18 BULLETIX 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 Syrphidas and to the genus Halictus, frequent 
sweet-clover flowers, but no records have been noted that 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^ 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. 
