SOME IMPORTANT DEGREE DAY PARAMETERS FOR ALFALFA SEED

Karen Strickler

Pollination Ecologist
University of Idaho

Alfalfa Leafcutting bees Degrees Fahrenheit Degrees Celcius Pteromalus venustus Degrees Fahrenheit Degrees Celcius Lygus bug Degrees Fahrenheit Degrees Celcius Alfalfa Degrees Fahrenheit Degrees Celcius A Degree Day Primer Degree-day concepts Developmental thresholds Degree-days Accumulated degree-days

*(Degrees Fahrenheit)*

Alfalfa Leafcutting bees

  Lower threshold: 60^o F
  Degree day accumulations required for 50% adult emergence:  493-560 DD F (at 85^o F:  85-60=25 degree days per day, or 21-22 days)

If bees must be cooled below 85^o F, you can calculate manually how many degree days have accumulated at the lower temperature.  Open a few cells to see what is the average condition of the pupae, and use the table below to calculate how many more degree days are required:

Pupal Stage	Degree Days to 50% emergence	Days to 50% emergence at 84o F
All white, recently molted	264	11
Eyes light pink to tan	234	10
Eyes dark red or brown	189	8
blackening pupa, body darkening	108	4-5
Black pupa, entire bodypigmented	72	3

For example, if the average stage of a sample of bees is dark-eyed, then an additional 8 days are required at 84^o F for 50% emergence.  If the incubator is held at 75^o F for three days, and then brought up to 85^o F, it will be 75-60=15 degree days per day for the three days at 75^o F = 45 Degree days.  That leaves 189-45=144 degree days to go, or 5.8 days at 85^o F.

Pteromalus venustus

  Lower threshold: 59^o F
  Degree day accumulations required for 5% adult emergence:  182-272
  Degree day accumulations required for 50% adult emergence:  ~292

Question: Can you calculate how many days it would take for 50% parasite emergence at an incubator temperature of 85^o F?  Consider that each day will be 85-59=26 degree days.

Lygus bug

  Lower threshold: 52^o F
  Degree day accumulations required for 50% egg hatch:  250^o
  Generation time, egg to adult:  623 degree days.
 

Alfalfa

  Lower threshold: 41^o F
  Degree day accumulations for bloom: ~1000-1600 ???
 
 

*(Degrees Celsius)*

Alfalfa Leafcutting bees

  Lower threshold: 15.5^o C
  Degree day accumulations required for 50% adult emergence:  274 - 311DD C
(at 30^o C:  30-15.5=14.5 degree days per day, or 19-20 days)

If bees must be cooled below 30^o C, you can calculate manually how many degree days have accumulated at the lower temperature.  Open a few cells to see what is the average condition of the pupae, and use the table below to calculate how many more degree days are required:

Pupal Stage	Degree Days to 50% emergence	Days to 50% emergence at 29oC
All white, recently molted	147	11
Eyes light pink to tan	130	10
Eyes dark red or brown	105	8
blackening pupa, body darkening	60	4-5
Black pupa, entire body pigmented	40	3

For example, if the average stage of a sample of bees is dark-eyed, then an additional 8 days are required at 29^o C for 50% emergence.  If the incubator is held at 20^o C for three days, and then brought up to 30^o C, it will be 20-15.5=4.5 degree days per day for the three days at 20^o C = 13.5 Degree days.  That leaves 105-13.5 =91.5 degree days to go, or 6.3 days at 30^o C.

Pteromalus venustus

  Lower threshold: 15^o C.
  Degree day accumulations required for 5% adult emergence:  105-151.5
  Degree day accumulations required for 50% adult emergence:  ~162

Question: Can you calculate how many days it would take for 50% parasite emergence at an incubator temperature of 30^o C?  Consider that each day will be 30-15=15 degree days.

Lygus bug

  Lower threshold: 11^o C
  Degree day accumulations required for 50% egg hatch:  140^o
  Generation time, egg to adult:  346 degree days.
 

Alfalfa

  Lower threshold: 5^o C
  Degree day accumulations for bloom:  ~555-890???
 

A DEGREE DAY PRIMER

From University of California Statewide Integrated Pest Management Project
 

Degree-day concepts

Temperature controls the developmental rate of many organisms. Plants and invertebrate animals, including insects and nematodes, require a certain amount of heat to develop from one point in their life cycles to another. This measure of accumulated heat is known as physiological time. Theoretically, physiological time provides a common reference for the development of organisms. The amount of heat required to complete a given organism's development does not vary - the combination of temperature (between thresholds) and time will always be the same. Physiological time is often expressed and approximated in units called degree-days (^oD).

Developmental thresholds

Upper and lower developmental thresholds have been determined for some organisms through carefully controlled laboratory and field experiments. For example, the lower developmental threshold is 51° F and the upper developmental threshold is 90° F for the San Jose scale (Quadraspidiotus perniciosus). Thresholds vary with different organisms.

 

The lower developmental threshold for an organism is the temperature below which development stops. The lower threshold is determined by the organism's physiology.

 

Degree-days

The total amount of heat required above the lower threshold for an organism to develop from one point to another in its life cycle is calculated in units called degree-days (°D). Sometimes called heat units, degree-days are the accumulated product of time and temperature between the developmental thresholds for each day. One degree-day is one day (24 hours) with the temperature above the lower developmental threshold by one degree. For instance, if the lower developmental threshold for an organism is 51° F and the temperature remains 52° F (or 1° above the lower developmental threshold) for 24 hours, one degree-day is accumulated.

Accumulated degree-days

Each developmental stage of an organism has its own total heat requirement. Development can be estimated by accumulating degree-days above the temperature threshold throughout the season. Each species requires a defined number of degree-days to complete its development. The accumulated degree-days from a starting point can help predict when a developmental stage will be reached. The date to begin accumulating degree-days, known as the biofix date, varies with the species. Biofix dates are usually based on specific biological events such as planting dates, first trap catch, or first occurrence of a pest. Accumulation of degree-days should be done regularly, especially when a control action decision is near.

For more information:
 

Karen Strickler,  pollination ecologist
University of Idaho
Parma Research & Extension Center
29603 U of I Lane
Parma ID 83660-9637
Phone: 208-722-6701
Fax: 208-722-6708
email: <karens@uidaho.edu>
 

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Revised May 12, 1998.
Copyright © 1995, Department of Plant, Soil and Entomological Sciences, University of Idaho. All rights reserved.