26 BULLETIN 1225, U. S. DEPARTMENT OF AGRICULTURE. 



was also considerable injury on the rows pruned in November and 

 December. The least damage was noted on those trees pruned dur- 

 ing the hot summer months. The thrips have a habit 01 congregat- 

 ing in large numbers along the sides of the stubs near the cut ends. 

 They also lay large numbers of eggs on the cut surfaces. Hence 

 these cut limbs are soon killed and are unable to put out new growth. 

 This experiment, like those previously recorded, showed that prun- 

 ing the trees to the level of the ground results in much less injury 

 from thrips and also produces a more vigorous growth then when the 

 trees were pruned by the branch method. 



TREATMENT OF ORNAMENTAL HEDGES. 



When grown for ornamental hedges, it is often desirable to prune 

 camphor trees frequently and thus keep the hedge in regular forma- 

 tion. Since this will result in injury from the pruning and also in 

 subsequent injury by the thrips, some experiments were conducted 

 on camphor hedges to determine whether they could be treated to 

 prevent dying back and also to prevent much of the large infestation 

 of thrips which usually follows pruning. A row of trees averaging 

 about 4 feet in height were cut 1 foot from the ground on December 

 29. The cut ends of one-third of the row were treated with commer- 

 cial or painter's shellac and one-third with concentrated lime-sulphur 

 solution and nicotine sulphate containing 40 per cent nicotine, diluted 

 1 to 200. The remaining one-third of the row was left without any 

 treatment whatever, as a check. As was to be expected, the thrips 

 soon deposited enormous numbers of eggs on the cut ends of the 

 untreated trees, but no eggs were deposited on the treated trees. The 

 trees treated with shellac did not die back at all and after a few 

 months put forth new growth near the cut ends. The shellac pre- 

 vented the cut ends from drying out and consequently there were no 

 unsightly dead branches. The lime-sulphur solution and nicotine 

 sulphate treatment also prevented the trees from dying back to a 

 large extent, but the growth started a little nearer the ground. The 

 untreated trees also put forth vigorous growth, but the shoots were 

 very close to the ground and unsightly dead branches were present in 

 great abundance. 



Therefore, if it is desirable to prune an ornamental hedge of camphor 

 trees so that they will retain their beauty, it is advisable to treat all of 

 the cut ends possible with some material that will seal them up and 

 prevent their dying back and also prevent the thrips from ovipositing 

 there. Shellac has proved to be the best material for this purpose. 



FUMIGATION. 



With the end in view of preventing the spread of camphor thrips on 

 nursery stock and small camphor trees, some fumigation experiments 

 were carried on to find a safe and effective method of treating the 

 trees. Previous work along this line reported by Berger (10) and 

 Newell (7) (9) showed that scrubbing and dipping the trees with 

 soap, oils, and other insecticides is not entirely satisfactory. Many of 

 the adults, larvse, and pupse can be destroyed in that way, but some 

 of the eggs are so protected in the bark as to escape injury. The 

 fumigation experiments were intended, therefore, to destroy the eggs 

 in the bark lesions. 



