10 farmers' bulletin 880. 



effectiveness of the gas and consequently lessens the possibility of 

 injury to the plants by burnmg. It is obvious, therefore, that in 

 order to increase the effectiveness of the fumigation the plants 

 should be syi'inged not less than four or five hours prior to the 

 liberation of the gas, to avoid undue absorption of the gas by the 

 water on the benches and walks. 



HUMIDITY. 



A relatively high humidity (98 to 100), with temperature varying 

 from 70° to 75° F., greatly increases the amount of injury to the foliage 

 of the plants, whereas plants in the presence of the same high humidity 

 (98 to 100), with a temperature of 60° to 65° F., will exhibit little 

 if any injury. 



ADVISABILITY OF A FUMIGATION BOX. 



A fumigation box is desirable for two reasons, namely, for testing 

 the amount of gas plants can stand without injur}^, and for ridding 

 a hmited number of potted plants of insects, and thus avoiding costly 

 and laborious hand scrubbing of such plants. The size of the box wiU 

 depend on the use to which it is to be put. A box with a capacity 

 of 200 cubic feet can be used advantageously for nursery stock, 

 palms, etc. 



Plants to be fumigated in a box in the daytime should remain in 

 the box with the door closed at least one hour before the gas is gen- 

 erated and should be shaded from the bright sunhght for at least 

 two hours after the completion of the exposure. 



HOW GREENHOUSES BECOME INFESTED WITH INSECTS. 



Doubtless many greenhouses become infested with insects through 

 the agenc}^ of plants commoidy referred to as "boarders." The prac- 

 tice of turning over home-grown plants to a florist to care for during 

 the absence of the owner on a vacation is prevalent over the entire 

 country, and often results in establishing pests not hitherto known to 

 occur on the florist's premises. If the trade requires such a practice, 

 plants of this character should be cleaned thorouglily of insect pests 

 before being placed with the regular stock of the greenhouse. 



Insect infestations in greenhouses have been traced to the following 

 sources: Infested plants brought in from coldframes or propagating 

 beds which have not received proper attention; cuttings, plants, and 

 buds received from other estabhshments ; and imported foreign or 

 domestic stock. Adults of the greenhouse white fly, grasshoppers, 

 beetles, aphids, etc., may enter through open ventilators from other 

 greenhouses or gardens; cutworms, wireworms, white grubs, etc., may 

 be brought into the greenhouse with the soil ; and roaches, ants, sow- 



