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



August 19, 1912. — All spikes examined. 



Nos. 1, 2, and 3 in good condition; no signs of thrips or other insects; no injury 

 from nicofume; good seed formed. 



No. 4. No injury from nicofume apparent; no evidence of thrips or other insects; 

 emasculated July 24, 1912; pollinated July 27, 1912; of 10 flowers pollinated 8 pro- 

 duced seed. 



No. 5. General condition similar to No. 4; emasculated July 24, 1912; not polli- 

 nated; 13 flowers emasculated; all remained sterile. 



No. 6. Similar to No. 5; all flowers remained sterile. 



No. 7. Check; spike dead. 



Tliis experiment shows that the treatment with nicotine solution 

 did not perceptibly injure beet flowers and that it at the same time 

 removed thrips from them. 



CONCLUSION. 



From these experiments it is seen that these minute insects, the 

 numerous species of Thysanoptera, some of which more or less injuri- 

 ously infest practically all our plants, are also active agents in pol- 

 lination. Among beet flowers they are frequently very numerous 

 indeed, effecting both close pollination and cross-pollination upon 

 them. However, after taking into account the various forms of 

 injury they do, it is doubtful whether the balance remains in their 

 favor in regard even to beets. Under ordinary conditions, in fields 

 of commercial seed beets, it is believed that on the whole their work 

 is beneficial; but should they become excessively numerous, they 

 sustain their reputation as one of our really destructive pests. To 

 the horticulturist and plant breeder they are pests of the worst type, 

 necessitating constant watchfulness and a refined technic in all 

 pollination work. 



The suggestion is ventured that certain supposed mutations may 

 really have been the result of unsuspected cross-pollination by means 

 of one or another species of thrips, whether in cereals supposedly 

 not susceptible to cross-pollination without the intervention of man 

 or in flowers which were thought to have been isolated against 

 cross-pollination. 



o 



WASHINGTON' : GOVERNMENT PRINTING OFFICE : 1914 



