282 
It follows that since the first and the second generations are almost 
complete in the southern half of Illinois, if the larve of the first genera- 
tion are observed to be entering the fruit in large numbers on a certain 
date, larvee of the second generation will be entering the fruit in large 
numbers after an interval during which the accumulated effective day- 
degrees have reached 1195, unless delaying factors, such as parasites, rainy 
weather at hatching time, or other cause which might occasion great mor- 
tality of eggs, larve, or pupz, should intervene. This would not hold 
true for the third generation, since it is not complete, for, at best, only a 
small proportion of the larve of the second generation transform. 
The maximum number of effective day-degrees which may accumu- 
late between the deposition of an egg of one generation and the deposi- 
tion of an egg of the next generation is about 1600. If a number of 
eggs are deposited on the same day the deposition of eggs by females 
developed from them will be extended over a period during which the 
accumulation of effective day-degrees ranges from 1000 to 1600, which 
corresponds to a period of about 20 days. This accounts largely for the 
overlapping of the second and third generations notwithstanding the fact 
that the first and second do not overlap. 
How To COMPUTE EFFECTIVE DAY-DEGREES 
One of the main objects of this investigation was to find, if possible, 
a single factor in terms of which the progress of the seasonal history of 
the insect could be followed and even predicted long enough beforehand 
to be of practical value. Apparently we have found that factor im tem- 
perature expressed in terms of effective day-degrees as the unit. Humid- 
ity, evaporation, and rainfall may affect the rate of development to some 
extent, but in the case of the codling-moth these factors have such a 
small influence under out-of-door conditions that for practical purposes 
they may be disregarded, leaving temperature as the single factor to be 
taken into account. 
The temperatures that are to be taken into account are the tempera- 
tures above 50 degrees and those above 86. To ascertain the number of 
effective day-degrees that have accumulated during any given day, first find 
the average degrees above 50 degrees F., then the average degrees above 
86 degrees, multiply the latter by two and subtract the product from the 
former. The result will be the effective day-degrees. 
To compute the effective day-degrees, accurate thermograph records 
should be kept. The average daily temperatures above 50 degrees F. can 
be computed by adding together the hourly readings above 50 and divid- 
ing the sum by 24, and the average daily temperatures above 86 can be ~ 
computed in the same manner. If the drum type of thermograph be 
used the temperature curve above 50 and above 86 may be integrated by 
means of a planimeter, expressing the result as hourly-degree units, then 
by dividing the totals for each day by 24, the average hourly-degrees 
above 50 and 86 may be ascertained, which are the day-degrees in each 
case. Subtracting from the day-degrees above 50 twice the day-degrees 
above 86 gives the number of effective day-degrees. 
