RAVAGES OF THK ARMORED SCALES. 47 



The long-coutiuued heat of summer acta as a check upou the advauce 

 southward of those species which inhabit the North, and is probably a 

 more im]>ortant factor in determiuing the geographical distribution of 

 many species than the frosts of the northern winter. 



Indeed tlie notion that Scale insects are destroyed by frosts is entirely 

 erroneous. Their eggs withstand any ordinary degree of cold, and tlie 

 insects themselves survive a freezing temperature that kills the plants 

 njion which they feed. The winter climate of a land in whicli the open 

 culture of the Orange is possible cannot be sutliciently rigorous to kill 

 even the young of IJark-lice. In Florida tiie coldest weather merely 

 serves to retard their development.* 



yatural Checls. — The i)arasites of Bark-lice, some of which have al 

 ready been mentioned, and the numerous enemies to be considered here 

 after, are similarly atfected by climatic conditions. Their broods in- 

 crease in number as they extend southwards, and in the main their 

 activity keeps pace with that of their prey. Ordinarily, therefore, 

 the various checks upon their increase are sufficient to jjrevent the 

 spreading of Bark-lice to an injurious extent, and, as we have seen, it 

 is only at times that they increase so rapidly as to entirely outstrip their 

 enenues and overrun the plant. 



* Mr. Joseph Voyle, in a report made to Professor Riley, and published in Bul- 

 letin No. 4, United States Department of Agriculture, Division of Entomology, 

 gives the results of an elaborate series of experiments made by subjecting Scale-in- 

 sects, with their young and eggs, to the action of low temperatures for varyiug lengths 

 of lime. In these experiments the Coccids were placed in a small tin cylinder and 

 surrounded with a freezing mixture of ice and salt. The time of exposure was never 

 less than one nor more than sixteen hours, and the temperatures ranged from 1(5- to 

 iUV 1'. Eggs were killed when the minimum fell below 25° and were not killed at 

 liighcr temperatures; the young Bark-lice were killed in every experiment. It is to 

 lie remarked, liowever, that an average time of nine days was allowed to elapse before 

 the results obtained were considered iinal. As Mr. Voyle himself suggests, "Some- 

 times larvaj retain for several days an apparently natural appearance, leaving it 

 doubtful whether their final death is the result of the temperature or want of food." 



In regard to the eggs also it is pi'obable that certain conditions not noted and not 

 taken into account in these experiments vitiated the results, since they do not cor- 

 respond with what takes place in the open air during severe frosts. 



On this point Mr. Voyle himself gives evidence when he says: "During the past 

 winler, lb32-'«3, by some special observations, positive evidence was obtained that 

 .often very little damage was done to scale insects by cold that killed the tender orange 

 >h()()t.s. On the morning of December 16, 1882, the thermometer was reported at va- 

 rious figures, from 19'^ to 2o'^ F. My own lowest readiug was 25°. On this morning 

 I (lit orange branches incrusted with scale insects, and found young migratory larvie 

 of Mytilanpl'i running about (luite lively." 



This discrepancy is remarked by Mr. Voyle, and the following explanation is sug- 

 u»\slcd : " There are conditions practically unattainable artificially, whc^-e the coccids 

 arc jtrotecled from the efi'ec s of such temperature as under favorable conditions 

 woul.i bo fatal to them. The loaves of the tree, the warm current rising from the 

 ground around the trunk of the tree, and the initial heat of the tree itself perform au 

 important part in modifying temperature for these insects. In a still atmosphere 

 this might become a perfect protection against a temperature much lower than would 

 prove fatal in other conditions." 



