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 



