12 BULLETIlSr 75, U. S. DEPAETMENT OF AGBICULTUBE. 



Tripped flowers bearing pods per cent. . 58. 06 



Flowers not tripped when inclosed bearing pods do 5. 26 



Average number of seeds per pod from tripped flowers 3. 53 



Average number of seeds per pod from flowers not tripped when In- 

 closed 1. 00 



The abundant data presented in Table II permit the following con- 

 clusions : 



(1) Flowers not tripped either artificially or by insects may produce pods. The 

 percentage of pods to flowers under these conditions varies from 11.63 to 0, the aver- 

 age for 77 plants being 5.55. 



(2) Flowers artificially tripped produce pods in percentages ranging from 25.33 to 

 44.57, the average for 9,074 flowers on 77 plants being 30.68. 



(3) Under natural conditions the percentage of flowers setting pods varies from 9.24 

 to 43.75. The average percentage of pods from 8,939 flowers on 77 plants is 16.76. 



(4) The number of seeds per pod in artificially tripped flowers is usually less than 

 in naturally fertilized flowers, the average number of seeds per pod for 77 plants being 

 1.72 in the former case and 2.22 in the latter. In caged plants not tripped either 

 artificially or by insects the pods averaged 1.78 seeds each. The larger number of 

 seeds per pod in the exposed portions of the plants is perhaps due to cross-pollination. 



RELATION OF INSECTS TO TRIPPING. 



To obtain data on the efficiency of insects in tripping alfalfa flowers, 

 observations have been made at PuUman, Wash.; Chinook and 

 Havre, Mont.; Chico, Cal.; and at Arlington farm, Virginia. No at- 

 tempt was made to secure a list of visiting species, the object being 

 rather to ascertain the relation, if any, of insect visitors to seed forma- 

 tion. 



BEES. 



Among the commonest insects which visit alfalfa flowers are honey- 

 gathering bees. The data from detailed observations made at PuU- 

 man, Wash., and Chinook and Havre, Mont., are shown in Table IV. 



Table IV. — Alfalfa flowers tripped by different honey-gathering bees. 



Year. 



Species. 



■\\'here obsen'ed. 



Total 

 number 

 of flowers 

 visited. 



Flowers tripped. 



Number. Percent 



1909..., 

 1910. . . 

 1909... 

 1909. . . , 

 1909... 

 1909... 



Apis melliflca 



do 



do 



Bombusspp.' 



Megachile latimanub. 

 do 



PuUman, Wash . 

 do 



Chinook, Mont.. 

 Havre, Mont . . . 

 Pullman, Wash. 

 Chinook, Mont.. 



318 

 189 

 126 

 268 

 52 

 45 



0.31 

 1.58 

 4.76 

 29.47 

 90.38 

 93.33 



• Four species of Bombus were found tripping flowers at Havre, viz, B. auricomus Robertson, B. sepo 

 ratus Cresson, B. bi/arius Cresson, B. borealis Kirby. 



It will be noted that the leaf-cutting bee (Megachile latimanus Say) 

 is by far the most efficient, tripping about nine flowers out of every 

 ten visited. Bumblebees are decidedly inferior to Megachile, tripping 



