Quantifying wing wear in bumble bees using fractal geometry

Wing wear in bees (Hymenoptera: Apidae) accumulates through flight activity and can serve as a proxy for foraging effort and age, yet methods for quantifying wear rely on coarse categorical scoring or simple geometric measurements that do not capture the complexity of wing edge degradation. We evaluate fractal dimension (FD) analysis as a quantitative morphometric for assessing wing wear and evaluate its relationship with visual scoring and body size. Unlike linear wing metrics, FD can capture subtle irregularities, yet no study has used FD to quantify wing wear in insects. We use forewing images collected from 263 workers of three North American bumble bee species (Bombus impatiens, B. bimaculatus, and B. rufocinctus) across meadows in Rouge National Urban Park (Toronto, Canada) to compare FD to traditional measures of wing condition, including wing wear scores and perimeter-area ratio. We found that FD of undamaged wings varied between species indicating interspecific variation in wing morphology. Furthermore, damaged wings correlated more strongly with visual wear scores than perimeter-area ratio, and unlike perimeter-area ratio, was not confounded by body size. Our findings highlight FD as a sensitive quantitative morphometric for wing wear and a promising tool for measurement of bee body condition, age, and foraging effort.