INJURIOUS INSECTS. 97 



if the air is warm for several days, the butterflies 

 emerge from the chrysalid shell, soon pair and begin 

 egg laying. The eggs, from 100 to 300 in number, 

 are deposited singly upon the under sides and edges 

 of leaves of old cabbage stumps, early cabbage, kale, 

 wild radish, cress or related plants. The egg laying 

 may last for three weeks, by which time the worms 

 from those first laid have hatched, fed, grown to full 

 size and are ready to pupate. The old butterflies 

 live for some time after laying their eggs, so that 



Fig. 15 Imported Cabbage Worm, Chrysalis and 

 Female Butterfly. 



before they have passed away those of the new brood 

 have come out and are ready to start a new genera- 

 tion. This overlapping of broods makes it impossi- 

 ble to destroy all the pests at one application of an 

 insecticide, no matter how successful; because any 

 poison would be eaten only by the worms, leaving 

 unharmed both eggs and mature insects. If all eggs 

 and worms could be destroyed by hot water or insect- 

 icides which kill by contact, the butterflies would still 

 remain, prepared to restock the plants with eggs. 



