104 VEGETABLE FORCING 
free from weeds and debris which may harbor foes of the 
vegetables grown under glass. 
Steam sterilization (Chapter VI) is universally ad- 
mitted to be the most effective preventive measure in 
controlling many of the insect foes of forcing crops. 
Fumigation with hydrocyanic gas, at the rate of five 
ounces of cyanide of potassium to 1,000 cubic feet of 
space, is destructive to all animal and plant life, but it 
should not be used when there are any crops in the 
houses. 
Care should be exercised to select for greenhouse pur- 
poses soil which is free from white grubs, cutworms and 
wireworms. If they are known to exist in the soil, thor- 
ough steam sterilization before the beds are planted will 
be a certain method of destroying them. Insect enemies 
may be introduced through manure, and it is therefore 
important to apply it to the beds before they are 
sterilized. 
Red spiders and various insects, like thrips, aphids, 
white fly and nematodes, may be transferred to the 
houses on plants. When this happens, the plants should 
be dipped, fumigated or perhaps destroyed, if they are 
badly infested. 
The rotation of crops is always helpful in avoiding 
losses from insect depredations. For example, it is much 
more difficult to control the white fly on tomatoes if the 
crop is grown throughout the year than it is if lettuce is 
produced a part of the year. 
Insect ravages are generally less harmful to crops that 
are making a vigorous growth. It is important, there- 
fore, to employ every possible means to promote rapid 
growth, avoiding at the same time the development of 
soft, tender plant tissues, which are preferred by insects 
and very susceptible to the attack of fungi. 
Steam sterilization is extensively used for the control 
