June, '15] 



SEVER IX-SEVERIN: IMPORTED OXIOX FLY 



345 



In all probability, the last mentioned parasite attacks both species of 

 Phorbia. 



In the field onion flies which had succumbed to the effects of a 

 fungus disease were frequently found adhering to onion leaves. In the 

 laboratory the disease was spread to healthy flies confined in breeding 

 jars. 



IV Probable Causes of Enormous Increase of 

 Imported Onion Fly 



After a large number of commercial onion fields were visited and the 

 methods employed by the growers were observed, it soon became 

 evident that there are a number of reasons why the onion fly has in- 

 creased to such enormous numbers in Wisconsin. The most favorable 

 conditions for the multiplication of the pest rests in the fact that no 

 steps are taken to destroy the maggot}^ onions in the fields. Most of 

 the onion growers pay no attention to clean field methods. 



As there was a low market for onions in the fall of 1912, many of the 

 growers stored their seeded onions in warehouses over winter, with the 

 expectation that a better price could be obtained in the spring. Dur- 

 ing the winter mam' farmers experienced difficulty with the onion rot 

 {Sclerotium cepivorum) and all diseased onions were dumped on the 

 land. In the spring there was no market for the onions and the bulbs 

 were dumped and plowed under in the fields. One grower used 

 18,000 bushels as a fertilizer. Some of these onions sprouted early in 

 the spring, offering the first brood of flies a most favorable opportunity 

 to oviposit before the onion seeds germinated. It was common to see 

 these onions growing here and there in sugar beet and potato fields. 

 In fields where onions had been grown the previous year and where 

 rotation of crops was practiced, hundreds of eggs of the pest were 

 obtained by removing the soil in contact with the onion stems. An 

 examination of the decaying bulbs plowed in the soil showed the pres- 

 ence of numerous maggots. Puparia were found in the soil beneath 

 these decayed onions and the second brood of onion flies emerged from 

 June 14-20; whereas, in the case of infested green onions under a cage 

 in the field, the second brood issued from June 28 to July 25, most of 

 the flies issuing from July 1-12. 



When the onions are so seriously infested that the crop is not worth 

 harvesting, the growers simply plow under the maggoty onions, thus 

 giving the pests a most favorable opportunity to complete their life 

 history. 



During the marketing of the crop many infested, decayed and in- 

 jured onions are thrown in the fields. In grading the onions, the men 

 throw out of the screen all infested, decayed and injured bulbs. 



