281 
MetuHopD oF ForRECASTING DATES WHEN THE DIFFERENT STAGES 
OF THE SEVERAL GENERATIONS MAY BE EXPECTED TO APPEAR 
AT ANY GIVEN PLACE IN A NorRMAL SEASON 
By ascertaining the normal daily effective day-degrees at any point 
in the state throughout the season and ascertaining the total sums of the 
daily accumulations for each day of the season and comparing the sums 
on the different dates with Table 44, we can ascertain the approximate 
dates when the different stages of each generation may be expected to 
appear. Table 45 gives the normal daily effective day-degrees, the total 
effective day-degrees, and the dates on which the first individuals of the 
stages of each generation normally appear at Olney, Illinois. 
This table was computed from the maximum and minimum read- 
ings of the U. S. Weather Bureau at Olney from 1901 to 1916 inclusive 
by the formule given on page 283. In all of these computations the 
daily effective day-degrees were taken as the average degrees above 50° 
F. diminished by twice the average degrees above 86. 
It is not to be expected that the first eggs, larve, pupz, and adults will 
appear in any locality on the exact dates shown in a table of the normal 
accumulations of effective day-degrees for that locality. Such tables will 
give only the approximate dates. There are many factors which may 
influence the time of appearance: there are always periods during every 
season when the temperatures are not normal; there are situations in 
every locality in which the temperatures differ from those of the place 
where the recording instrument is located, such as steep northern and 
steep southern slopes, and differences in altitudes in hilly countries; and 
pupe on the south side of the trunks of trees will be subjected to greater 
heat than those on the north side. 
There is no practical method by which all the local factors may be 
taken into account, but the seasonal variations may be anticipated by 
keeping a daily record of the total accumulation of effective day-degrees 
in each of the fruit-growing sections; and dates when the first larve of 
each generation may be expected to enter the fruit may be anticipated 
however abnormal the weather may be. When the accumulations 
amount to 390, 1290, and 2280 the first larve of the first, second, and 
third generations, respectively, may be expected to enter the fruit in 
about 10 days. (See Table 51, p. 289.) 
Table 43 (p. 280) shows that the average accumulation of effective 
day-degrees for the complete life cycle, as ascertained by a study of the 
different stages, is 1176, the maximum being about 1567, and that by a 
study of the life history the average accumulation is 1195, the maximum 
being about 1600. These differences arise partly from the fact that in 
the former case 86, 85, and 87 degrees F. were used as the degrees of 
maximum rate of development of the egg, larva, and pupa, respectively, 
whereas in the latter case 86 was used for each of the stages. Since it 
is much more convenient to use 86, 1195 should be used as the approxi- 
mate average. 
