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Alfalfa Leafcutting Bees

Megachile rotundata
The alfalfa leafcutting bee, Megachile rotundata, is a valuable pollinator of alfalfa grown for seed in western Canada as well as in the western USA.

The alfalfa leafcutting bee is a solitary bee meaning that each female lays eggs and provisions her own nest. Although the bee lives alone and provision its own nest, it is also gregarious preferring to live close to its own kind. The fact that this bee is gregarious and will live in man made shelters is why this insect can be reared and managed as a crop pollinator. This bee prefers sunny dry warm days with temperatures exceeding 69 degrees F.

Alfalfa Leafcutting bees are used primarily for alfalfa seed production but they have been found to pollinate other forage legumes and Canola Brassica campestris effectively as well as other crops such as blueberries.















Alfalfa Leafcutting Bee Adult - Megachile rotundata

 
LIFE CYCLE OF THE ALFALFA LEAFCUTTER BEE
Understanding the life cycle of the leafcutter bee is essential to proper management. The following information was summarized from a literature review conducted by E.C. Klostermeyer, Washington State University.


The alfalfa leafcutting bee, Megachile rotundata (Fabr.), is one of the few insects that can be considered domesticated. The bee was accidently introduced into the U.S. from Eurasia on numerous occasions. The bee was probably widely distributed during and after World War II through movement of nests in crated war materials and developed large populations under the favourable conditions in the western U.S. When its potential as a pollinator was suggested, a new pollinator industry was developed. The bee is now used to produce alfalfa seed in western U.S., Canada, New Zealand and in Southern American and European countries.


The Catalog of Hymenoptera lists pertinent details and literature references on the species under the name Megachile pacifica Panzer. The International Commission on Zoological Nomenclature suppressed pacifica in favour of Megachile rotundata (Fabr.). That, and the common name, alfalfa leafcutting bee, was accepted as the approved names by the Entomological Society of America. However, the common name "Alfalfa leafcutter bee" is still favoured by growers and used by some scientists. The chromosome number is 32 for the female and 16 for the male. There is only slight genic variation in enzymes in the bee, perhaps as a result of the commercial distributions of populations from a limited gene pool.


M. rotundata overwinters as a mature larva (prepupa). For bees overwintering under ambient conditions, spring emergence coincides with early bloom of uncut alfalfa. A chilling period is needed to break diapause. (Diapause is the suspended state of development which delays the emergence of the adult bee until the following season. Second generation bees are those in a population that do not enter diapause, and therefore continue developing steadily.) Without exposure to cold, emergence did not occur for 98 days and only 80% had emerged after 365 days. With prepaupae held under refrigeration, as is the usual management practice, at least 3 months at 4C, is needed for optimum emergence. When incubated at a temperature of 30C, pupation begins in about a week. Emergence of a population begins in 18 days and is completed in 34 days. Males emerge first, since they are in the outer cells of the nesting tunnel, with a peak at about 3 days from first emergence. Female emergence peaks at 7 days. The latest produced bees of either sex in a nesting tunnel tend to emerge first even when the cells are removed from the nesting tunnel. Emergence day is related to bee size with the smallest bees emerging first. The mandibles of the males have a prominent tooth, perhaps adapted for chewing through the closing leaf plugs. Bees usually emerge in early morning.


Mating occurs on and around the nests. Males pounce on females as soon as they emerge and are resting in the sun. A male lands on the female's back and attempts to pull the tip of her abdomen up with his hind legs. If receptive, the female everts the sting, allowing coupling of the genitalia. Mating lasts less than a minute. Males continue to approach mated females but there is no evidence that the female mates more than once. Only 3 of over 800 females examined at a nesting site failed to have sperm in the spermatheca. Unmated females produce only males. Unmated caged females nested normally but all 504 progeny of 65 such females were males. Males cluster in nests or other cavities at night, but male populations dwindle quickly, perhaps by dispersal, as females begin nesting. Females may reuse the holes from which they emerge or select new ones. They clean out old leaf pieces and other debris before using a tunnel. Under natural conditions, beetle holes in wood are used as nests but bees utilize any cavity of the right size, including old wasps nests, nail holes, drain tubing, car radiators and folds in cloth. Horizontal holes are most often occupied. In a mass of holes, such as in commercial bee boards, bees tend to place their nests first along an edge or near a knothole or other marker. This led to the practice of painting a pattern of landmarks on nesting boards.


Females spend the night in the nest, faced inward. As temperatures rise in the morning, they turn around and face the entrance but do not come out and fly until the temperature exceeds 20C and the sun's radiation reaches 0.7 langley. Bees foraged at 1075 lux when the temperature was 25C but needed 6450 lux at 17C. Bees stopped foraging in the evening when radiation dropped to 0.3 langley and also stopped if clouds reduced radiations to that level.


Females construct thimble shaped cells of leaf pieces. ( Figure 1) The first cell is usually in the bottom of the hole, but in deep holes, the cell may be placed at any point. The bee may use a variety of leaves but chooses older, limp leaves that she can easily cut and manipulate. The old, lower leaves of alfalfa are often used, but sweet clover, Lamb's quarters, and leaves and flower petals from many other species of plants may fly long distances for suitable leaves. Bees in an alfalfa field flew over 300 feet past obstructions to gather rose petals. Bees have also used thin paper and polyethylene plastic sheets in the absence of suitable leaf material.











Figure 1

The Female cuts the leaf with her mandibles, (jaws) rolling it between her legs for transport.  It has been suggested that sensory hairs on the tarsal claws served as a tactile sensor in cutting. In the first cell the bee first brings in small, round leaf pieces, then larger oval pieces, and finally 1 to 3 smaller oval pieces that form a smooth base in the bottom of the cell and which may serve to make the provision level. As the bee places the leaf she chews the edge. Plant juices, and perhaps the bee's glandular secretions serve to hold the pieces together. If the bee has difficulty in arranging a leaf, she may pull it out of the nest, drop it and obtain another. This is a common occurrence during hot dry weather.


An event recorder and a balance was used to number and time the various aspects of nesting and to weigh the leaf material and provisions collected by the bees. In 4 mm diameter holes, bees used an average of 9 leaf pieces per cell; in 4.8 mm holes, 11 pieces; and in 6.2 mm holes, 14 pieces but as many as 27 leaf pieces were recorded. Leaf loads average about 17% of the body weight. Collecting a leaf took 2 - 5 minutes and arranging leave in the cell, 4 - 8 minutes. Cell construction times average 2 1/2 hour but were as brief as 20 minutes under favourable weather conditions and with a nearby source of leaves. There was evidence that the bee took time to feed during nest construction.


After a cell is completed, the bee gathers nectar and pollen as food for the larva. M. rotundata commonly chooses the legume flowers, particularly alfalfa and sweet clover. She pries open the keel on such flowers while inserting the proboscis to suck nectar. In doing so, the stamens and pistil hit the bee on the front legs and under the head. While sucking nectar she transfers accumulated pollen to the middle legs. The pollen is then removed by the hind tarsi (legs) and transferred to the scopa (rows of hairs on the underside of the abdomen) while she flies to the next plant. Flower visitation rates vary depending on the weather and other factors, but on alfalfa, 8 to 10 blooms visited per minute is average.


The provisioning process can be observed by cutting out the leaves from the sides of the cells made in transparent tubes. Bees usually repaired such cuts even if they had begun provisioning but eventually resumed provisioning. The female enters the nest head first and regurgitates nectar, mixing it with her mandibles into any previously collected pollen. She then cleans her antennae and proboscis by pulling them through the antenna cleaners on her front legs. She somersaults in larger holes or, in small holes, backs out to turn around. When the tip of the abdomen touches the provisions she braces herself by pressing the ends of the tibia of the front and middle legs against the walls and uses the hind tarsi to brush off pollen from the scopa, tamping it into the stores with her abdomen. Afterwards she flips her abdomen rapidly up and down to rearrange the hairs.


The initial provisioning load consists of about 80% pollen. Subsequent loads contain increased amount of nectar. The final load or two is of nectar only and is not mixed with the pollen. As a result, the bee larva is provided with a differential diet. The first instar feeds on the pool of nectar on which the egg is laid. As it eats its way into the provisions the larva gets a progressively greater proportion of pollen to meet its nutritional needs. The average provision mass is 64% nectar and 34% pollen by fresh weight.


When ready to lay an egg, the female vigorously cleans the tip of her abdomen with her hind legs, then backs into the cell with her stinger extended nearly vertically and lays the egg in a series of abdominal contractions. If the egg is unfertilized (male) egg laying is completed in 10 - 25 seconds. If the egg is to be fertilized (female) the bee pauses for several seconds when the egg is 2/3 extruded. The egg probably is fertilized during this pause. Following the pause, egg laying is completed quickly. In all but 1 of 9 observed ovipositions, eggs laid with a pause were female. All observed ovipositions without a pause produced males.


After laying the egg, the female checks the egg with her antennae. She chews on the leaf pieces at the outer edge of the cell, then brings in one or more round leaf pieces that she wedges into place as a cap and to form the base of the next cell. She then constructs, provisions, and lay eggs in additional cells until the hole is nearly full. The entrance plug consists of pieces of leaf of various shapes, but usually round. Only a few pieces may be used, but some nests have over 50. Plugs usually are flush with the surface. As soon as she has plugged the nest the bee searches for another hole.


Females completed a cell in an average of 7 1/2 hours. They usually lay one egg a day, but under favourable conditions can complete 2 cells. One caged bee laid 35 eggs in 19 days, including 2 days when she laid 3 eggs in a day. The usual nesting pattern is for the bee to complete a cell and partly provision it on one day and to finish the process usually before noon on the following day. Males live 3 - 4 weeks and females 5 - 6 weeks but under field conditions probably live less than a month. The average number of eggs laid by 38 bees was 28 with a maximum of 55.


Bees accept the smallest hole diameter fitting their body dimensions but may omit a leaf liner in very small holes or will use additional, loosely place leaf pieces in large holes. The amount of provisions and therefore the size of the progeny is related to the hole diameter. Thus, a small bee may produce large progeny in large holes and large bees may produce small progeny in small holes. A bee nesting in a 6.2 diameter hole stored 10 mg more provisions per cell than when she nested in a 4.8 mm hole. The number of bees and females is proportional to the depth of the hole: 1 bee in the first 2 cm of depth and an additional bee for each additional 1.3 cm; 1 female for the first 5 cm and an additional female for each additional 2.5 cm. Numerous nest modifications were made to determine the effects on sex determination. Moving partly completed cells forward from a deep to shallow position resulted in fewer females than expected. Moving the cell to a deeper position resulted in more females. Most bee populations have fewer than 40% females. The percentage of females per hole increased from 28 in 4 mm holes to 39 in 5 mm and 40 in 6 mm. In 5 mm diameter holes, those 2 cm deep had only 6% females compared with 21% in 4 cm, 38% in 8 cm, and 39% in 16 cm.


The egg (Figure 5) hatches in 2 - 3 days and develops through 4 instars with the following development times at 28C and 55% RH: egg 2.2 days; 1st and 2nd instars 2.1 days; 3rd 1 day; 4th 3.5 days feeding and 2.5 days quiescent, cocoon spinning 1.5 days for a total of 12.8 days. Larvae began defecating 5 - 8 hours after the final molt. Development time at 35C was 9.6 days. Non-diapausing larvae developed at a faster rate (15.4 days at 22C and 8.7 days at 29C). Development rates for males and females were about the same. The larvae are among the most efficient invertebrate converters of energy with gross ecological efficiencies at 58.5% for energy and 87.6% for nitrogen.


While most larvae enter diapause, some bees develop to adults in 23 - 28 days from the time the eggs were laid. In Alberta, over 50% of the cells that were completed by July 13th produced 2nd generation bees, but only 1.4% produced 2nd generation adults by the end of July. However, 0.6% were still being produced at the end of August. In Washington, 70 - 90% of bees nesting in June produced emergent progeny, 33 - 53% in the 1st half of July, 12% in the last half of July, and 0 in August. More second generation bees were produced from the first laid eggs, regardless of when they started nesting. Hobbs and Richards (2) selected a univoltine strain that produced less than 1% second generation bees for 3 years. Because of the second generation, bees in Washington may be found nesting from June until October.


The alfalfa leafcutter bee adapts to a wide variety of climatic, nesting, and foraging conditions and maintains populations through the onslaught of predators, parasites, and insecticide poisoning. Because of its gregarious habit, alfalfa pollinating efficiency, ease of management, and usually rapid increase in populations the bee has attracted the interest of layman and scientist as few other insects have. Yet there are still gaps in our knowledge of the bee. These include internal morphology, physiology, diapause, and genetics. In addition, a population model would be helpful in making the most effective use of the bee as a pollinator.


LOOSE CELL MANAGEMENT - AN OVERVIEW
Loose cell management refers to the system used by Canadian leafcutter bee keepers where the cells are removed from the nesting material during the annual management cycle. This system was developed in order to control the potential build up of diseases and natural insect enemies of the bee. The loose cell system allows for good control of the incubation process so that adult bees can be released into the field in a timely manner. The system also allows for accurate sampling of the seasons production of cells. Sampling and testing of cells is important for monitoring parasites and disease, as well as when bees are being bought and sold. The loose cell system is opposed to one that is used in some areas of the United States where bees are stored and returned to the field intact in the nesting material. This "solid" system does not allow any adequate disease or parasite control, and should not be considered for use. This section gives a brief outline of the annual management procedures using the loose cell system. The remainder of this chapter deals with each aspect in detail.


The leafcutter bee cells (Figure 2) are removed from the nesting material in the fall or early winter of each year using a mechanized cell remover. The type of cell remover will depend on whether wood laminates of polystyrene is used as nesting material. Once stripped, the cells are run through a tumbling machine to remove excess leaf material and surface moulds. The tumbling procedure also breaks the "fingers" of cells into smaller segments, and this aids in the emergence of bees during incubation. A representative sample of cells should be taken during the tumbling procedure and submitted to the Canadian Cocoon Testing Centre in Brooks, Alberta for analysis. This analysis provides information on the quality of the cells, including percent live prepupae, live count per pound, and degree of parasitism and chalkbrood disease if present. This information is essential when buying and selling bees, and is also useful for evaluating and planning management practices aimed at improving the quality of bees.


The cells are stored at about 5 - 8 or 10C and 50% relative humidity for the remainder of the winter, and until incubation is begun in early June. During this time the bees are in the prepupal (mature larva) stage, in a natural state of arrested development called diapause. In general, about three months of cold storage is required to adequately break diapause. Shorter periods of storage result in a delayed and drawn out emergence. Care must be taken to store cells in such a way that mouse damage and the growth of moulds is prevented (Figure 6)


Incubation of the cells to synchronize the emergence of the adult bees with 10% bloom in the field begins in late May to early June. An incubation room, in which the temperature and relative humidity can be accurately controlled, is used to complete the development of the prepupae (Figure 7).


Two types of incubation systems are in general use - the tray system and the bleedoff system. The incubation procedure is similar for both systems but the method in which the adult bees are taken to the field differs. The tray system employs a single room in which the cells are incubated in shallow trays with removable screen lids. Once emergence of the adults is well underway the trays are taken to the field and the bees are released (Figure 13). Unemerged bees generally complete their development under the field conditions, but trays may also be brought back to the incubation room.


The bleedoff system employs a two storey structure - an incubator on top and a cold room underneath. Cells are incubated in trays without lids. As the adults emerge they are attracted to black lights placed near funnel shaped chutes built into the floor of the incubator. The bees fall down the chutes and into store containers in the cold room. Excelsior (stringy wood shavings) is placed in the storage containers to prevent smothering of the bees.  Adult bees are immobile at the temperature used in the cold room (10C) and may be stored for up to one week before release into the field. Both systems have advantages and disadvantages in regard to the amount of labour required and the flexibility of bee storage and release.


After normal winter storage conditions, the emergence pattern of adult bees during incubation can be accurately predicted. At a temperature of 30C and a relative humidity of 70% the first males will emerge on about the 18 - 19th day and the first females on the 21 - 22nd day. By the 25th day, 95% of the males and females, respectively, will have emerged. Incubation is therefore started approximately three weeks before the alfalfa field is expected to reach 10% bloom. During incubation, cool wet weather may delay the onset of bloom and prevent bee release. Under these circumstances the temperature may be lowered after the 14th day of incubation to slow development and delay the hatch.  


Parasites of the leafcutter bee may cause heavy losses, and must be controlled. Pteromalus venustus, a small jet black wasp, is the most common and serious pest. Pteromalus lay eggs on the full sized bee larva, and these hatch leaving an intact cell full of Pteromalus prepupae (Figure 11). On the 8th or 9th day of incubation these will emerge as adults, immediately mate, and re-parasitize cells if left uncontrolled. Control measures include the use of dichlorvos (Vapona) in the incubator, vacuuming, light traps, and well constructed nesting material to prevent their invasion.


Nesting material and shelters are placed in the field to provide a place for construction of new cells. The nesting material may be either wood laminates, polystyrene laminates or polystyrene blocks. Each has advantage or disadvantages in terms of price, durability, attractiveness to bees, and the quality of cells produced. The choice of nesting material will depend on the growers preference and particular circumstances. Nests are placed in the field shelters prior to bee release, and approximately 1 1/2 tunnels are supplied for each female adult to be released. The nest surfaces are painted and a blue design lis later stencilled on to serve as orientation markings for the nesting females. Field shelters are used to protect the bees and nesting material from the weather. Various types of shelters have been constructed in attempts to increase nesting activity and bee increase. Shelters are placed in the field with the opening facing east so that the morning sun will warm the bees and nesting material. Approximately one shelter is used for every three acres to be pollinated, but have been varied up to 10 acres with satisfactory coverage if bee stacking rate is high enough. Shelters are usually painted to serve as orientation markings.


Bees are released to the field during early morning when the temperature is cool and bee activity is minimal. Trays should be placed inside the shelters in an area where the will be protected from wind, rain and direct sunlight. If using the bleedoff system, bees will cling to the excelsior and this may be placed on tope or inside the shelter. Stocking rates of bees generally falls between 10,000 and 30,000 per acre. The lower rate results in high bee increase but less than adequate pollination, while the higher rates results in excellent pollination but lower increase. A rate of 20,000 per acre is usually considered a good compromise. Once bees are released mating takes place, and within a few days females begin constructing and provisioning cells. The nesting material is inspected periodically during the summer and usually removed when about 3/4 of the tunnels have been capped. Empty replacement nests are usually provided in place of the full nest.


Full nests are brought to a storage room and are held at 20 - 22C for about three to six weeks to allow time for the eggs and young larvae to develop to the prepupa stage. The nesting material may be pulled about one inch away from the backing to help dry down the cells. Parasites are sometimes a problem during this storage period and some form of control is necessary. Once the cells have dried down they can be stripped from the nests, and the annual management cycle is repeated.


Chalkbrood is a serious fungal disease which has hampered alfalfa seed production in the United States. Sodium hypochlorite beach solutions have been effective in killing chalkbrood spores found on nesting material, shelters, incubators and other equipment. New methods of control are presently being investigated, including fumigation with paraformaldehyde. Paraformaldehyde is now registered and very effective.



A CALENDAR OF INCUBATION FOR ALFALFA LEAFCUTTING BEES
(Variations occur - should be used as a guide only)

Day 1 Cocoons are at 30C. All are in the prepupal stage. Black lights and water traps are inplace, and the thermostatically controlled alarm system is working.

Day 3 Parasites undergo their final molt into the pupal stage.

Day 7 Place Vapona into the incubator at the recommended rate of 3/4 stip per 1000 cu. ft. If the incubator is only partially full use a lower rate.

Day 8 Leafuctter bees begin to undergo their final molt into the pupal stage. At this stage they are very sensitive to temperature fluctuations so maintain an even temperature -- no cooling and holding at this time!

Days 8 - 9 First Pteromalus begin to emerge. If the Vapona is working they should die in the trays rather than making it to the water trap.

Days 9-12 Pteromalus continue to emerge.

Day 10 Leafcutter bee pupae begin to show some eye colour -- the pink eyed stage.

Day 12 Leafcutter bee pupae continue to darken in colour in the eyes and over the back.

Day 13 Remove the Vapona strips as soon as hatch is complete. Actively air the incubator thoroughly and completely, using an exhaust fan and circulating fans, for 24 to 48 hours. Maintain the 30C temperature if possible and desirable.

Day 14-15 Leafcutter bee pupae continue to darken in colour. If cooling occurred during airing after Vapon removal, bring the incubator temperature bact to 30C for continuation of development.

Days 14-15 Native leafcutter bees emerge. This may cause concern to the new beekeeper that the timing is off and the bees are beginning to emerge several days early.

Days 14-22 At any time during this period, if incubation must be slowed due to weather or bloom, the cocoon temperature can be lowered to 10 - 15C for up to two weeks. This virtually stops development. Once the temperature is increased, development resumes until the hatch is complete. Remember that the bee temperature must be at 10 - 15 , not just the air temperature.

Day 16 Early hatching pupae (mainly males) are completely dark in colour; later hatchers (mainly females) are darkening.

Day 18-19 Males begin to emerge. Remember that the bees are very susceptible to high temperatures! Make sure that your alarm system is working!

Days 21-22 Females begin to emerge. Male emergence peaks. Second generation Pteromalus begin to emerge.

Days 23-24 Female emergence peaks.

Days 23-24 Trays are taken to the field for adult bee release once the females are 75% hatched. Trays may be returned to the incubator for ompletion of female hatch under controlled temperature conditions, then taken to the field again after two or three days.

Day 28 Hatch is virtually complete at 30C.

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