12 BULLETIX 1093, U. S. DEPAETMEKT OF AGRICULTURE. 



to the square foot would materially reduce the crop and would 

 necessitate flooding or spraying in order to prevent damage 



Plate VI, Figure 1, shows a section of the surf ace of the "vdnes in the 

 above-mentioned pen and when compared with Plate VI, Figure 2, 

 showing a similar area just outside of the pen, on the bog proper, 

 with the vines in full bloom, the loss of the new growth is manifest. 



MORTALITY OF FIRST-STAGE LARV^. 



General observations on Muddy Pond Bog, particularly in connec- 

 tion with the pen experiments, have shown that there is a varying 

 percentage of mortality among the several larval stages of the gipsy 

 moth, after they reach the cranberry vines. This mortality is prob- 

 ably greatest in the first stage, owing to several factors. The princi- 

 pal factor is reduced vitality,, since larvse hatching from egg masses 

 that were deposited upon conifers or other nonfavorecl food species 

 had used up a considerable percentage of their A^tality in searching 

 for food before they were blown from the tree onto the vine surface. 

 Their vitality was furtlier reduced in crawling over the vines before, 

 approaching starvation finally forced them to feed upon the buds or 

 new stalks of the cranberry vines. While in this weakened condi- 

 tion the larvfe are more susceptable to cold, particularly when ac- 

 companied by rain. That there is quite a difference between the 

 temperature at the tops of trees and that at the bog surface was 

 demonstrated by the use of recording thermometers. The platform 

 at the top of the tower constructed around tlie white oak tree on 

 which feeding observations were made was about 50 feet above the 

 bog level. A comparison of the thermometer records made at the 

 top of the tree and at the surface of the bog shows that the tempera- 

 ture averaged 5.2° F. cooler at the bog surface than at the top of 

 the tree for the night period during the 6 days of heaviest wind dis- 

 persion, viz, May 25 to 30, inclusive. From 6 p.m. May 29 to 5 a. m. 

 May 30 the average temperature was 11.3° F. -lower at the bog sur- 

 face than at the top of the tower. At 1 a. m. May 30 it was 15° F. 

 lower at the surface of the bog than in the tree top, the maximum 

 difference in the locations during the period. This was the coldest 

 night of the period, and gave the greatest range of temperature. 

 The above temperatures were obtained from the thermometers in 

 latticed shelters. 



There is also a high mortality among first-stage gipsy moth larva3 

 on cranberry bogs due to disease, predacious enemies, and other 

 causes that are not well understood. All of these agencies, together 

 with the rather unfavorable nature of the cranberr}^ foliage as a food, 

 combine to bring about an enormous reduction of the wind-blown 

 larvse. 



