1924 ENTOMOLOGICAL SOCIETY 41 



has not been satisfactory. In 1923, for instance, in one field in which the poison 

 bait was set out in pans ten feet apart, so as to give the method every possible 

 chance of success, the plantation suffered a 12 per cent, loss which was second 

 highest degree of infestation noted in the Ottawa district this year. In this 

 connection it is well to emphasize the extreme difficulty in obtaining satisfactory 

 check-fields. Sufficient attention has not, it is believed, been given to such 

 factors as dates of planting, soil fertility, soil moisture, and the existing degree 

 of infestation in relation to. the results obtained from experimental work and life- 

 history studies. 



As eggs may be taken in the field on May 24th in a normal year, poison bait 

 cans would necessarily need to be set out about May 15th in the Ottawa district, 

 or conjointly with the dates of germination of the onion seeds and the pans 

 would have to be kept filled until about June 30th, or after the termination of the 

 first-generation egg deposition period. From our egg records there was no dis- 

 continuance of the egg deposition during June, hence at least eight refillings of 

 the poison-bait cans would need to take place during a year of heavy infestation 

 to obtain satisfactory results. Unfortunately, since the writer undertook these 

 studies, he has not experienced a year of heavy attack during which close obser- 

 vations were kept, and has therefore not been able to demonstrate this method 

 of control applied under conditions entirely in harmony with the life-history 

 records. 



As regards the trap crop or cull onion method of control developed as a 

 result of studies made in British Columbia by Messrs. R. C. Treherne and M. H. 

 Ruhmann, the light infestation records at Ottawa during the last few years have 

 not enabled us to pronounce upon the value of this method which, however, 

 undoubtedly shows much virtue as the following records show. 



As a possible improvement in the trap-crop method of control, as described 

 on page 33 of the Fifty-second Annual Report of the Entomological Society of 

 Ontario for 1921, a number of cull onions were planted in soil in flat boxes, ten 

 inches wide and of a convenient length, with sides six inches high and fitted with 

 galvanized wire screen bottom. These flats were set thirty feet apart through 

 two fields of seedling onions during the summer of 1923. The percentage of loss 

 (estimated by comparing the number of original seedlings in the row with the 

 number of infested plants from an actual count), at the close of the season in one 

 field was 6.55 per cent., and in the other 1.08 per cent. Both fields were heavily 

 infested during 1922, the former being more heavily infested than the latter. 

 In three untreated fields during 1923 losses of 25 per cent., 12 per cent., and 6.8 

 per cent, were observed as bases of comparison. An average count of eggs taken 

 from ten trap boxes equalled the number of eggs collected from 376 feet of 

 seedling rows between May 26th and June 30th. 



As previously mentioned, the loss in a certain baited field was 12 per cent. 

 This may be regarded as a fair basis of comparison with the other fields men- 

 tioned. The other records obtained in baited fields were interfered with by 

 secondary factors which made them unsafe to use in a comparison of this descrip- 

 tion, but in each case the degree of infestation was higher than in the fields where 

 bait onions were used. 



Evidence is now available from a long series of notes which we could present 

 if time permitted, that in the use of trap onions we have a remedy which is 

 apparently more satisfactory than poison bait. A combination of the two 

 systems, using a poison bait with the cull onions in flats, may prove ultimately 

 to be the solution of the problem on onion-maggot control. 



