# The Concept of Physiological Time

Since insects are cold-blooded animals (poikilotherms) their rate of growth and development is proportional to ambient temperature.  At very low temperatures, there is no development at all.  As temperature increases, there is some point (different for various species) at which development begins to occur. This low-temperature “gate” is called the Developmental Threshold.  As temperatures increase above the developmental threshold, the rate of insect development gets faster.  At high temperatures, development rate levels off and then drops quickly near the upper limit of survival.  Figure 1 illustrates the typical relationship between temperature and development time:

Similar insects reared at different temperatures will require different amounts of time to complete development.  Cooler temperatures retard development; warmer temperatures stimulate development.  Since the relationship between temperature and development rate is linear through a wide range of temperature, it is possible to predict how long development will take at each different temperature.

1.   Suppose an insect species develops two times (2X) faster at 20 degrees C than at 10 degrees C.  If complete development takes 10 days at 20 degrees C, how long would it take at 10 degrees C?
Five Days
Ten Days
Fifteen Days
Twenty Days

The concept of Degree-Days is a convenient way to track development at different temperatures.  One degree-day is the amount of development that occurs in one day (24 hours) when the temperature is one degree above the development threshold.

2.  Suppose an insect’s development threshold is 45 degrees F.  How many degree-days accumulate each 24 hours when the insect is reared at 65 degrees?
Five degree-days
Ten degree-days
Fifteen degree-days
Twenty degree-days

Degree-days accumulate over time.  Each day the temperature is above the developmental threshold, more degree-days are added to the accumulation.

3.  For an insect with a developmental threshold of 10 degrees C, which of the following would equal 15 degree-days?
24 hours at a constant temperature of 25 degrees C.
48 hours at an average temperature of 20 degrees C.
12 hours at a constant temperature of 30 degrees C.
36 hours at an average temperature of 15 degrees C.

Each insect species requires a certain total number of degree-days to complete its development.  This is also known as the PHYSIOLOGICAL TIME of development.

4.  A seedcorn maggot needs 375 Celsius-degree-days above its development threshold of 4 degrees C to complete development from egg to adult.  If eggs laid on July 1 were reared at a constant temperature of 29 degrees C, when would they emerge as adults?
July 13
July 15
July 20
July 23

Constant temperatures are uncommon in the natural environment where most insects live and grow.  Changes in environmental temperature cause changes in development rate from day to day and even from hour to hour.  In order to apply the concept of physiological time to insect populations that develop in natural environments, we must estimate cumulative degree-days under fluctuating conditions.  This often means computing an average value from a set of hourly observations or calculating the area under a curve (remember anything from calculus?). 5.  How many degree-days accumulate on Oct. 15 (see graph) if the developmental threshold equals 40 degrees F?
Five degree-days
Ten degree-days
Fifteen degree-days
Twenty-five degree-days

6.  How many degree-days accumulate on Oct. 15 (see graph) if the developmental threshold equals 35 degrees F?
Five degree-days
Ten degree-days
Fifteen degree-days
Thirty degree-days

7.  How many degree-days accumulate on Oct. 15 (see graph) if the developmental threshold equals 45 degrees F?
Five degree-days
Ten degree-days
Fifteen degree-days