317 



"Two questions of special practical interest present themselves: one, 

 the number of generations in a year ; and the other, the time when the 

 eggs of each generation hatch to give out the young worms. To these we 

 may add a third question, as to variations in the number of generations 

 and the times when the young larvae of each appear in different parts of 

 the state, and in successive years of unlike weather conditions."* 



Weather conditions, especially temperature, humidity, rainfall, and 

 sunlight, have a great deal to do with the rate of development of the 

 codling moth, with the time when the different generations make their 

 appearance, reach their largest numbers, and disappear, and with the size 

 and importance of tiie last or third generation of the year. The course 

 of these events must be carefully and intelligently observed in order that 

 spraying operations may be properly timed — to put the eft'ective poisons 

 on the apples when the larvae of each generation are to appear. 



Measurement of development. Each stage in the life-cycle of an 

 organism requires a certain period of time depending on weather condi- 

 tions. The better the conditions, the shorter the time, and vice versa, 

 within certain limits set by the nature of the organism. If development 

 went on always at the same rate, the number of days or hours from the 

 beginning to any point in the stage would be a direct measure of the 

 amount of development which has been accomplished to that point. This 

 is implied in such common expressions as "a year's growth" or "a day's 

 growth," in which time alone is used as a measure of development on 

 the assumption that the rate of growth is constant over a number of days 

 or years. But rates of growth, or velocities of development, vary with 

 conditions, so that it is necessary to refine this method b}^ taking into 

 account all factors affecting the process. 



In attempting to predict the time of appearance of insect pests, to 

 estimate the abundance of a pest or its enemies, and to arrange spraying 

 schedules, phenologists have taken account of temperature as well as 

 time by using "degree-days" in estimating development. They com- 

 monly get a total number of "degree-days" for a stage of development 

 by taking for each day from the beginning to the end of the stage the 

 number of degrees which the day's mean temperature shows above a 

 certain assumed starting point, or "threshold", and summing the number 

 of degrees thus obtained for all the days to the end of the stige. This 

 "summing of eft'ective temperatures in degree-days" is sometimes fairly 

 useful for estimating development within certain limits of temperature. 

 Glenn ('22) made corrections for high temperatures but not for low 

 temperatures and for variations in humidity and other factors. This 

 method is never very accurate, however, because medial tempera- 

 tures and humidities (where the rate of development is directly propor- 

 tional to teiuperature ) are exceeded almost every day in our climate, 

 and because development takes place at temperatures lower than the 



* Quoted from a paper "On the life history of the codling moth", by Stephen A. 

 I"orbes and Pressley A. Glenn. 29th Report of the State Entomologist of Illinois (1916). 



