n, WINGS-Det incorporates three complementary components. C3k2LGL combines large-kernel local aggregation with sparse global self-attention to capture local structures and long-range spatial dependencies. SCMIFF performs selective cross-scale interaction between hierarchical features to strengthen the fusion of spatial details and semantic information. C2WSA introduces wave-aware frequency modeling to preserve global structural information while enhancing high-frequency boundary and texture cues. A four-category cotton dataset containing 13,771 images and 27,352 annotated instances was constructed for evaluation. WINGS-Det achieves 98.6% Precision, 98.2% Recall, 98.9% mAP50, and 75.7% mAP50:95 with only 2.38 M parameters and 5.3 GFLOPs, while achieving 163 FPS on an RTX 4090 with native PyTorch FP32 inference. On the newly constructed four-category cotton dataset, WINGS-Det improves mAP50 and mAP50:95 over the YOLO26n baseline by 4.7 and 3.8 percentage points, respectively. Extensive evaluations across target scales, object densities, image perturbations, network architectures, hardware platforms, and external datasets further demonstrate the robustness, transferability, and generalization capability of WINGS-Det. The results indicate that the proposed framework achieves an effective balance between fine-grained detection accuracy, computational efficiency, and practical deployment.