Hilgendorf. — On Plutella cruciferarum. 145 



Art. IX. — Life-history of Plutella cruciferarum, Zeller. 



By F. W. Hilgendorf, M.A., B.Sc, Canterbury Agricul- 

 tural College. 



[Read before the Philosophical Institute of Canterbury, 3rd October, 



1900.'] 



The diamond-back moth is referred to in every work on agri- 

 cultural entomology, but the best notices occur in " Farm 

 Insects" (Curtis, p. 85), and in a "Special Eeport to the 

 Board of Agriculture, London," by Charles Whitehead, 1891. 

 In neither of these, however, is the life-history given with any 

 degree of completeness, and to supply this omission is the 

 object of this paper. 



The diamond-back moth and the ravages of its larva on 

 cruciferous crops are well known. I found larvae, pupae, and 

 adults on cabbage-plants all the year round. In winter their 

 numbers are small, and do not increase rapidly till December 

 and January, when the turnip-crops become available as food. 

 Bape is hardly touched, but in February and March of this, 

 year clouds of the moths infested the turnip-fields on the 

 College farm. The larvae destroyed all the foliage, so that the 

 turnips put forth new leaves, and this checked the root-growth 

 so seriously as to diminish the crop by nearly 75 per cent. 



In the adult the male and female can be distinguished, with 

 the aid of a pocket-lens, by the fact that the male has the an- 

 tennae more setose, and therefore less distinctly notched, than 

 the female. Of the specimens I employed for breeding, 

 gathered in late autumn, the females were about twice as 

 numerous as males. The adults lived, on an average, four- 

 teen days when confined in a glass box 1ft. by 3 ft. by 2 ft. 

 in which turnip-plants were growing. For purposes of closer 

 observation the specimens were kept in bell-jars about 1 ft, 

 high, and here their average life was ten days, the longest life 

 being that of a female who lived for thirteen days, in com- 

 pany, most of the time, with another female. 



When a male and a female were placed under a bell-jar, 

 with a piece of cabbage-leaf to encourage laying, the following 

 was found to be the normal course of events : Copulation 

 took place on the second day of adult life ; eggs (about seven 

 in number) were laid on the third day ; copulation again on 

 the fourth day ; eggs (averaging six in number) laid on the 

 fifth day ; copulation again on the sixth day ; eggs (about four 

 in number) laid on the seventh day. One or two more eggs 

 may be laid, but the death of both male and female occurred 

 10 



