86 THE REPORT OF THE No. 33 



in colour or become mottled with black, green, yellow and grey and remain 

 attached to the plant, soft and limp, or hang suspended by the prolegs, a grayish 

 or blackish fluid exuding from the mouth and anus. The body contents become 

 liquid, and dead larvae soon blacken and collapse; their juices spread over 

 the leaves and together with their excrements serve to disseminate the disease 

 among healthy larvae which ingest the virus when feeding. 



In 1886 Forbes 3 experimented with this disease in Illinois. He spoke of it 

 "as a frightfully contagious and destructive disease of the European cabbage 

 worm," and came to the conclusion that the causal organism was a micrococcus. 

 Glaser and Chapman, 4 working more recently with a very similar disease 

 affecting larvae of the gypsy moth, found numerous micrococci, but concluded 

 that the disease was due to a filtrable virus. During the past season (1923) 

 this disease killed large numbers of larvae used in life-history and host selection 

 experiments, but was rarely met with in the field until late in the season, when 

 dead larvae became common. 



In addition to disease the cabbage worm is subject to the attacks of two 

 important hymenopterous parasites, Apanteles glomeratus L. of the family 

 Vipionidce, and Pteromalus pupariim L., a small Chalcid fly. Of these the 

 latter is probably the more useful. 



Pteromalus puparum overwinters in the larval condition within its host, 

 the adults emerging early in June, as many as 43 having been counted from a 

 single cabbage butterfly pupa. Some of the flies reared in the insectary and 

 fed on a weak molasses solution lived well over a month. 



The host is parasitized while in the larval stage, but pupates before dying, 

 the flies during the summer months emerging two to three weeks later through 

 a small hole punctured in the wall of the chrysalid. Large number of cabbage 

 worms are undoubtedly destroyed by this insect, but this fact is not readily 

 apparent in the field because parasitized larvae on nearing pupation migrate to 

 locations where they are not easily found. 



Apanteles glomeratus also attacks the larva of the cabbage butterfly, such 

 larvae being left shrivelled and dying before reaching maturity. The parasites 

 spin up in small lemon-yellow elongate-oval cocoons held together in an irregular 

 pile by a fine meshwork of silk close to the dying host. This year parasitized 

 larvae were found from July to October, but only in small numbers. 



Another larval parasite is the Ichneumon fly, Itoplectis conquisitor* Say., 

 a specimen of which was observed on September 1st from a cabbage butterfly 

 chrysalid at Ottawa. 



Certain species of common wasps attack the larvae, Polistes pallipes* LeP. 

 and Vespula germanica* Fab. having been observed in early August devouring 

 almost full grown specimens on cabbage foliage. 



Spiders also take toll of the butterflies, many being devoured in our experi- 

 mental cages during the season. 



References 



1. Scudder, Mem. Boston Soc. Nat. Hist. IV, 3, 1887. 



2. Chittenden, U. S. F. B., 766, 1916. 



3. Forbes, Bull. 111., State Lab. Nat. Hist., 1886, Vol. II, pp. 260-276. 



4. Glasser & Chapman, Jr. Econ. Ent. VI, 479, 1913. 



*Species kindly determined by Mr. H. L. Viereck. 



