Weather conditions affect pollinator activity, fruit set rate, and yield in Japanese apricot

Japanese apricot blooms in mid-winter, and thus severe weather conditions, including low temperatures, sometimes limit pollinator activity and consequently decrease fruit set rate and yield. Plant physiology is also affected by weather conditions. In this study, we aimed to evaluate the effect of pollinator activity on fruit set rate and yield, in comparison with the effects of meteorological factors. First, we conducted field experiments in 2021 and 2022 at Wakayama prefecture. The results showed that honey bee visitation increased as ambient temperature increased. Fruit set rate also increased under warm conditions when Japanese apricot was naturally pollinated by honey bees. In contrast, hand-pollinated flowers were not affected by ambient temperature. These results revealed that low temperature during the flowering period decreased pollinator visitation but did not affect plant fertilization ability. To compare pollinator activity with meteorological factors over a long period, including the pre- and post-flowering periods, we used a state-space model describing the time series of fruit set rate. The model using fruit set rate data and meteorological records over 15 years (2007–2021) at the Japanese Apricot Laboratory showed that the pre-blossom temperature and number of days suitable for pollination significantly affected the fruit set rate. Another model for the time series of fruit yield using the fruit yield records over 46 years (1976–2021) in Wakayama prefecture showed that fruit yield was affected by number of suitable pollination days, pre-blossom temperature, and sunshine duration before harvest. Moreover, the introduction of Western honey bees into orchards had considerable effects on fruit yield. These results indicate that pollinator activity (which is responsive to weather conditions) and pre-blossom temperature have a significant influence on fruit set rate and yield in Japanese apricot.