HATCHING OF GIPSY-MOTH EGGS. 11 



EFFECTIVENESS OF NONHATCH. 



Nonhatch as an agenc}' in the natural control of the gipsy moth 

 reaches the maximum of its importance in those sections of the in- 

 fested area which have, some time during each winter, cold severe 

 enough to kill all eggs exposed to its action. 



The principal protection from this killing cold is afforded by the 

 snow ; and the upper limit of this protection, measured from the 

 ground, has been designated as the snow-protection line. It must 

 not be supposed that this line has any definite limits, for the depth 

 of snow is an extremely variable factor. 



To determine the value of nonhatch as an agency in gipsy-moth 

 control it is therefore necessary to determine the distribution of 

 the egg clusters on the trees, the factors which influence this dis- 

 tribution, and the proportion placed above the snow protection line. 

 Fortunately a considerable amount of information along these very 

 lines has been obtained during the progress of the " observation 

 point " investigations. 



The count of egg clusters made at each " point " was divided into 

 two sections. The dividing line was marked at 5 feet from the 

 ground on the trunks of the trees. The count of clusters above this 

 line was known as the high count, of those below as the low count. 

 Such a division was necessary on account of the work involved, 

 which made impossible the complete counting of an entire point at 

 one time. Many times also snow would prevent a low count but 

 would not prevent the high count being taken. Five feet was chosen 

 as a convenient point well above the usual snowfall. 



As the clusters were recorded with reference to their position above 

 or below this 5-foot line, it will have to be the dividing line consid- 

 ered in studying the distribution of clusters on the trees. A depth 

 of snow to the extent of 5 feet is almost unknown in most sections 

 of the gipsy-moth area, so we may safely consider that all the ex- 

 posed egg clusters above 5 feet will be killed if the temperature drops 

 to —20° F. or lower. At the same time there will probably be a 

 considerable number of clusters between the 6-foot level and the 

 top of the snow during at least one period of severe cold, so that we 

 are conservative in using the high count as a basis for figuring 

 benefit derived from nonhatch. 



A careful consideration of the egg-cluster records from the 

 " point " notes, which were taken during seven consecutive- j^ears, 

 shows that on the average TO per cent of the clusters are laid above 

 6 feet. An average of nearly 900 individual counts showed 72 per 

 cent. 



The deposition of clusters is influenced largely by the ground 

 conditions. If there is no underbrush and if debris, such as dead 

 wood, bark, etc., is not present or if the ground is wet, most of the 

 clusters will be well up on the trees. On the other hand, if debris 

 is abundant, if there is much undergrowth, or if, as often happens 

 in New England, there are stone walls running through the woods, 

 a large proportion of the clusters will usually be found close to the 

 ground. 



It is not possible to say just what causes the differences, but the 

 " point " notes show that they are as stated. In addition, the records 



