December, '21] horsfall: onion thrips infestation 495 



One serious outbreak of thrips in a field of set onions near Davenport 

 was directly traceable to the fact that adjoining the sets the grower 

 had six rows of perrenial multiplier onions. The thrips had wintered 

 over on these and thus had an opportunity to begin breeding early. 

 By June 19, they had spread over the first twenty rows of set onions 

 seriously checking their growth. The further spread in this field was 

 checked by the use of a Hardie barrel sprayer with hose extension to 

 run between the rows. Nicotine sulphate-soap formula was employed. 



Another source of spread was the greenhouses of the Davis Floral Co. 

 near Davenport, Iowa. This concern is a large producer of hothouse 

 tomatoes and cucumbers. Here the thrips had a chance to breed on the 

 cucumbers through the winter and in the spring they spread from the 

 houses to onion fields in the neighborhood. The first infestations noted 

 in the fields in three different directions from the greenhouses were, 

 in every case, on the side of the field nearest the greenhouses. Then, 

 too, these attacks occurred from two to three weeks earlier than the in- 

 festations in other sections of the onion growing district. Attempts 

 made by this company to grow onions in the* near vicinity of the houses 

 failed because of this early infestation by thrips which had emerged 

 from the cuomiber houses after breeding there during the winter. 



One field of seed onions was visited in Mitchell County, Iowa, on 

 August 26 where the source of infestation was very evident. This 

 field of 10 acres was bordered along the east, south, and west sides by 

 spruce trees forming a windbreak. At the east end of the field, well 

 imder these spruce trees, was a long pile of refuse where the tops and 

 screenings of the 1918 season had been thrown when hauled from the 

 field. An area heavily infested by the thrips was found to extend 

 around the three sides of the field bordered by the spruce, but this 

 area extended in the farthest at the east end. The thrips had evidently 

 wintered in this pile of refuse and in the matted grass under the spruce. 



A few other sources of infestation might be listed in addition to those 

 already enimierated. In all cases they were factors because they had 

 furnished protection to hibernating adults during the winter. In one 

 field an area bordering an implement shed was the first part to become 

 affected by the thrips. The tops of these onions were noticeably yellow 

 while the rest of the field was still green. Upon examination, the tops 

 of the onions near the sheds were found to be swarming with thrips 

 whereas these insects could only be found deep in the sheaths of the 

 onions over the rest of the field. In a field adjoining an orchard, the 

 semi-circular region of "blast" appeared bordering the orchard. Where 

 bluegrass and weeds along creeks, roads, or railroads were i)bt burned, 



