Table VII.--_Influence of the weather at blossoming time upon the yield of sweet -clover seed, at Ames. Iowa, in 1915._
-------+-------------------------+---------+-------+-------+----------- | | |Number | | | | | of | | Percentage Date, | Weather conditions. | Insect |flowers| Pods | of flowers 1915. | |visitors.| that |formed.| that | | |opened.| | matured. -------+-------------------------+---------+-------+-------+----------- Aug. 16|Cloudy and showery | Very few| 102 | 18 | 17.6 Aug. 17|Rain all day | None | 69 | 4 | 5.7 Aug. 18|Cloudy most of the day | Very few| 60 | 20 | 33.3 Aug. 19|Clear and cool | Numerous| 94 | 53 | 56.3 Aug. 20|Mostly clear and warm | do | 61 | 38 | 62.2 Aug. 21|Clear and warm | do | 81 | 44 | 54.3 Aug. 22|Partly cloudy and warm |} | | | Aug. 23| do |} do | 181 | 100 | 55.2 Aug. 24|Cloudy till mid-afternoon| Few | 37 | 12 | 32.4 -------+-------------------------+---------+-------+-------+-----------
The data given in Table VII show that the percentage of effective pollination is much higher in clear weather, when insects are active, than in cloudy or rainy weather, when but few insects visit the flowers.
On account of the ease with which the heavy flow of nectar of sweet-clover flowers may be obtained many insects visit the flowers, thereby pollinating them. While the useful insect visitors of flowers of red clover are limited to a few species of Hymenoptera, those pollinating sweet-clover blossoms are many and belong to such orders as Coleoptera, Lepidoptera, and Diptera, as well as to the Hymenoptera. However, in the United States the honeybee is the most important pollinator of sweet clover. In many parts of the country the different species of Halictus, commonly known as sweat bees, rank next in importance. The margined soldier beetles (_Chauliognathus marginatus_ Fabr.) were very active pollinators at Arlington, Va., in the latter part of June and first part of July, 1916, but the woolly bear (_Diacrisia virginica_ Fabr.) was the only night-flying insect found working on sweet clover at Arlington.
Insects belonging to the genera Halictus, Syritta, and Paragus were very active pollinators at Ames, Iowa, in 1916, and ranked next in importance to the honeybee. In fact, the results obtained in the cage where the plants were protected from visitation by insects that could not pass through a screen having 14 meshes to the linear inch showed that these small insects were able under the conditions of that experiment to pollinate practically as many flowers as larger insects.