52 FUMIGATION" FOR THE CITRUS WHITE FLY. 



injury to orange trees and apparently never to tangerine and grape- 

 fruit trees. The liability of injuring trees through the emptying of the 

 contents of the jars after fumigation close to or upon the base of the 

 trees will be referred to under the subject of precautions. The injury 

 to orange trees from the gas itself has never in the writer's experience 

 been sufficient to offset the benefits of destroying the white fly and 

 scale insect pests. Nevertheless the subject is one of considerable 

 importance. The experiments conducted in January and February, 

 1907, demonstrated the practicability of destroying the white fly 

 with hydrocyanic-acid gas without injury to citrus trees. 



The fumigation of nearly 4,000 trees in the winter of 1907-8 has 

 greatly extended our knowledge of the effect of fumigation upon the 

 trees, but there remain several unsolved problems in this connection 

 which it is hoped will be elucidated by future experience. The work 

 of fumigating a grove should be completed if possible before the new 

 growth appears in the spring. Under certain temperature conditions 

 successful fumigations luay occasion no injury to new growth, but 

 there is danger of destroying the first spring shoots which normally 

 produce the greater part of the blooms. When affected by the gas 

 new shoots wilt and turn dark, appearing as though affected by frost. 



Under certain conditions there is more or less shedding of the old 

 leaves following fumigation. The loss of 10 or 15 per cent of the old 

 foliage can not be considered an injury, inasmuch as even more than 

 this proportion is usually shed during the winter or in the spring. In 

 fact, it has been demonstrated by experiments conducted by Mr. 

 Yothers and the writer in February, 1908, that the leaves shed by 

 fumigation when the percentage of the whole does not exceed 15 per 

 cent are among the leaves which would normally drop in the course 

 of a few weeks. 



In the experiments with the sheet tent of 8-ounce duck summarized 

 in Table IV, the most extensive shedding occurred in experiments 

 40.14. In this it was estimated that about 50 per cent of the leaves 

 were shed. The tree was fumigated on January 29, beginning at 

 4.07 p. m., about one-half hour before sunset. No shedding was 

 observed until the morning of February 2, when it was estimated that 

 from 15 to 20 per cent of the leaves dropped. On February 4 it was 

 estimated that 50 per cent of the leaves had fallen, after which date 

 the amount of the shedding was inappreciable. The winged petioles 

 of the leaves remained attached to the tree in most cases and the 

 fallen leaf blades showed distinct brownish areas due to burning by 

 the gas. The tree consisted of five stems growing from the roots of a 

 tree frozen to the ground in 1895. One of these stems was affected 

 by foot rot or mal-di-gomma, and the defoliation of this was nearly 

 complete, materially increasing the percentage of shedding from the 



