81 
pp. 63-75) of the effects of cold and of parasitic attack in limiting the 
increase of these insects. This work has been continued, with some 
results which appear to justify the furnishing of more details. 
The study of extreme cold and its effect upon insects affecting 
crucifers was continued until late in December, after which time it 
usually happens that we have severe freezes which put a practical end 
to the breeding of most insects. Some species were actually found 
breeding upon winter cabbage as late as December 24, and this in spite 
of the fact that, with the exception of perhaps seven days distributed 
at intervals through November and December, there had been con- 
tinuous nightly frosts from the time when observations began in the 
last week of November until their completion. Observations were 
conducted in the District of Columbia and at near-by points in Mary- 
land. The species under particular observation were five in number. 
There was no great difference as to the number of individuals or 
injuriousness. The approximate order, however, was as follows: The 
cabbage plant-louse (Aphis brassicde Linn.), diamond-back moth (Plu- 
tella cruciferarum Zell.), harlequin cabbage bug (Murgantia histri- 
onica Hahn.), imported cabbage butterfly (Pieris rapae Linn.), and the 
cabbage looper (Plusia hrassicss Riley). Of these the diamond-back 
moth was the most active, and the looper and the larva of the imported 
cabbage butterfly the most injurious. 
Like man}^ introduced, and Southern forms of insects which have 
recently migrated northward from the South, these species remain 
feeding in the field long after mo^t of our strictly native forms, or those 
which have long been established in the District and vicinity, have 
sought winter quarters. 
The Imported Cabbage Butterfly (Pieris rapse Linn.). — Larvae were 
noticed the last week of November feeding with the others which have 
been mentioned on late cabbage. The work of this species and the 
cabbage looper was noticeable on all old leaves. Many larvae were not 
above half grown at this time, showing that egg deposit had taken 
place not earlier than the last week of October, and perhaps in early 
November. Larvae taken at this time fed freely on cabbage, and most 
of them attained maturity during the second week of December. 
It was quite noticeable that when rains and freezing weather 
occurred during December, the larvae crawled deeper into the large 
heads of cabbage, where they appeared to be abundant^ protected. 
It w r as noticed throughout the season, and particularly in late autumn 
and early winter, that this species was remarkably free from disease 
as compared with Plusia occurring on the same beds and same cabbage 
plants, a fact, however, that has been observed by others. 
The Diamond-back Moth (Plutella cruciferarum Zell.). — In recent 
years this species has always been found in about the same abundance 
in spring and summer, but it sometimes occurs, like the other species, 
23987— No. 33—02 6 
