30 BULLETIN 124, IT. S. DEPARTMENT OF AGRICULTURE. 



flies to feed upon and to the fact that the greater part of the fields 

 was kept closely grazed, making the alfalfa less favorable for the 

 la3 T ing and development of the eggs. Under such conditions the 

 number of eggs deposited in a given field is greatly reduced. Many 

 of the eggs laid on the young growth under such conditions are 

 destroyed by the grazing of the stock, and the percentage that de- 

 velops is kept at a minimum. Later on it was noted that on the 

 stock ranches visited the disease previously mentioned, which is com- 

 mon to lepidopterous larvae, was more prevalent in pastured ranches 

 than in hay ranches. The prevalence of the disease in such fields 

 is due to the fact that usually a few days after stock are turned in 

 the alfalfa becomes trampled. The ground and the alfalfa are very 

 moist, there being more or less dew every morning, and droppings 

 from the cattle bring about a foul condition in the field, thus assist- 

 ing in the retention of moisture, which, in turn, is conducive to the 

 development of the disease. 



If fields can be systematically and carefully pastured, damage from 

 the caterpillar will accordingly be at a minimum. Cattle should 

 never be allowed on a field when wet nor for too long a period, 

 say from 24 to 35 days, and disking or renovating should always 

 follow so as to loosen the soil and place it in a receptive condition 

 for future irrigation. 



It is on ranches and fields from which successive crops of hay are 

 taken that the height of the damage is reached. In such fields the 

 conditions for the development of the species are as nearly ideal as 

 possible, and here the worms are ordinarily unmolested in their feed- 

 ing and growth. The period elapsing from the time that one crop is 

 cut until another is ready for harvesting so nearly coincides with the 

 length of the period necessary for the development of any one gen- 

 eration of the butterfly that the cutting of the hay, as ordinarily 

 carried on, does not reduce their numbers or disturb their work, 

 since the worm will likely be in the adranced stage, or, perhaps, have 

 passed into the pupal stage, before the crop is cut. 



CONDITIONS AFFECTING INJURY. 



As has been pointed out, this insect is ordinarily kept in control 

 by its natural enemies, such as insect parasites and diseases, and it is 

 only upon the occurrence of conditions unfavorable to the development 

 of these enemies that serious outbreaks occur. It has also been noted 

 time and again, both by the writer and others, that the seriousness 

 of the damage quite often depends upon the farming methods used 

 by the individual whose fields are attacked, or upon certain other 

 conditions, such as character of soil, quantity of water for irrigation, 

 location of land, etc. The former are conditions that the individual 

 may remedy by changing his methods, while the latter may be prac- 

 tically alleviated by proper handling of the farm in question. 



