36 BULLETIN 124, U. S. DEPARTMENT OF AGRICULTURE. 



every growing sprig in the field. They are then sent to another field, 

 and so on and on, the owners in this way keeping their alfalfa ahead 

 of the butterflies, and by the clean-up method few larvae are allowed 

 to develop to adults. Of course, not everyone can have stock available 

 at just the right time, but this is another example of what clean-up 

 methods will do. 



IRRIGATION AS A FACTOR IN CONTROL. 



As has been stated in a previous paragraph, moisture is conducive 

 to the development of the disease which plays an important part in 

 the control of this insect. A number of experiments were therefore 

 tried by which, with the use of irrigation water, an attempt was 

 made to supply moisture artificially so that the worms would be- 

 come diseased. This was found to be quite successful. In fields 

 where clean methods of cutting are used at haying time and this is 

 immediately followed by irrigation, there seems little doubt that a 

 part of the mortality of the larvae is due to the effect of irrigation. 

 The beating sun, of course, kills a great many, and then, as has been 

 shown, under such a procedure the food supply is cut off and the de- 

 cayed remains of larvae are found hanging in great numbers to the 

 alfalfa stubs about two days after such a procedure. This led the 

 way for other experiments ; accordingly, during the summer of 1913, 

 Mr. T. Scott Wilson made a number of observations on irrigation of 

 alfalfa at a time when the worms were beginning to appear numer- 

 ously, and he found that invariably this gave the disease the neces- 

 sary moisture and the worms soon died. For a rancher to take ad- 

 vantage . of this would, of course, mean that he must have water 

 available any time he wants it, which is not the case in all irrigated 

 regions, as water is usually distributed in turn. However, in cases 

 where the time for irrigation corresponds with the occurrence of an 

 outbreak the water can be utilized and the worms killed. 



VALUE OF DISKING AND RENOVATION. 



It has been suggested before that an alfalfa field should be disked 

 or renovated annually, or oftener, in order to keep the sod in good 

 loose condition, so that it will take water readily and be aerated, 

 and also to kill weeds. If teams are available, the best procedure 

 would be to renovate several times, or at least twice a year. The 

 usual method is to renovate once, and this during the winter. Now, 

 if the alfalfa can be renovated in August, immediately after the 

 third crop is removed, not only will the ground be placed in an 

 excellent condition and weeds killed, but any larva? or pupa? on the 

 ground will be killed and future crops protected from damage. 

 Some ranchers do this already and claim great results for it, and 



