224 VEGETABLE FORCING 
Fig. 77.—Pot grown plant ready for market. 
the crop as well as in marketing it, and can be used only 
for local markets. 
Insect enemies.—The green fly is recognized as the 
most serious insect enemy of lettuce. It is controlled 
almost exclusively by fumigating with tobacco or nico- 
tine preparations, instructions for the use of which are 
given on page 105. Fumigations should be made at regu- 
lar intervals so there will be no possibility of the insects 
becoming sufficiently numerous to damage the crop or to 
impair its selling quality. 
The nematode sometimes attacks the roots of green- 
house lettuce, though it is not regarded as a serious 
enemy of this crop. 
White grubs may be troublesome in soils which have 
not been steam sterilized. This is the larval stage of the 
June bug or May beetle. Old compost heaps and manure 
piles are favorite breeding places. The eggs or young 
larve may be introduced into the houses by infested soil 
and manure, and the grubs may cause considerable injury 
