CIS 538 rev. Part 3

Spring Management

Being able to place bees in the fields when the alfalfa is in bloom is one major advantage of incubation after refrigerated, overwinter storage. 

  1. Incubation calendar
  2. Incubation of bee nests after non-refrigerated storage
  3. Use of lights over pan traps in incubation room
  4. Manipulation of incubator temperatures to delay emergence 
  5. Use of dichlorvos (Vapona pest strips)
  6. Further Information


I.  Incubation of bee cells at 85F after refrigerated storage will result in the following developmental sequence:

(see an updated incubation calendar at the IPS web site)

Day 1. Incubation begins. Bees are in a worm-like larval form, totally white in color.

Day 9. Bee larvae begin to change into white pupae.

Day 10. Pteromalus and Tetrastichus parasites begin emerging from parasitized bee larvae. They appear as tiny black wasps. (Eves et al. 1980)

Day 14. Monodontomerus parasites begin emerging from parasitized bee larvae. They appear as tiny black wasps.  (Eves et al. 1980)

Day 15. Bee pupae are totally black in color.

Day 17. First adult males are fully formed in the cells.

Day 18. Males begin emerging. All cells still in nesting material (boards, etc.) should be placed in the field.

Day 20. The first female bees emerge.

Day 22. In loose cell management, incubation trays should be taken to the field and the lids removed so the bees can escape; 30 to 50 percent of the females should have emerged by this time.

Day 28. Most bees should have emerged.

Advantages of Controlled Incubation:

  1. Bee emergence can be accurately predicted if bees have been in cold storage for about 250 days.
  2. Most incubated bees emerge within 2 weeks after being placed in field shelters.
  3. Many bee enemies can be collected and destroyed during the incubation and phase out process.


  1. Bee cells require daily attention during the incubation period.
  2. The cost of the incubator and its operation raises operating expenses.
  3. Controls may malfunction resulting in an excessively high temperature that can kill bees.
  4. Loose cells in incubation trays must be specially protected from parasites during the incubation process. Pesticides may harm the bees.
Top of page 

II. Incubation of bee nests at 85F after non-refrigerated storage.


  1. Bee emergence will begin in 19 days or sooner, depending on pre-incubation storage temperatures.
  2. All advantages listed for incubation after refrigerated storage with the exception that emergence will not be as uniform.


  1. All disadvantages listed for incubation after refrigerated storage.
  2. Nest-destroying enemies multiply any time storage temperatures exceed 40F. Consequently, more nest destroyers may be present during incubation, which is a favorable period for feeding, growth and reproduction of these bee enemies.
Top of page 

III. Manipulation of incubator room temperatures to delay bee emergence during period of cold, wet weather that delays alfalfa bloom in the spring.


  1. During the normal 19-day incubation period, at 85F and 50 to 70 percent relative humidity, temperatures may be lowered and maintained as low as 79F  if necessary to slow bee development.  Use degree day accumulations to estimate  how much development has taken place.
  2. After day 14 of the normal incubation period, bee emergence can be delayed as much as an additional two weeks by holding the temperature between the narrow limits of 50 and 59F). These temperatures stop immature bees from developing. Development continues when temperatures are raised above 60F, but typically growers continue development at 85F. Use degree day accumulations to estimate  how much development has taken place, and how long to emergence.


  1. Bee development can be delayed so emergence coincides with the availability of alfalfa bloom and/or favorable weather.


  1. During the last half of the incubation period, temperatures lower than 60- 65F may cause some bee mortality.
  2. During this same period, temperatures above 67F, especially above 70F, will permit some bees to emerge within the incubation room. These bees will die from starvation.
Top of page 

IV. Use of lights over oil or water pan traps in incubation room.


  1. A combination of incandescent and fluorescent "black lights" attracts many nest-destroying insects and parasites emerging during incubation. Many of these emerge from nests earlier than bees and are easily destroyed in oil or water-filled pans positioned directly under the lights.
  2. The use of water pans will help maintain relative humidity between the required 50 and 70 percent during incubation.


  1. Extra labor is necessary for daily maintenance.
  2. Light fixtures must be purchased and installed.
  3. The light-proof room must have temperature and relative humidity controls.
  4. Leafcutting bees are also attracted to these lights. Early emergence or unexpected delay in placing bees into field shelters can lead to some bee mortality.
Top of page 

V. Use of dichlorvos (Vapona pest strips) in closed storage rooms.


  1. This pest strip kills many nest-destroying insects quickly. Dichlorvos can be used in closed storage and incubation rooms after bees have finished emerging in the spring to eliminate most remaining pests.


  1. This pesticide can kill bees in closed storage rooms and incubators, especially when humidity is <70% . (read  Wayne Goerzen's research report on this subject)
  2. These strips are not legally registered for use in rooms containing alfalfa leafcutting bees.
Top of page 

Further Information

IPS incubation calendar 
Degree day accumulations  

Eves, J.D., D.F. Mayer, and C.A. Johansen 1980.  Parasites, Predators and Nest Destroyers of the alfalfa leafcutting bee, Megachile rotundata.  Western Regional Extension Publication (WREP) 32.  (Contact your county extension office to order a copy).

Stephen, W.P. and C.E. Osgood. 1965.  The induction of emergence in the leaf-cutter bee Megachile rotundata, an important pollinator of alfalfa.  J. Econ. Entomol. 58: 284-286.

Whitfield, G.H. and K.W. Richards. 1987.  Postdiapause development and adult emergence of Pteromalus venustus Walker (Hymenoptera: Pteromalidae) during alfalfa leafcutter bee incubation.

Richards, K.W. and G.H. Whitfield. 1988.  Emergence and survival of leafcutter bees, Megachile rotundata, held at constant incubation temperatures (Hymenoptera: Megachilidae).

Peterson,S.S., C.R. Baird, and R.M. Bitner. 1991.  Variation in weight and postdiapause development among groups of alfalfa leafcutting bees, Megachile rotundata (F.), reared in different locations.  Bee Science 1:230-236.

Kemp, W.P. and J. Bosch.  2000.  Development and emergence of the alfalfa pollinator Megachile rotundata (Hymenoptera: Megachilidae).  Ann. Entomol. Soc. Am. 93:904-911.

If you have comments, corrections, or additions to the information provided in this document, contact Karen.
Top of page 
CIS 538 Contents:
Phase out
Shelter Size and Design
Nesting Material
Buying bees and/or loose cells
Avoiding theft; Purchase precautions.
Pollination Ecology

Revised Nov. 18, 2000.