Effects of honey bee (Apis mellifera L.) colony size on the pollination of greenhouse-cultivated watermelon (Citrullus lanatus L.) under forcing cultivation

Abstract

We investigated the effects of honey bee (Apis mellifera L.) colony size on the pollination of greenhouse-cultivated watermelon grown under the forcing cultivation system. The highest pollination activity of bees was observed ($14.3{\pm}5.0$ honey bees/day) when the bee colony size was 10,000 followed by 7,500 and 5,000 honey bees. There was a positive correlation between the bee colony size and pollination activity (R = 0.262) but insignificant difference in fruit set with different honey bee colony sizes (88%-91%). Evaluation of physical properties revealed that the weight and shape of watermelon were also not significantly different among different colony sizes. However, larger the bee colony size, higher the number of seeds were fertilized and rate of seed fertilization (p > 0.05). Number of seeds and content of sugar were negatively correlated (R = -0.714). Fertilized seeds showed a significant increase in mealy flesh, which has a negative effect on fruit quality, compared with that of the unfertilized seeds. Overall, we found that a colony size of 5,000 honey bees was the most effective for the pollination of watermelon grown under forcing cultivation. A comparison of the effects of bee pollination with those of artificial pollination suggested that artificial pollination can be effectively replaced by bee pollination in the forcing cultivation of watermelon, because fruit set, weight, and shape by bee pollination were similar to those achieved by artificial pollination.

Fig. 1. Relationship between pollination activity and different colony sizes.

Fig. 2. Relationship between the pollination activity of bees for a colony size of 5,000 bees and distance from the colony box in (A) and between the pollination activity of bees and distance from the colony box in (B).

Fig. 3. Fruit set of watermelon in terms of different pollination methods and colony sizes of honey bees. Different letters indicate significant differences among the pollination methods, as determined by one-way ANOVA and Tukey’s HSD (p < 0.05).

Fig. 4. Regression analysis between the content of soluble solids and number of fertile seeds (black circles).

Fig. 5. Comparison of the size of mealy flesh around the seeds between the fertile seeds and sterile seeds. “*” indicates significant difference at p < 0.05 (t-test) between the fertile seeds and sterile seeds.

Table 1. Pollination activity of bees per day in the greenhouse according to the distance from the colony box

Table 2. Correlations between pollination activity in the greenhouse according to different colony sizes and distances from the colony box

Table 3. Fruit set of watermelon in the greenhouse according to the distance from the colony box

Table 4. Effects of different colony sizes on the physical properties of non-cleaved watermelon

Table 5. Effects of different colony sizes on the physical properties of cleaved watermelon