Proceedings , 1920. 93 



separated by icxD yards of an alfalfa-field, objected to the process of rethin- 

 ning owing to the increased cost. He was prevailed upon, however, to do 

 the rethinning on 5 acres, leaving the remaining 3 acres untouched. The 

 results of this work were most surprising. The rethinned 5 acres yielded 

 a crop at the end of the year of 25 tons to the acre, while the untouched 3 

 acres produced a total of 900 lb. 



At the close of the year the statement was made in our annual report that 

 results from the use of the poisoned bait had been unsatisfactory, but that 

 the bait was still believed to be satisfactory. It seemed essentially necessary, 

 however, to conduct a close study of the life-history of the fly in order to 

 correlate its habits with the use of the bait. The onion-growers, further, 

 were very anxious to obtain more information leading towards control ; 

 hence it was decided to include the study of this fly in the outline of experi- 

 ments for 1919. 



In 1919 an extensive experiment with the use of sodium-arsenite and 

 molasses bait (^ oz. to ^4 pint) was carried on at Vernon Specially con- 

 structed cans were made, filled with water, which allowed a certain quantity 

 of water to filter down on to felt pads arranged in trays. The felt pads, 

 having been soaked in the bait and a few ounces of the poisoned liquid 

 poured over them, retained their moisture for four to ten days under all 

 temperatures. Twenty of such bait-cans were placed to the acre between 

 the rows of onions and 23/2 acres in an 8-acre field in all were treated in 

 this experiment. The bait-cans were set on May 14th, five days before any 

 eggs were laid in the field, and they were maintained continuously until the 

 end of June, and again during the first two weeks in August. 



Samples of the poisoned liquid were taken two or three times a week 

 from the trays and fed to flies. The adults were observed to be in distress 

 within five minutes of actual feeding and were completely stupefied in thirty 

 minutes, and this condition prevailed throughout, despite frequent dilutions 

 from rain and from the water in the reservoirs above the felt pads. Flies 

 left to their own devices in the laboratory in the presence of drops of 

 poisoned bait gave a loo-per-cent. mortality in less than twenty-four hours. 

 Circular fly-traps arranged in the field, each baited with different essential 

 oils and mixtures, undoubtedly showed that molasses was the most suitable 

 substance to attract the flies. Despite these two favourable circumstances, 

 we are unable to claim that the poisoned-bait method is a satisfactory control 

 under such field conditions as prevailed at Vernon, B.C., in 1919. Oviposi- 

 tion took place on seedling onions and on volunteer onions (from the crop 

 of the previous year) in such numbers that it was impossible to state that 

 the poison bait had any effect in checking oviposition. In several instances 

 volunteer onions gave as many as 500 eggs to the plant during May and June. 



The influence of a suitable type of volunteer onions as a trap-crop for 

 the first generation presents a phase of onion-maggot control that must be 

 regarded very highly. In the 8-acre experiment of 1919 a number of sec- 

 tions of onion-seedling rows were examined daily for oviposition, and 

 it may be stated that there was no material difference in oviposition in the 



