38 THE REPORT OF THE No. 33 



Several experiments were conducted to determine the method used by the 

 maggots to get into the onion bulb. Seedling plants were used and newly- 

 hatched maggots were placed within half an inch of the plant. Within two and 

 a half hours all the maggots had burrowed into the soil but none were found 

 within the bulb in less than twenty-two hours. The first maggots entered the 

 bulb at a point about 6 mms. above the root attachment. The puncture was 

 roughly oval in shape, about 1.5 mms. by .7 mms. After the opening had been 

 used by the maggots and the tissues had shrunk, it materially increased in size. 



Only one case was observed where the maggots entered by the leaves. The 

 eggs had been laid well up on the plant at the junction of two leaves. When the 

 larvae hatched they penetrated the leaf and worked down the inside to the bulb. 

 Twenty-four hours after penetrating the leaf the maggots were about 5 cms. 

 below the point of entry and forty-eight hours after hatching they had reached 

 the bulb. 



Control Experiments 



During the season of 1922 crude naphthalene was used on rows of onions 

 after the damage had begun to appear. The results were rather surprising as 

 the mixture of naphthalene and furnace ash had only been cultivated into the 

 soil close to the plants and had killed over 80 per cent, of the maggots in four days. 

 In 1923 it was decided to try some more experiments with this material, but 

 unfortunately most of the onions were seeded before the work could be started. 

 Mixtures containing crude naphthalene were worked into the soil in the prelimin- 

 ary cultivation, but it was only in the case of a small late-seeded plot, and the 

 results were not satisfactory. 



Other mixtures were tried after seeding, and when the damage first appeared. 

 Some of these showed promise, and will be tried again next year, using larger 

 plots. Each plot this season consisted of a single row of onions with check rows 

 between the plots, and the damage was very nearly the same on all the rows 

 with a few exceptions. It is very evident that to get any definite results from 

 soil-fumigant work the plots must be larger, and separated by a greater distance 

 than exists between rows as laid out in the ordinary garden. 



The fact that the volunteer and early-sown onions were most heavily in- 

 fested, and that eggs were found on them earlier than on any of the other plants, 

 demonstrates the possible value of using onions in a trap-crop method of control. 

 The later sown onions were almost free from eggs until about the end of the 

 oviposition period of the first generation. 



The females almost invariably select plants of heavy flaccid growth for 

 oviposition. Volunteer onions or sets that are rather deep in the ground, so that 

 the leaves branch before reaching the surface, will carry the bulk of infestation. 

 Plants that have a pronounced neck with the leaves branching an inch or more 

 above the surface of the soil are seldom chosen by the females for oviposition, 

 and an examination of several plants showed this type of onion to be almost free 

 from infestation. 



There is a distinct relationship between rainfall and oviposition which 

 requires further study. This season it was observed that oviposition dropped 

 off materially before a storm and ceased until a day or so afterwards. It is hoped 

 that another year will amplify our records on the life-history and natural control 

 factors. 



