TYPICAL LESSON PLANS 143 



leaves it will be hot enough to kill the worms but not injure 

 the plants. 



What natural foes do the cabbage butterflies and their 

 larvae have ? Many birds eat both worms and butterflies. 

 See if you can find any birds feeding among the cabbage. 

 A common foe is a small insect known as a braconid fly. 



The children will often find on the cabbage leaves a 

 mass of yellow bodies that look like eggs. Bring some of 

 these into the schoolroom. Put them into a tumbler and 

 tie a piece of cheese cloth or netting over the top. In a 

 few days the glass will be swarming with tiny creatures 

 not much larger than gnats. Where did they come from ? 

 Examine the yellow mass. What do you find ? A hole in 

 each of the egglike bodies. When you look closely you 

 will see that these are not eggs but cocoons made out of 

 silk. The flies hatched from the cocoons. They are 

 parasites that help to keep the cabbage butterflies in check. 

 They lay the eggs just under the skin of the cabbage worm. 

 These eggs hatch into tiny white grubs which feed upon 

 the tissues of the worm. They are careful not to touch 

 any of the vital organs such as the heart, digestive system, 

 etc. Why? When they are ready to spin cocoons and 

 change to pupae, they burst through the skin of the worm 

 in a group, killing it instantly. 



The braconid flies are relatives of the bees and wasps 

 and are not true flies. There is another parasite, a near 

 relative of the braconids known as chalcis fly, that feeds 

 upon the pupae. So sometimes you may find a swarm of 

 chalcis flies emerging from the pupa skin instead of a 

 butterfly. 



The history of the cabbage butterfly in America is in- 



