Suppression of the Gipsy and Brown-tail Moths. 5 



from place to place on shipments of lumber and other ma- 

 terial, and that in cases where heavy infestations occurred 

 the caterpillars might be carried a considerable distance on 

 vehicles. It is a common habit of the larvaB, if they are dis- 

 turbed, to spin silken threads which are attached to the 

 trees and in this way lower themselves to the ground. Since 

 the gipsy-moth campaign first began, an unprecedented de- 

 velopment in means of rapid transportation has taken place. 

 At first and for several years motor vehicles were practically 

 unknown, but for the last few years the increase in this mode 

 of transportation has been enormous. It has been found, 

 however, as a result of much work and many experiments, 

 that if the roadways are kept clear from heavy infestation 

 the number of caterpillars distributed by motor vehicles is 

 very small. A number of years ago the results of the scout- 

 ing work, which consists of examining roadways, orchards, 

 and wooded areas for infestation, indicated that many 

 colonies were present the occurrence of which could not be 

 explained by any known means of spread. Woodland in- 

 festations were found in places that were infrequently visited 

 by men or animals. This led to a thorough study and a long 

 series of experiments which proved conclusively that the 

 small caterpillars, immediately after hatching, may be blown 

 long distances by the wind. It has been proved that spread 

 often occurs for a distance of from 12 to 20 miles in this 

 way. These facts would seem to make the prevention of 

 spread of the insect hopeless, if not impossible. But the 

 same factors, namely, temperature and wind direction, which 

 have brought about the greatest drift of infestation by the 

 brown-tail moth toward the seacoast, are equally effective 

 in connection with the spread of small gipsy-moth cater- 

 pillars. During the period when these minute larvae can 

 be blown by the wind it is necessary for the temperature to 

 range from 60 F. upward, the higher temperature increas- 

 ing the activity of the insect. This comparatively high tem- 

 perature must be accompanied by strong winds if spread for 

 any great distance is to result, and when the combination 

 of high temperature and strong wind occurs in New Eng- 

 land in the month of May the wind usually blows from the 

 south or southwest. Variations in this general rule occur, 

 depending on how far the locality is removed from the sea- 



