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.