24 



MANUAL OF VEGETABLE-GARDEN INSECTS 



Fig. 21. — Wingless vivip- 

 arous female of the 

 cabbage aphis (X 8|). 



parthenogenetically, no eggs are produced and the young are 

 born alive. From April first to October first, sixteen genera- 

 tions have been known to develop. From 

 time to time when the food supply be- 

 comes limited, either from overcrowding 

 or from some injury to the plant, winged 

 forms (Fig. 22) are developed which fly 

 to other plants and start new colonies. 



Cabbage plants often become infested 

 in the seed-beds or very soon after 

 transplanting. The lice soon become 

 closely packed together in dense masses, 

 often hiding the leaf from view. Their 

 presence causes the leaves to curl, some- 

 times forming deep pocket-like depressions, the inner surface 

 of which is completely covered with lice. The aphids also 

 cluster in the forming head. Badly infested plants cease to 

 grow, the larger leaves die, the heads do not develop and in 

 some cases the plant is killed outright. Figure 23 shows a 

 badly infested radish seed-stalk. 



Late in the fall, true males and females are produced. The 

 female is wingless and the male is winged. x\fter pairing the 

 female soon begins to deposit her eggs on the petioles and under 

 surface of the cabbage 

 leaves. When first laid, 

 the eggs are pale greenish 

 3^ellow in color but usually 

 turn black in a few days. 



Fig. 22. — Winged viviparous female 

 of the cabbage aphis (X 8-2). 



Control. 



Although cabbage lice 

 are easily killed when hit 



by ordinary contact insecticides, it is difficult to control them 

 economically under commercial conditions. The lice are pro- 



