When size matters: effectiveness of three endemic African stingless bees as watermelon pollinators

Despite the significant contribution of stingless bees to pollination and conservation of the environment, information on their effectiveness as pollinators is insufficient. Therefore, we tested their effectiveness using watermelon, which is one of the most economically significant global food crops in terms of production volume and production value. Our study aimed at determining how bee body size, visitation rate, and bee-flower size matching ratio affect pollen deposition. We measured pollen deposition in combination with the morphometrics of 30 bees each and measurements of 30 corresponding flowers they visited. We calculated the bee-flower size matching ratio, the relationship between the bee size, and the flower measurement to gauge the effectiveness of each bee species on pollen deposition and distribution among the three watermelon stigmatic lobes. Our findings elaborate on how the different sizes of various bee parts majorly impacted the amount of pollen deposited. Besides, the number of visits, probing time, and bee behavior when handling the flower also played a role. Bees with a size matching ratio approaching 1 deposited more pollen on a single visit. However, pollen distribution highly depended on the number of visits. Among the stingless bee species, Meliponula ferruginea was the best performer. Though the honey bee species Apis mellifera was the best pollen depositor, its performance was almost similar to that of M. ferruginea. In fact, upon a single flower visit, M. ferruginea performed better than A. mellifera. Based on our findings, it can be concluded that stingless bees, particularly M. ferruginea, are effective pollinators of watermelon flowers. Furthermore, traits like probing time, multiple visits, or frequent visitation are not enough to gauge pollinator effectiveness, but there is also a need to include other traits like the size matching ratio between the bee body and flower head.