Bumble bees versus honey bees: A comparison of pollination success in Oregon cranberries

Cranberries (Ericaceae: Vaccinium macrocarpon), an important crop of cool, cloudy, and windy Southwestern Oregon, face the obstacle of pollination, the necessary precursor to fruit-set. Honey bees (Apis mellifera), well known as fair-weather foragers, are rented by farmers at two hives/acre to saturate the area with pollinators. Their increasing price, decreasing availability, and questionable pollinator efficacy are motivating Oregon cranberry farmers to seek alternative pollinators. Native bumblebees (Bombus sp.) forage on cranberries even under unfavorable weather conditions that keep honeybees indoors. However, their impact on cranberry pollination under Oregon conditions is not known.

Our objective was to compare pollination by honey bees and bumble bees in Oregon cranberries. Colonies were connected to 1m x 1m cages and the following treatments were compared: 1) bumble bees; 2) honey bees; 3) control (no bees); and 4) open pollination (all bees). The experiment was set up in two separate cranberry beds with four replications.

Pollination success was evaluated using the following parameters: berry yield (g/m2); number of berries/m2; size of berries; and number of seeds/berry. An analysis of variance indicated that for all parameters considered, honey bees and bumble bees did not differ significantly from each other (p values > 0.05). However, due to cage effect, yield in bee pollinated cages was lower than yield in open pollinated plots. Options for build-up of bumble bee populations for enhancing cranberry pollination will be discussed.