Composition and foraging activity of flower-visiting insects
Throughout the 208 censuses (104 hrs.) across five terms in 2017 and 2018, 15,127 insects were recorded on the flowers of 105 plant species. Hymenopteran insects occupied 30.4%, dipteran insects 62.7%, coleopteran insects 3.8%, lepidopteran insects 2.2%, and other insects 0.8% (Table 3). Major insect groups were non-syrphid flies (40.4%), bumble bees (24.9%), and syrphid flies (21.2%), while the frequencies of other insect groups were < 5%.
Seasonal patterns of visitation frequencies highly varied among major visitor groups (Fig. 4). The visitation frequency of bumble bees increased with seasonal progress in both years with a peak in the middle to late season (a positive coefficient of the linear term and a negative coefficient of the quadratic term of day number in the GLM; Table 4a). Worker bees occupied 98% (= 3710 / 3770) of all bumble bees. Thus, the seasonal trend of hymenopteran visitors reflected the population dynamics of bumble bee workers. In contrast, seasonal trends in the visitation frequency of dipteran insects highly varied between years. The abundance of syrphid flies was relatively high during the early half of the season and decreased in the later season in 2017, while it tended to increase with seasonal progress in 2018 (a negative and a positive quadratic term of day number in 2017 and 2018, respectively; Table 4b). The abundance of non-syrphid flies was large early in the season in 2017, while it was large late in the season in 2018 with lower values in the middle of season (positive quadratic values in both years; Table 4c). These results indicate that there is a clear seasonality in bee activity, while the seasonal trend of flies was unpredictable. The GLM results indicate that relative humidity was negatively related to the abundance of all insects (Table 4). The mean relative humidity during the censuses was 68%, ranging from 38 to 100 %. In contrast, the effects of ambient temperature varied among insect groups; bumble bees responded positively, syrphid flies responded negatively, and non-syrphid flies were less sensitive (Table 4). The mean ambient temperature during the visitor censuses was 17.9 ºC, ranging from 11.5 to 24.9 ºC.