292 University of California Publications in Agricultural Scienoes [Vol.3 



In many plants insect injuries have been shown to he the cause of 

 abscission. The case of the cotton boll weevil is perhaps the best known 

 example, though it is likely that young fruits of the plum and apple 

 react to injuries due to the curculio 16 and codling moth 17 in much the 

 same way. In view of these observations it is interesting to find that 

 oranges are a marked exception to the rule, the young fruits being 

 particularly resistant to the effects of insect wounds. 18 In the San 

 Joaquin Valley there are two insects at least which cause serious injury 

 to the fruit. The work of Scirtothrips citri results in an -extensive 

 though superficial scarring of the fruit, yet the fruit develops to 

 maturity. The nymphs of the fork-tailed katydid, Scudderia furcata, 

 eat holes in the young fruits (see pi. 32), the holes sometimes extend- 

 ing entirely through the orange. This insect produces traumatic 

 stimuli of the first magnitude, yet they do not result in abscission. 

 Large numbers of the chewed, deeply scarred and distorted fruits 

 develop to maturity only to be discarded by the pickers at harvest time. 



Mechanical shock produced by transplanting trees or the root 

 pruning incident to heavy spring plowing, such as is necessary to turn 

 under a rank-growing cover crop, is usually followed by more or less 

 dropping of the leaves and fruit. It is believed, however, that this 

 may be accounted for by the disturbance of the w 7 ater relations which 

 follows root pruning rather than by the mechanical shock alone. 

 Balls, 10 by root pruning cotton plants in Egypt, was able to cause 

 abscission of the bolls, which he explained on the ground of water 

 relations rather than shock. This particular phase of the problem will 

 be again referred to later. 



Am Temperatures and Light Changes 



Abnormally high air temperatures or sudden changes in the tem- 

 perature are by some investigators considered the cause of abscission 

 in certain cases. It is evident that the question of the influence of air 

 temperature is so involved with other important questions, such as the 

 influence of humidity, air movement, transpiring power and the like, 

 that it is inadvisable to assign specific influences to this factor alone. 

 The same is true of changes in light intensity. Suffice it to say, how- 



i6 The Plum Curculio, U. S. Dept. Agr. Bur. Ent., Circ. 73 (1906), p. 4. 



i? The Codling Moth, ibid., Yearbook (1887), p. 90. 



is True in California, though Hubbard mentions the punctures of two insects, 

 Dysdercus suturellm and Leptoglossus pltyllopus, as causing the dropping of 

 mature oranges in Florida. Hubbard, H. S., Insects Affecting the Orange, U. S. 

 Dept. Agr., Div. Ent. (1885), pp. 167-69, 



i9 hoc. cit., p. 68. 



