Bumble Bees as a Tool for Enhancing Apple Yield Quality and Quantity

most apple cultivars are self-sterile and completely dependent on cross-pollination from a different cultivar in order to set fruit. various insects may be pollinators, but the main one is the honey bee (apis mellifera; figure 1a+b). however, despite the advantages of the honey bee as pollinator of many plants, it is a relatively inefficient pollinator of apple flowers. the main reason for this is the tendency of honey bees to visit the apple flower from the side (sideworker), thus “stealing” nectar without touching the flower’s reproductive organs – stamens and stigma. in contrast, a bee that visits the flower from the top (topworker) contacts the flower’s reproductive organs, which results in better pollination. due to the low pollination efficiency, few seeds are formed, and often the resulting fruit is too small to be of commercial value.

increasing size of pears and number of seeds

experiments conducted in the galilee and in the golan over the last few years have shown for the first time that adding bumble bees (bombus terrestris; figure 2) into pear orchards improved cross-pollination, thus increasing the number of seeds and subsequently fruit size. the main advantages of bumble bees are that they are active at ambient temperatures below 15° c, which is the threshold for honey bees, their fast flight, which allows them to visit more flowers per time, their high efficiency as pollinators due their large body that does not allow them to be sideworkers, and their limited communication system, which retains bees in the orchard even if nestmates have found a highly attractive competing bloom elsewhere.

figure 3

figure 4

increasing size of apples and number of seeds

the goal of the present work was to test the hypothesis that adding bumble bees to apple orchards may improve cross-pollination, not only due to an additive effect from the addition of pollinators, especially those that are active at lower ambient temperatures, but also due to a possible synergistic effect on honey bees. such an effect would increase the activity and efficiency of the honey bees as topworker pollinators, thereby increasing the number of seeds and fruit size. we found that adding bumble bees to the honey bees in the apple orchard improved pollination in all tested cultivars, especially in ‘gala’, which naturally suffers from relatively few seeds in the fruit. it appears that the addition of bumble bees did not only increase the number of pollinating insects in the orchard that could perform cross-pollination, including in the cool mornings and in adverse weather conditions, but that it also changed honey bee foraging behavior, which resulted in improved cross-pollination and increased efficiency, and subsequently more seeds and larger fruit (figures 3 and 4). the improved pollination was due to the greater mobility of honey bees between rows of pollinated cultivar and pollenizer (figure 5), and due their greater efficiency as pollinators while foraging mainly as topworkers (figure 6).  

figure 5	figure 6