December, '21] horsfall: onion thrips infestation 493 



SOURCES OF INFESTATION OF THRIPS TABACI IN IOWA 



By J. L. Horsfall, Dubuque College, Dubuque, Iowa 



While employed as entomological assistant by the Iowa Experiment 

 Station during the summer seasons of 1917-'1S-'19 the writer was 

 engaged in investigations of the Onion Thrips. During these investiga- 

 tions the following data were of particular interest in their bearing on 

 the problem.. In the vicinity of Davenport, Iowa, som.e five hundred 

 acres are devoted to onion culture. From five to ten percent of this 

 acreage is planted in set onions for the early m.arket while the larger 

 amount is grown from. seed. 



Records show that the thrips will establish themselves and begin 

 breeding on set onions from two to three weeks earlier than on seed 

 onions. From our life history studies we found fifteen days to be the 

 average life cycle for Thrips tabaci during the last two weeks in June. 



Thus the thrips have an opportunity to produce an early generation 

 on the set onions. These will infest the seed onions in far greater 

 numbers than would be the case were there no set onions grown in the 

 neighborhood. Consequently, wherever set onions are planted in the 

 vicinity of seed, they have proven a source of infestation for the later 

 crop and it is doubtful whether the larger returns realized from the 

 early crop compensate for this damage. 



On June 24, 1919, the writer was called to investigate a report of thrips 

 infestation in a 5 acre field of seed onions near Davenport. The onions 

 in an area covering 4 square rods in the western comer of the field 

 showed the characteristic yellowish "bHght" which always indicates a 

 severe infestation of thrips. This same field had been visited in the 

 summers of 1917-'18 and was practically free from thrips both years. 

 Across the fence from the infested comer was a 10-acre field of alfalfa 

 which had been planted in 1918. This field had been in corn in 1917. 

 The nearest field of onions to the west was two miles distant. On 

 June 22, two days before the visit mentioned, the first cutting of alfalfa 

 had been taken off. While the alfalfa was being cut the air was filled 

 with flying thrips. This was mentioned especially by the farmer across 

 the road who noticed them alighting on his clothing. The prevailing 

 wind that day was from the east. Considering the above facts, it is 

 evident that the alfalfa must have been the place of hibcmation for the 

 thrips and when this was cut they were induced to migrate. Since 

 these insects fly wath the wind they would only be carried over the west 

 comer of the onion field. The resulting spread in this field was traced 



