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Modified from Dorothy Murrell1,2 and D. W. Goerzen2

By Karen Strickler


Second generation

Removing Nests from the Field

Storage Prior to Punching Cells

Fall Parasite Control

Punching, Tumbling, and Storage of Cells


Loose cells of Megachile rotundata

By mid-August, the leafcutting bee females begin to die off and the population gradually decreases. Each female has produced many cells for offspring and filled and capped several tunnels, which is evidenced by the capped nests piled in the storage shed. The goal of fall and winter management is to get bee larvae into diapause for the winter, and to keep them safely in diapause until incubation in the spring.


The leafcutting bee offspring progress through the egg and larval stages, spin their cocoons and defecate, and go into a state known as diapause. Diapause is a condition characterized by an increased concentration of glycerol (anti-freeze) in the body tissues which enables the prepupae to survive low temperatures.

Under laboratory conditions, diapausing Megachile rotundata have the ability to withstand temperatures of -17oF and -27oF.3,4 In colder parts of the Northwest and in Canada, M. rotundata would probably not survive the winter without protection from cold temperatures. In milder climates, protection from the weather is still recommended to avoid naturally fluctuating temperatures that can be detrimental to the bees. In particular, temperatures above 65oF in the fall may delay diapause, while in early spring warm temperature may cause M. rotundata to break diapause, becoming more susceptible to cold snaps. Furthermore, larvae exposed to 65 oF or more for longer than 45 days in the fall, or exposed to 77 oF or more for longer than 20 days may not successfully emerge in the spring5, presumably because of reduced energy reserves.

Many insects that go into diapause, including M. rotundata, require a cold period to break diapause and allow further development. If bees are stored at room temperature, the requirement for cold exposure is not met. Emergence is therefore greatly delayed and drawn out. The cold storage period serves both to shorten the incubation time and to synchronize emergence, so that once incubation at 85oF begins, the bees will all emerge within the 19-25 day period. Johansen and Eves6 collected filled nests from the field and stored them at 82 oF for two weeks before subjecting the cells to various storage durations at 35oF. Incubation then occurred at 88 oF and 55% RH with the following results:

Table 1
Days at 35 oF Days to first
Days to 50%
Length of emergence period (days)
0 98 - 107 210 - 241 321 - 342
30 95 - 102 203 - 219 204 - 312
90 30 - 35 111 - 137 124 - 224
150 23 - 27 28 - 31 32 - 46
210 18 - 24 23 - 27 7 - 19
270 14 - 21 16 - 22 3 - 6

The data in table 1 suggest that bees should get a minimum of 5 months of continuous cold storage to shorten and synchronize their emergence time. Seven months is even better.

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Second Generation

The alternative to diapause is continuous development into a second generation adult. M. rotundata is normally a multivoltine species (i.e., it is capable of producing two or more generations per year). In some regions of the northwestern United States (e.g. California), two full generations and a partial third generation are produced. In the Northwestern USA and Canada, one full generation is produced, often with a partial second generation depending on latitude, nesting material and weather regime.

The determining factors for development vs. diapause are unclear. Some researchers attribute the number of generations to temperature7, while others have proven a genetic involvement8. More second generation bees emerge in a hot, dry summer than in a cooler year, but we are not sure if this is a response of the adult female or of the developing larva. It is likely that both environmental and hereditary factors play a role.

The net effect of second generation in Northwest leafcutting bee populations is probably to reduce the number of cocoons for storage. Whether second generation females have time to produce progeny and set seed depends on how soon they emerge, on how much bloom remains, and on the weather when they emerge. In central Washington adults emerged within 38 days (1968) and 23 days (1970) from the time eggs were laid6; the earliest adults emerged in mid to late July in the warmest seed-producing areas. Most leafcutting beekeepers have observed second generation adults emerging from capped nesting boxes in storage prior to punching, as well as pupae in various stages of arrested development in their samples. A large round hole in the cap of a previously completed tunnel indicates that emergence has taken place. Whether many have hatched previously and have been left in the field is hard to determine. The presence of males in August is a good indication of a second generation, since the first generation males die off during July.

Second generation adults develop from the earliest eggs laid, and little can be done to control this phenomenon. Once the first nesting boxes are brought into storage these emergent bees will continue their development as long as the storage temperature permits. After diapausing individuals have spun their cocoons and the storage temperature is decreased, second generation bees will stop developing and emerging. However, they will not survive the winter, and will die during the storage period. Thus cooling the early nests halts emergence, but not development, of second generation adults.

In most cases all of the progeny in a tunnel will be either emergent second generation or diapause individuals6. Second generation individuals rarely develop at the bases of their tunnels and then chew through diapausing sisters and brothers to escape. Therefore, the loss to the beekeeper will probably be closely equivalent to the number of emergent individuals. However, this loss is difficult to quantify. Samples taken after punching and tumbling the cells will no longer contain cells from which bees have emerged.

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Removing Nests from the Field

Nests that are 70-80% capped are generally removed from shelters to protect them from weather, rodent and bird feeding, and reopening capped tunnels by female leafcutting bees. Nesting boxes should be removed from the field during the heat of the day in order to ensure that the adults are absent from the tunnels. Depending on the date and numbers of bees still present these nests may be replaced with empty nests. This is a management decision that becomes easier with experience. Too much nesting space will result in many partially completed tunnels, necessitating much extra work punching these nests for little return. Too little nesting space, on the other hand, may result in loss of production. In Canada where bee returns tend to be 1.5 fold greater than the numbers released, or more, most producers allow two or three tunnels per female and generally add nests after the first capped nests are removed.

Once the alfalfa bloom is finished and the bee population is noticeably decreasing, most of the nests may be removed from the field. The least capped nests can be left out to the last. In a cool wet season, polystyrene and styrofoam nests should be brought in without delay, as mold problems will quickly develop; there is more leeway in hot, dry weather. However, after about September 15, bees should not be left in the field or in a warm storage facility for longer than 45 days at temperatures above 65 oF, or for longer than 20 days at temperatures above 76 oF, or emergence in the spring may be reduced5. Rapid temperature swings may also be detrimental to bees.

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Storage Prior to Punching Cells

Temperature management is the key when bringing in boards and storing them. There are three stages:

  1. Complete larval development in the last nests of the season at warm temperatures.
  2. Dry the cells at the lowest temperature possible, preferably below 59 oF.
  3. Maintain a constant 40 oF in storage.
Once removed from the field, nesting boxes that have recently been filled should be cross-stacked and stored at 68oF for about three weeks (two weeks at 82oF). This period allows egg hatch, larval feeding and cocoon-spinning to be completed. This takes about 209 degree days, base temperature of 59oF.

Boards that have been filled for some time, or that have completed the three week period, may be cooled to 45-59oF. Nest backs may be removed to enhance drying of the leaf material only at the lower temperatures, or if Pteromalus are not present. If polystyrene blocks are used, they must be actively dried prior to cell removal. The drying process involves cross-stacking the filled blocks and running air through the stacks with overhead fans, while exhausting the moisture-laden air using a wall-mounted exhaust fan. Humidity may be monitored during the process, and when room humidity stabilizes at 25-30% the blocks may be dry. Dehumidifiers may be necessary in some storage facilities and climates. Temperature should not exceed 59oF for any length of time or leafcutting bee energy reserves may be depleted, and parasite and predator activity will continue. Generally blocks are dried for 1-2 weeks prior to the cell removal process. The cells should then be hard enough to withstand normal punching procedures. Cells may be checked periodically by punching out or pulling apart some nesting boards and rolling the nests between the fingers. They should feel dry and hardened, not spongy, soft or moist. Excess moisture may enhance fungal growth, which is detrimental to the health of both leafcutting bees and beekeepers. Samples with a large percentage of pollen balls will take longer to dry than samples with few pollen balls.

Polystyrene blocks easily re-imbibe moisture, and some growers who have left them in winter storage until May find that they are at that time again very moist and difficult to strip, with noticeable crush damage to cells. Grower experience suggests that the best time to strip blocks is between November and March. They must be stored until punching at normal winter storage temperatures of 40oC both before and after cell removal, in order that incubation time and emergence are not affected.

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Fall Parasite Control

Parasites in fall, as in spring, represent a threat to the leafcutting bee population, since they will mate and attempt to parasitize developing larvae, causing both a decrease in live count and an increase in wintering parasite numbers for the next season’s incubation. Unfortunately, the very practices that help to prevent mold buildup allow easy access by parasites to potential hosts, by exposing the backs of the boxes and cracking the leaf seals around the tunnels. Exposing the back of the board is not recommended until after the three week warm storage period is completed and temperatures have been cooled to below 50oF.

Both vacuuming and water traps under black lights will help to control parasites in the fall. For black lights to be really effective the storage facility should be insect- and light-proof, so that the black light is the only light source. Adult bees will also fly to the light traps, as will moths, wasps, and other insects. Insect zappers are not effective against parasites.

Parasite levels are less likely to build up in the blocks during the summer if they are tightly sealed, with no cracks that would allow parasite entry. The inclusion of a compressible foam material further reduces parasitism by eliminating any crawl space between nest block and nest back. If you have a parasite problem, experiment with ways of sealing the back of the nest block. Blocks with different seals can be punched out separately, and examined and compared for parasite level in the fall or winter.

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Punching, Tumbling, and Storage of Cells

The cells should be punched, tumbled, and placed in cold storage (40oF) as soon as possible to avoid problems with mold, dermestid beetles, parasites and mice. Temperature fluctuations, which may affect the live count, must also be avoided.

There are many methods of punching cells from nest laminates and blocks, from hand-strippers through mechanized models. The main points to watch for are that cells are not crushed or damaged and that the nesting laminates or blocks are not chipped or broken. A producer who is interested in purchasing puncher and tumbler should check with other producers and discuss the various good and bad aspects of other setups. Suppliers often bring a demonstration model to the Northwest Alfalfa Seed Grower’s Association Winter Seed School.

Tumbling helps to remove empty cells and leaf litter, thus reducing volume. It also helps to remove chalkbrood cadavers, checkered flower beetle larvae, and other pests. The major problem with most tumblers is the dust and mold spores that fill the air. Many producers become more sensitive to this fine dust each year; in some cases serious allergies have developed. Thus it is strongly recommended that tumbling be done outside or in an open shed with good ventilation, or that the tumbler be hooked up to a vacuum system that vents out-of-doors. Face masks or respirators should also be worn for protection.

A cell breaker machine is helpful for breaking sequences of cells into single cells, on the premise that emerging leafcutting bees will be able to chew out of their individual cells without passing through a sequence of cells and possibly contacting chalkbrood cadavers. Since leafcutting bees often chew out through the sides of their cells, the cell breaker’s usefulness may be more specifically to further reduce volume, by removing more leaf and empty cells and by crumbling pollen balls, other debris and chalkbrood cadavers. Samples of single cells for cutting or for x-ray analysis are also more representative of the entire lot of bees than are sequences of cells, each from the same nest. The cell breaker must be carefully adjusted to avoid denting and crushing cells. Cells that pass through a cell breaker should be dipped in a bleach solution the following spring, to destroy chalkbrood and mold spores liberated during the breaking process.

Once punched and tumbled, cells should be placed in containers, closed up and put into cold storage. Containers left overnight in a warm building may be heavily parasitized by morning. Large garbage bags, plastic pails, and cardboard drums and boxes have all been used successfully. Containers should have an adequate seal so that the cells do not absorb moisture. Containers of cells must be stacked to allow flow among containers. A heap of containers may heat in the center, causing loss of energy reserves, and bee and parasite development. In late winter, warming the cells could break diapause. Once diapause is broken, cooling the cells again can cause mortality to the prepupae.

Cells should be stored at 40oF to maintain diapause, preserve energy reserves and render pests such as dermestid beetles inactive. Storage chambers range from refrigerators to garages, basements and root cellars to specially built walk-in coolers. For small numbers refrigerators work well if not too full; otherwise the cells in the centers of the containers may heat up and cause such problems as cessation of diapause, mold growth, and death. Garages and basements, while convenient, may require cooling and/or heating to maintain the proper temperature. A walk-in cooler may be a modification of the incubation chamber, and will be best for temperature and humidity control, but will probably be quite expensive. The storage facility depends on the size of the operation, what is available, and what is affordable.

Stored leafcutting bee cells should be checked often and regularly for problems with mold, mice and temperature. Should cells become damp or moldy they may be spread out to dry and repacked when they feel dry and hard again. Mice may be trapped or poisoned with bait. High temperatures may cause diapause to break, or may be lethal. Very cold temperatures during storage and shipping are also lethal.

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  1. Murrell, D. 1997 Fall and winter biology and management of alfalfa leafcutting bees. In: D. W. Goerzen, ed. Alfalfa Leafcutting bee management and alfalfa seed production producer manual, Saskatchewan Alfalfa Seed Producers Association.
  2. Murrell, D. and D.W. Goerzen. 1997. Alfalfa Seed and leafcutting bee production in Saskatchewan. Saskatchewan alfalfa seed producers association extension publication No. 97-01.
  3. Krunic, M.D., M.M. Brajkovic and K.W. Richards. 1982. Some aspects of cold hardiness in Megachile rotundata Fabr. Proc. 1st Int. Symp. on alfalfa leafcutting bee management. Ed. G.H.Rank.
  4. Gusta, L.V. 1982. Supercooling in Megachile and Pteromalus larvae in relation to temperature. Proc. 1st Int. Symp. on alfalfa leafcutting bee management. Ed. G.H.Rank.
  5. Stephen, W.P. 1995. How and where were they raised? – Critical management considerations in Megachile. 26th Northwest Alfalfa Seed Growers Winter Seed School. 27-35.
  6. Johansen, C.A. and J.D. Eves. 1973. Effects of chilling, humidity and seasonal conditions on emergence of the alfalfa leafcutting bee. Environ. Entomol. 2:23-26.
  7. Stephen, W.P. and C.E. Osgood. 1965. The induction of emergence in the leafcutter bee Megachile rotundata, an important pollinator of alfalfa. J. Econ. Entomol. 58(2): 284-286.
  8. Hobbs, G.A. and K.W. Richards. 1976. Selection for a univoltine strain of Megachile (Eutricharea) pacifica (Hymenoptera: Megachilidae). Apidologie 9:273-290.

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Revised Aug. 2, 2000.
Copyright ©
2000, Karen Strickler. All rights reserved.