Studies on Eggs of Apple Plant Lice 19 



approximately 50 per cent of the eggs of this species hatched, while 

 in 1918 about 30 per cent hatched, a difference of 20 per cent. 

 Among selected eggs used in the checks for experiments conducted 

 out-of-doors from February 19 to April 1, an average of 56 per cent 

 of the eggs hatched in 1917, while 44 per cent hatched in 1918, a 

 difference of 12 per cent. The above difference in the percentages 

 of hatch for the two seasons is believed to be due to the decided 

 difference in the weather. In 1917 we experienced many wet and 

 cold days throughout the last two weeks in February and almost 

 all of March. The last two weeks of March in 1917 were par- 

 ticularly wet (IMarch 15-31). In 1918 the later part of February 

 resembled the same period in 1917, but the entire month of March 

 was comparatively dry, especially the last two weeks, and this 

 weather was very conducive to high evaporation. Unfortunately, 

 only occasional records were made of the humidity during these 

 two years, but in 1918 a number of days in the last weeks of March 

 showed 40 per cent of moisture at mid-day. Table 3 shows tke 

 rainfall for the two seasons covering six weeks, February 15 to 

 March 31. A comparison shows immediately a decided difference 

 in the amount of rainfall, especiall.y for the last weeks in March, 

 and these weeks are the most important for at this time the greatest 

 number of eggs have their outer coverings split, thus exposing the 

 permeable pigmented layer to evaporating factors such as l<-»w 

 humidity, high temperature and wind velocity. During March 15 

 to March 31, 1917, 1.59 inches of rain fell, 8 days out of 19 gave 

 over 0.01 inch of rain and 9 days out of 15 were cloudy or partly 

 so, while in 1918, 0.22 inch of rain fell, and only 3 days out of 

 15 were partly cloudy. During March 15-31, 1917, there was a 

 mean maximum of 53°F., and a mean minimum of 32.1°F., while 

 in 1918 the mean maximum was 59.3°F. and the mean minimum 

 33.6°F, thus in 1918 there was an increase of 6.3°F. in the mean 

 maximum and 1.5°F. in the mean minimum. This difference 

 in temperature along with the decided difference in rainfall and 

 cloudy days means that the humidity was probably greater in 1917 

 than in 1918. All the above characteristics of the weather for 

 March, particularly March 15-31, show that the evaporation of 

 moisture from the split eggs would be much greater in 1918 than 

 in 1917, and this being the case the percentage of hatch would be 

 lower for 1918. This is actually true, as shown above among eggs 

 collected from various orchards and also among selected eggs used 

 as checks in various experiments for the two seasons. 



