18 BULLETIN 844, U. S. DEPARTMENT OF AGRICULTURE. 



This experiment was also conducted at Ames. One plant was 

 protected from insect visitation at night in August, 1916, with the 

 result that it produced 418 racemes, with an average of 41.11 pods 

 each. 



PLANTS PROTECTED FROM ALL INSECTS THAT COULD NOT PASS THROUGH A WIRE 

 SCREEN HAVING 14 MESHES TO THE LINEAR INCH. 



It is well known that many small insects, and especially those 

 belonging to the family Syrphidae and to the genus Halictus, frequent 

 sweet-clover flowers, but no records have been noted tha.t show how 

 important these insects are as pollinators of this plant. In order to 

 obtain data on this subject a cage 12 feet square and 6 J feet high, 

 made of wire screen having 14 meshes to the linear inch, was placed 

 over a few plants at Ames, in July, 1916, before they began to bloom. 

 The base of the cage was buried several inches in the soil, so that no 

 insects could get into it. As these plants were growing in a field 

 where there was a sufficient supply of moisture at all times, they made 

 a growth of 5 to 6 feet. For this reason all the racemes were collected 

 from only a portion of one of the plants instead of from the entire 

 plant, as was done with the smaller ones inclosed in the cheesecloth- 

 covered cages. The branches selected contained 224 racemes, with 

 an average of 24.53 pods each. Many insects that were able to pass 

 through the wire netting were observed working on the flowers of the 

 inclosed plants. 



A check plant, subject to visitation by all insects and growing 

 within a few yards of the cage, contained 264 racemes, with an average 

 of 28.23 pods each. 



This experiment shows that small insects are efficient pollinators 

 of sweet clover and that the plant to which all insects had access 

 produced an average of only 3.7 pods to the raceme more than the 

 one inclosed in the cage. As these plants were growing close to a 

 strip of timber and some distance from a field of sweet clover, it is 

 probable that more small insects worked on the flowers than would 

 have been the case if the cage had been located in the center of a 

 field of sweet clover. Though these results show that small insects 

 are able to pollinate sweet-clover flowers freely, it is very doubtful 

 whether insects of this kind would be numerous enough to pollinate 

 sufficient flowers in a large field of sweet clover for profitable seed 

 production. The honeybee is the most efficient pollinator of this 

 plant, and it is believed that in many sections it is responsible for the 

 pollination of more than half of the flowers. 



SUMMARY OF INSECT-POLLINATION STUDIES. 



The data secured in the different experiments where sweet-clover 

 flowers were subject to insect visitation at one time or another are 

 presented in detail in Table V. 



