10 BULLETIN 104, U. S. DEPARTMENT OF AGRICULTURE. 



pollination and subsequent fertilization when transferred from one 

 flowering beet to another, but that they voluntarily travel from plant 

 to plant and carry pollen throughout a sustained flight. 



From this evidence there can remain no doubt that these insects 

 are capable of playing an important role in the pollination of beet 

 flowers. May it not be a very significant one ? It is known that in 

 certain parts of Europe and the United States beets have sometimes, 

 perhaps frequently, failed to produce seed, although an abundance 

 of bloom appeared. It seemed possible that the presence or absence 

 of thrips in great number might in part be responsible for this phe- 

 nomenon. Subsequent observations, however, afford evidence in dis- 

 proof of this theor}^. It is safe to say that thrips undoubtedly assist 

 in the pollination of beet flowers, perhaps to a greater extent than 

 any other species of insect. It can scarcely be doubted that they 

 perform a like service for many other plants. 



INJURY TO SEED BEETS. 



Thrips feed avidly upon the nectar and pollen of beets, but beet 

 pollen is so abundant that unless thrips be present in enormous num- 

 bers they apparently do no damage to the floral organs, preferring as 

 food the nectar and pollen. However, should they become extraor- 

 dinarily numerous, as was the case at Ogden in 1912 during the 

 latter part of the season, it would seem that the nectar and pollen 

 are not sufficiently abundant to supply their truly voracious appe- 

 tites. They then attack the more delicate and succulent parts of the 

 flowers. Sometimes the styles are cut through at the base, but 

 more frequently the papillae, with which the lobes of the stigma are 

 thickly studded, are torn to pieces. Furthermore, they may devour 

 so much pollen as to interfere with both wind and insect pollination 

 by too greatly diminishing the supply. Thrips move actively from 

 flower to flower of the same spike, from spike to spike and stem to 

 stem of the same plant, and in this way bring about much more close 

 pollination than cross-pollination, and in fact effect all the close polli- 

 nation and fertilization of which the plant is susceptible. This in 

 itself is undesirable and even harmful. Close fertilization has been 

 shown to cause degeneration among beets, even in the sense of pol- 

 lination and fertilization between different individuals of the same 



progeny. 



TROUBLE TO PLANT BREEDERS. 



The writer has experimentally shown that the larvae of thrips in 

 all stages readily pass through the meshes of fine silk chiffon and 

 much more readily through the net, cloth, and sheeting frequently 

 used by horticulturists and plant breeders to isolate flowers designed 

 to be hand-pollinated. He has also been able to demonstrate that 



