Diurnal and geographic variations of pollinator importance for Cucurbita maxima Duchesne

Despite growing awareness of the importance of monitoring wild crop pollinators worldwide, there are still few reports, especially in East Asia. Considering ongoing global warming may change the distribution range and diurnal activity of pollinators, it is necessary to describe current geographic and diurnal patterns. We clarified pollinators of Cucurbita maxima Duchesne (Cucurbiales: Cucurbitaceae) in three geographically distinct (>350km, minimum) areas in Japan, focusing on diurnal variation. Apis mellifera L. (Hymenoptera: Apidae) and Halictidae (Hymenoptera) were observed in all of the experimental gardens. Apis cerana japonica Radoszkowski (Hymenoptera: Apidae) were mainly observed in Mie and Kagoshima, while Bombus diversus diversus Smith (Hymenoptera: Apidae) were observed only in Ibaraki. The peak time of flower visits depended both on bee taxa and area and, interestingly, did not necessarily synchronize with the timing of highest pollen loads and the probability of stigma contact. In particular, visits and probability of contacting stigmas of Halictidae tended to increase as time passed, whereas pollen grains on their bodies sharply decreased with time; only a few individuals of Halictidae spp. that visit early can become effective pollinators. There were no differences in yields between supplementary hand- and natural-pollination in all areas, and flower-enclosure experiments using different mesh sizes clarified that small insects that can go across an approximately 4mm mesh may not transport sufficient pollen for fruit set. Our study demonstrated that pollination effectiveness, which is usually regarded as a static value, within a taxon can fluctuate in the space of just several hours. Considering such diurnal patterns can be altered by climate change, we need to carefully monitor the diurnal temporal patterns of pollinators worldwide.

尽管全球范围内对野生作物传粉昆虫（wild crop pollinators）监测工作重要性的认知不断提升，但相关研究报道仍较为匮乏，东亚地区尤为如此。鉴于持续的全球变暖可能改变传粉昆虫的分布范围与昼夜活动节律，阐明当前传粉昆虫的地理分布与昼夜活动模式已成为必要之举。本研究以日本3个地理间距超350km（最小间距）的独立区域为研究对象，聚焦昼夜活动节律，明确了笋瓜（*Cucurbita maxima* Duchesne，葫芦目Cucurbiales、葫芦科Cucurbitaceae）的传粉昆虫类群。
所有试验样地中均观测到西方蜜蜂（Apis mellifera L.，膜翅目Hymenoptera、蜜蜂科Apidae）与隧蜂科（Halictidae，膜翅目）昆虫。日本蜜蜂（Apis cerana japonica Radoszkowski，膜翅目蜜蜂科）主要在三重县与鹿儿岛县被观测到，而异色熊蜂指名亚种（Bombus diversus diversus Smith，膜翅目蜜蜂科）仅在茨城县被记录到。
访花高峰时段同时受蜂类类群与采样区域的双重影响，且值得注意的是，该高峰未必与花粉载量峰值以及柱头接触概率峰值同步。具体而言，隧蜂科昆虫的访花频次与柱头接触概率随时间推移逐渐升高，但其体表携带的花粉粒数量却随时间显著下降；仅少数早期访花的隧蜂科个体可成为有效的传粉者。
各区域的人工辅助授粉与自然授粉的坐果率均无显著差异；此外，采用不同网孔尺寸的花套袋实验证实，能够穿过约4mm网孔的小型昆虫或许无法携带足够花粉以保障坐果成功。本研究表明，通常被视作静态指标的传粉有效性（pollination effectiveness），在同一类群内仅在短短数小时的时间跨度内就可能出现波动。考虑到这类昼夜活动模式可能因气候变化发生改变，我们亟需在全球范围内对传粉昆虫的昼夜活动时序开展精细化监测与研究。