October, '16] quayle: scale dispersion 489 



ent exposures, and the fact that infested trees occurred on more than 

 one side in some cases, no definite conclusions are drawn from the above 

 figures as to the number of scales captured in relation to the prevailing 

 direction of the wind. 



In the case of the black scale, which has a more or less definite annual 

 period of young production, dispersal would be expected to occur 

 largely during that period. The only other time would be when the 

 scales are migrating from the leaves to the twigs, or when they have 

 detached themselves from the plant for any other reason, and at such 

 times, on account of the increased size of the insects, their dispersal by 

 the wind would not be so great. 



The above conclusions are supported by our experiments as shown 

 by the last four sheets referred to in the table. These sheets were 

 exposed on September 1, after the hatching of the scales had ceased, 

 and they were settled on the leaves and twigs. The sheets were placed 

 in the same grove as those of June 28, and 15 feet was the greatest 

 distance away from infested trees. The average number captured on 

 these sheets was 2| scales as against an average of 346 scales on the 

 sheets placed on June 28. The important period of dispersal of the 

 black scale, as well as of the citricola scale and others having usually 

 but one generation a year, is, therefore, from April to September, or 

 the period when the great majority of active young appear. This 

 fact would not apply so strictly to scales having three or four genera- 

 tions a year, as the red, yellow, purple, and soft brown scales, since 

 young may be present in greater or lesser numbers throughout the 

 warmer portion of the year, or from March to December in California. 



The data thus far secured have reference to the black scale, but Mr. 

 Bishop of Orange County and his inspector, Mr. Paddock, had tangle- 

 foot sheets so placed as to capture the red scale, Chrysomphalus aurantii. 

 These sheets were submitted to the writer for examination. The dis- 

 tance ranged from 6 feet to 150 feet, and young red scales were found 

 on most of the sheets. 



In the experiments with the tanglefoot paper noted above, there was 

 no obstacle between the sheets and the infested trees to interfere with 

 the free carrying of the scales. In order to determine what happens 

 under normal conditions in the grove, the following experiment was 

 carried out : 



A four-acre block of grapefruit was selected that was fumigated in 

 1914 with the result that practically 100 per cent of the black scale 

 were killed. A careful examination of the block in April, 1915, re- 

 sulted in finding no scales, and in midsummer all of the scales present 

 were young scales indicating that they had not come from parent scales 

 on the same trees, but from neighboring trees. This block of clean 



