140 THE EEPOET OF THE Xo. 36 



THE CABBAGE MAGGOT IN BEITISH COLUMBIA (Phorbia brassicce). 

 The Natural Control by Parasites and Predacious Insects. 



E, C. Treherne, Field Officer, Dominion Entomolpgical Laboratory, 



Agassiz, B. C. 



The Cabbage Maggot fly is a very serious pest in the Lower Fraser Valley of 

 British Columbia. Variations in prevalence occur one year with another, and cer- 

 tain locality differences are observed in any given season. These changes are not 

 accounted for with any degree of satisfaction, but it is certain that autumn tem- 

 peratures and precipitation play an important part in the conditions that arise the 

 following spring. 



The importance of this insect may be gauged by its long period of activity. 

 Eggs may be frequently observed deposited on the stems of plants during the first 

 week of April, and oviposition may continue intermittently but continuously until 

 well in October. Nearly full-grown larvse have been taken from roots in the closing 

 days of April, and they may be found at all times until November and sometimes 

 •as late as December. These records were taken in the Lower Fraser Valley of Li/uisii 

 Columbia and apply only to that district, which ranges about 100 miles east of 

 Vancouver. They are, further, notes gathered from three years' work with the 

 fly, and are only given in this connection to indicate the serious possibilities thai 

 may follow an attack by the pest. 



During the past summer an attempt was made to estimate the egg-laying 

 proclivities of the fly. A number of plants, as indicated below, were examined 

 €very day, at the same time, from April 17th until October 26th. At each examin- 

 ation all eggs were removed and counted, so that as a result we find we liave an 

 accurate record of the total number of eggs laid per day throughout the summ,er. 

 As an indication of tlie seasonal prevalence during the past summer, I may S!>y 

 that in a large experiment on control measures, out of 315 cabbages, untreated and 

 used as checks, only 26 died strictly by reason of maggot attack (12.1 per cent.), 

 and out of 210 cauliflowers, of the same nature, only 24 died (11.4 per cent.). Of 

 ■course many plants were attacked and a diminution of weight was noticed at hari- 

 vesting, but they survived the ordeal of the attack and a certain weight was recorded 

 to their credit at the close of the season. 



The fertility of these eggs, of which at least 2,500 were tested over the whole 

 of the above period of time, was shown to be well over 80 per cent. This would 

 indicate that if all the eggs as laid persevered through to puparia, the percentage 

 of sound plants at the end of the season would be practically nil, despite the fact, 

 as above noted, that the season was light in comparative prevalence. 



Our field records, however, from careful root examination of both cabbages and 

 cauliflowers, show clearly that during the past season rarely, if ever, were more 

 than 25 larvae and puparia found at any one time. In other years I have taken 

 as many as 100 larvae and puparia from single roots of cabbages, but not so this year, 

 Avhich is comparable to the egg deposition records in Table A. 



The question then arises: What happens to all these eggs and small maggots? 



One answer is that it is probable the larval mortality is high in the very early 

 ■stages. I have experienced difficulty in bringing through young larva from the 

 ■eggs under laboratory conditions, and further from observations taken on the move- 

 ments of newly-hatched larvos on the surface of the soil, I am convinced a great 

 "many never reach the roots at all. However, I am not prepared to say much on 

 this point. 



