32 THE EEPOET OF THE No. 36 



in the form of coarse drops and placed in open "pie-dishes" finds favour in many 

 sections of North America. In such sections this bait is regarded as the most 

 satisfactory commercial control operation, but it must be conceded that even it 

 has not always been successful in preventing infestation. Too much rain during 

 the spring oviposition period or adverse climatic factors are offered as excuses for 

 such failures. Nevertheless, judging from information at our disposal, onion 

 growers, particularly in Eastern Canada, and United States, are being advised 

 to use the sodium arsenite bait. Under conditions that j)revail at Vernon, B.C. 

 and doubtless applicable to all "dry belt" conditions, we are clearly of the opinion 

 that this bait is o.f no practical value, possibly owing to the presence of irrigation 

 water in ditches or furrows. This statement is based on four years' records in its 

 use at Vernon. In 1918, the ordinary "pie dish" method of application was 

 employed and the growers took a decided loss following its use. In 1919, an 

 improved "pie-dish" was devised containing a bait-saturated felt pad over which 

 was arranged an air tight reservoir capable of holding a half gallon of water. 

 The felt pads were thus maintained in a continuous moistened condition for about 

 two months with occasional renewals of water and bait. The same dishes were 

 used in 1920 and 1921 and records on oviposition in baited and non-baited areas 

 were carefully kept each year. No variable weather condition affected these baits 

 and twice-a-week tests of liquids yielded 100%' mortality with flies when fed in 

 confinement. Dead flies were also seen in the field, but the simple fact remains 

 that under field conditions results of value were not obtained, and. we are now 

 "forced to abandon the poisoned bait control measure as a recommendation to 

 growers in the "dry belt" of British Columbia. 



We have, however, been able to devise another control operation which 

 judging from results thus far obtained, is more efficient and less expensive. 

 This operation takes advantage of the selective habits of the adults for ovi- 

 position. It was noticed that any volunteer onion growth present in the field 

 invariably attracted the first flies during the May oviposition period and further 

 it was observed that the stronger growing seedling onions, near an irrigation 

 ditch, attracted the adults during June. As pointed out in the life history 

 notes oviposition commences in the middle of May when the onions are only VA 

 inches high. At this time the volunteer onion growth was four to eight times 

 that of the seedlings. In other words, the volunteer onion growth of May was 

 equal to the seedling onion growth of June and both attracted flies for the pur- 

 pose of oviposition. 



"We learnt, therefore, to recognize the fact that these volunteer onions were 

 a powerful factor in trapping the first generation flies for oviposition and, on 

 developing this idea, we also found out that a great deal depends on the type of 

 growth produced. We have already mentioned the fact thai; in mid-summer, 

 during the flight period of the second generation flies, the weakest plants or those 

 often previously attacked, proved more attractive to the adults than the strong 

 growing bulbs. This observation, as it applies to the mid-summer habits of the 

 flies also applies to the spring habits as it relates to the type of volunteer onion 

 growth. In short, the volunteer onion which shows a bulb or "neck" with the 

 leaves arising some distance from the soil was not chosen for oviposition. The 

 type of groAvth which proved most attractive was produced by a cull bulb in 

 which the leaves grew flaccid and which arose at the ground level. As a matter 

 ■of experience less than 10% of the volunteer onion growth under normal conditions 

 yielded the most suitable type of growth, but on such' plants, practically the entire 



