Stephen S. Peterson, Craig R. Baird and Ron M. Bitner Parma and Caldwell Idaho
Bee Science 2:135-142. 1992
To receive a reprint of this article, please request from Stephen Peterson:
steve@pollinationsystems.ca
Background, History and Biology
Alfalfa seed is an important crop in western North America with approximately 46,400 ha (114,500 acres) produced in the United States and 21,800 ha (53,800 acres) produced in Canada in 1990 (AOSCA 1990) (Fig. 1). The alfalfa leafcutting bee, Megachile rotundata (F.)(Fig. 2), is managed specifically for alfalfa pollination in the Pacific Northwestern States of Oregon, Washington, Idaho, Montana, and Nevada where most of the winter hardy seed is produced (Bitner 1982). Each female bee is capable of pollinating up to 0.1 kg (1/4 lb.) of seed, making yields up to 2200 kg per ha possible with this bee (Johansen 1991). In 1990, 24,900 ha of seed were produced in the northwestern United States, requiring ca. 838,000 l of leafcutting bees (ca. 2.2 billion bees) at a value of $10.9 million for the bees alone.
Unlike the honeybee, the alfalfa leafcutting bee is undeterred by the tripping mechanism found in alfalfa flowers and prefers this crop over competing bloom from nearby crops or weeds. In California, the honeybee is the major pollinator employed in alfalfa seed because the crop can be pollinated over an extended period of time in a longer growing season. Also, the flowers tend to trip themselves in hot, dry conditions. There is increasing interest in the alfalfa leafcutting bee in California because of the potential for shorter pollination periods, higher yields, and because of rising honeybee management costs from tracheal and varroa mites and concerns over the expected influx of Africanized honeybees into that area.
The alfalfa leafcutting bee was introduced to North America from Eurasia sometime after the mid-1930’s (Stephen 1961). It is a solitary bee, but nests gregariously. In the wild, it nests in pre-existing holes in wood or in soil in cut banks. Hole diameters of 5-7 mm are readily accepted by leafcutting bee females (Stephen and Osgood 1965, Gerber and Klostermeyer 1972, Rothschild 1979). Females make thimble shaped cells, ca. 8 mm in length, using an average of 15 oval-shaped leaf or flower petal pieces per cell (Fig. 3)(Gerber and Klostermeyer 1972). Each cell is provisioned with a pollen mass moistened with nectar, onto which an egg is laid just before the cell is capped. Cells are arranged linearly, filling the hole (Fig. 4). In a series of larval cells, the female offspring are normally placed deepest in the hole with the males towards the outside. Males usually outnumber females by 2 to 1. Although females are capable of producing 30-40 cells, under field conditions a typical female will complete 12-16 cells (Gerber and Klostermeyer 1972). Most bees undergo diapause in the prepupal stage; however, some become adults without diapausing. These adults, produced during the same season are known as second-generation adults.
The advantages of using leafcutting bees as pollinators were recognized by the late-1950’s (Stephen 1961, Bohart 1972). Some of the attributes that made leafcutting bee management feasible and profitable were: (1) Females are oligolectic, i.e., they forage on just a few plants, with a preference for alfalfa. (2) The bees collect pollen to provision their nests and thus rapidly and efficiently pollinate the crop. (3) Leafcutting bees are gregarious in artificial above ground nests, which makes their nests easy to service and move. (4) The bees forage near the nest when blossoms are available. (5) The alfalfa leafcutting bee readily breaks diapause after a sufficient cold treatment and emerges predictably, as needed. (6) The bees are relatively easy to manage, requiring a moderate investment of time and money.
The evolution of management practices for the leafcutting bee has depended on improvements in nest materials. In the early days of leafcutting bee management, wooden boards were drilled by hand and placed in fields for wild bees to use as nest sites. Later, machine-drilled wooden boards became the nesting material favored by bee producers. Soon it was discovered that paper soda straws were readily accepted as nesting material (Stephen 1961). Today, various polystyrene, paper, and wood-laminate nest materials are available to growers, in addition to the standard drilled wooden boards (Richards 1978, Parker et al. 1983).
From the beginning of leafcutting bee use in alfalfa, the loss of bees due to pesticide poisoning has been an important issue. Several serious alfalfa pests including lygus bugs, pea aphids, alfalfa weevil and various minor pests require control during the bloom period. During the 1960’s and early 1970’s, it was common for six to eight sprays to be applied to most alfalfa seed fields. The development of an integrated pest management program for seed alfalfa reduced the number of sprays to one to three per season. As a result, bee losses from pesticides sprays were dramatically reduced. Field scouting and action thresholds for spray applications are now used by most growers. A major benefit of improved pest management has been the protection of leafcutting bees and other pollinators (Johansen and Eves 1973).
Economics of Leafcutting Bees
For new seedings, 50,000 bees per ha (20,000 bees/a) are recommended and for established stands, 100,000 bees per ha (40,000 bees/a) are required for good pollination (Baird and Bitner 1991). There are usually about 2,600 bees per l (10,000 bees/gal) of loose cells. A bee board will average about 8000 bees, thus, 6.25 boards would be needed per hectare. The cost of bees varies, but in 1991 bees averaged about $13 per l ($50/gal). Thus, an established stand would require 38.5 l of bees per ha (100,000 bees per ha/2,600 bees per l) at a cost of $500 ($200 per acre) in 1991. Typical cost inputs for alfalfa seed in Idaho, excluding pollination, totalled about $1430 per ha ($580/a) in 1991 (D. Bolz, N. Rimbey and R. Smathers, unpublished data). Consequently, about 26% of the total expense is on bees. In recent years, some bee producers have begun to provide custom pollination service, wherein the required number of bees are provided to other seed producers. In exchange for the bees and pollination management, the seed producer is charged 25% of the ultimate crop value (Fig. 5).
Current Practices and Problems
Leafcutting bee management progresses through the year in distinct stages. In the spring, bees are moved from cold storage (5°C) to incubation (30°C) approximately 21 d before peak bloom is expected. Incubation takes place in darkness, at 40-60% relative humidity. The base temperature for development is 16°C for leafcutting bees (Richards and Whitfield 1988), thus at 30°C, 14 degree-days accumulate each day. Peak emergence occurs after 270-310 degree-days have accumulated or 19-22 d at 30°C (Peterson et al. 1991).
Summer management begins once the adult bees have emerged. If weather is cooler than normal and blossoming is expected to be delayed, the bees can be held mid-incubation during days 15-19 at 15-20°C, without adverse effects (Rank and Goerzen 1982). Leafcutting bees should be placed in the field when temperatures are warm and winds are low. In cool conditions, bees are inactive and may be preyed upon by birds. High winds blow the bees off course, and result in orientation difficulties and high bee losses.
Leafcutting bee nest sites must be protected from direct afternoon sun and should be somewhat moveable. The domiciles or shelters that are used are quite variable in their design (see Stephen 1981), but typically have three sides, a roof and a floor. Stephen (1981) recommends that shelters be at least 6 m long and 3 m high so that the bees can visually orient to them from a distance, and they should contain a minimum of 20,000 nesting females with 60,000-80,000 nesting holes. With smaller shelters, bees may have difficulty finding their shelters and may drift to stronger nest sites. Shelters built on trailers with wheels are popular because they can be moved to and from the field easily, and can also be moved to other late-blooming fields as the season progresses. Nest materials are usually placed against the interior walls of the shelters and are often stacked back-to-back in rows within the shelter. There should be at least 60 cm between facing nest sites, otherwise the bees may have orientation problems (Stephen 1981). The faces of the nest materials should be painted with shapes or symbols ca.10 cm in diameter, to help female bees recognize and find their nest holes quickly (Osgood 1968).
The openings to the bee shelters are oriented south or southeast to take advantage of early morning sun for rapid warming and to provide shade during the hot afternoon hours. Birds and rodents are sometimes attracted to nest shelters where they feed on bee larvae. To deter bird activity, mylar strips that move in the breeze are often hung from field shelters. Chicken-wire screens are effective in preventing bird entry but these can cause injury to bees as they fly to and from nest sites (Stephen 1981). Plastic screens are now also available to exclude these unwanted predators.
Fall and winter management begins with the removal of the bees from the field in September after the seed crop is pollinated. Bees are normally held at ambient temperature for 2-3 weeks after removal from the field to allow immature bees to reach the prepupal stage and spin their cocoons. Some growers prefer to incubate the nest materials for about 1 wk to speed larval development in the fall. Samples of bee cocoons can be examined by means of x-ray at this stage to obtain estimates of mortality (Stephen and Undurraga 1976). They are then placed in cold storage until next season. Cold storage conditions should be 5°C with 40-60% relative humidity. The whole process starts over about 3 wk before bloom, the next spring.
Two common leafcutting bee management schemes are currently in use: (1) the solid wood/phase-out system and (2) the loose-cell system. The solid wood system is the most common practice in the Pacific Northwest with ca. 75% of the seed growers using this method. In the solid wood system, ponderosa pine wood boards (120 by 15 by 7 cm) with »2000 drilled holes per board (5 mm diameter, 65 mm depth), are used as nesting material. The diapausing larvae remain in the wood boards during storage. The x-ray technique is not practical for wood boards, but a screw-in bee board probe is available to sample these boards. Because bees are attracted to previously nested boards (Stephen 1961), boards can be reused again and again. Although desirable in some respects, this reuse of nesting materials promotes an exponential increase in disease and insect enemies from year to year. The elimination or “phasing-out” of used nesting materials (as recommended by Homan et al. 1990) reduces rate of the increase of leafcutting bee enemies. Phase-out emergence traps have been developed which allow the bees to leave, but prevent reentry to old nesting materials. New nest materials are provided outside the phase-out trap to attract the bees to clean nest sites.
The loose-cell system allows better control of many leafcutting bee enemies in the fall and spring, reduces the cold storage space required, and reduces the spread of disease (Bohart 1972, Baird and Bitner 1991). The system does require more labor and equipment, however. In this system, the cocoons are mechanically stripped or punched out of the nest material at the end of the growing season. The loose cocoons are kept in boxes, cans or bags in cold storage. These cells are easily removed because the leaf pieces are held together by the silken cocoons spun by the prepupae. Nest materials must have holes completely through the block so that cells can be punched out, or be made of grooved laminae, which can be taken apart and stripped of cells. Because leafcutting bees do not nest in holes that have both ends open, removable backs must be attached to these materials. Punch-out wood boards and wood laminates, molded polystyrene (laminates or solid blocks) and rolled fluted paper nest materials are available for the loose-cell system. After removal from nest materials, the cells are sifted in a screened tumbler to remove excess leaf pieces and nest-destroying or predacious insects (Richards 1984). The clean cells are then placed into tight-fitting, covered storage containers. In the spring the loose cells are then incubated in enclosed, screened trays at 30°C. Accurate temperature regulation is especially critical in the loose-cell system. The cocoons are much more vulnerable to temperature and humidity extremes because they are less insulated than cells that remain in boards. Undurraga and Stephen (1980) showed that a 0.5 hr exposure to 50°C will cause complete mortality and if pupae are exposed to 45°C, emergence is delayed.
When most adults have emerged, the trays are moved to the field shelters where the bees are released. One difficulty with placing any unemerged bees in the field is that cool or wet weather delays emergence of those bees still in their cells. Several days of cold weather often increases bee mortality. An adult emergence technique, known as the “bleed-off” system has been developed to minimize this difficulty (Stephen 1981). In this system, a chilled room is established adjacent to, or below the incubator. Adult bees are attracted by a light to this room through a small opening. Bees can then be held until conditions are favorable for release. This system allows all bees to complete development in the incubator and it minimizes the movement of bee enemies and pathogens to the field.
Probably the greatest current concern to bee producers is a chalkbrood disease. The pathogen is a fungus, Ascosphaera aggregata Skou (Vandenberg and Stephen 1982). It is similar to honeybee chalkbrood, but is specific to Megachilidae. Infected larvae die before reaching maturity. They become hardened, and appear chalky, cream colored, gray or black (Fig. 6). The disease was first noted in Nevada in 1973 and has since spread to most areas in western North America (Stephen et al. 1981). Because the disease is less common in Canada, most leafcutting bee production now takes place in British Columbia, Saskatchewan, Alberta and Manitoba. While some seed growers in the United States are able to replace their bee stock in a good year, a loss of 50% or greater is more typical. Alfalfa seed growers in the United States buy most of their bees from Canadian sources each year. There are now concerns that chalkbrood may be increasing in Canada. If this disease becomes endemic in Canada, leafcutting bee production could be jeopardized.
Several management practices can reduce the incidence of chalkbrood. Phasing out used nest material is important in the solid wood system to limit spread of chalkbrood. Dipping nest materials in a 3% solution of sodium hypochlorite has been shown to reduce the losses from chalkbrood (Mayer et al. 1988). Used solid wood boards or laminates can be heated in a kiln at 120°C to kill chalkbrood spores (Kish and Stephen 1991). The fumigants paraformaldeyde and methyl bromide have been shown to kill chalkbrood spores and other microorganisms without adverse affects on nesting (Goerzen and Watts 1991, Mayer et al. 1991) and paraformaldehyde has been registered for this use in Canada (Goerzen 1992); however, these chemicals will probably never be registered in the United States for this use. Researchers have also found that certain fungicides, such as Captan, Benlate and carbendazim, can reduce chalkbrood when incorporated into food provisions (Youssef and McManus 1985, Fichter and Stephen 1987, Parker 1988), but these chemicals are not yet registered for this use.
The second most important problem facing leafcutting bee management is losses from parasitism and predation. At least 8 parasitic species and 28 predator or nest destroyer species are known to infest leafcutting bees or their nest materials (Eves et al. 1980, Waters et al. 1980)(Fig. 7). Parasitoids from four genera (Pteromalus, Monodontomerus, Tetrastichus and Melittobia) attack the alfalfa leafcutting bee. They emerge before the bees, usually from about day 8 to day 13 of incubation, and if undisturbed, they will heavily parasitize the developing bees, especially in the loose-cell system.
An ultraviolet light placed over a pan of water on the floor of the incubator will attract and drown most of the parasitoids (Waters 1970). A few drops of liquid soap in the water reduces the surface tension of the water so that when a parasitoid falls in, it is immediately wetted and unable to escape. These light traps show the importance of managing parasitoids. It is not uncommon to find them 1 cm deep on the floor around a trap in a large incubator. Research has shown that dichlorvos resin strips (VaponaR) are effective in controlling parasitoids and safe for bees when used from day 8 to 13 followed by 24 hr of active ventilation (Hill et al. 1984). The repellent N, N-diethyl-m-toluamide (DEET) was also shown to reduce parasitism when loose cells were dipped in dilute solutions or covered with DEET treated vermiculite (Parker 1979).
Adults of one parasitoid, Sapyga pumila Cresson (Sapygidae), were found to spend the night in leafcutting bee holes, but when offered a choice, preferred smaller holes (Torchio 1972), therefore, a night station trap that takes advantage of this behavior was developed. This trap is a hollow plastic pipe which has small (2.5 mm diameter) holes. Adult Sapyga are attracted to these smaller holes at night, but once inside the trap, the wasps are killed by an insecticide that is placed inside.
The checkered flower beetle is a serious predator of leafcutting bee cells. For this predator, a highly effective trap has been developed which uses an aromatic attractant. This trap is capable of killing up to 94% of the emerging beetles (Kious et al. 1978).
The third difficulty is the loss of bees from insecticide poisoning. Bees can be killed by insecticides applied to alfalfa for pest control, especially those sprays that are applied during bloom (Johansen et al. 1983). Bees are also exposed to insecticide residues in nectar, pollen and on leaf pieces. There is also the danger of bee losses when VaponaR strips are used carelessly during incubation. The alfalfa leafcutting bee is more tolerant to most insecticides than the honeybee (Torchio 1973). However, the leafcutting bee has a larger surface-to-volume ratio than the honeybee, allowing it to accumulate a lethal dose more efficiently (Johansen et al. 1983). The ongoing evaluation of insecticides for their toxicity to leafcutting bees, honeybees and alkali bees continues to provide valuable information to seed producers (Johansen et al. 1983, Johansen and Mayer 1990).
The pyrethroid insecticide bifenthrin (CaptureR, FMC Corporation) has recently become available for use on alfalfa seed in the northwest. One pre-bloom application of this pesticide is highly effective against lygus bugs, alfalfa weevil, and aphids. Reductions in the number of spray applications, particularly for lygus, have also reduced cases of bee poisoning. Careful resistance management, including the use of recommended rates only once per season, will be required to prolong the useful life of this pesticide.
Fourth, the second-generation phenomenon in leafcutting bees is an undesirable trait for bees used in the Northwest and Canada. The cause of second-generation emergence is unknown at present. These bees destroy some diapausing bees as they chew their way out of the nest and late hatching second-generation bees also tend to leave the area, because bloom is scarce (Tepedino and Parker 1986). The sex ratio of second-generation bees is also skewed towards females (Tepedino and Parker 1988). Second-generation production thus compounds the male-dominated sex ratio normal in the first-generation. It has been demonstrated that diapause is maternally influenced (Bitner 1976, Parker and Tepedino 1982). This has raised interest in breeding a 100% diapausing strain of leafcutting bees in order to eliminate the second-generation problem. (e.g. Parker and Tepedino, Rank) Multiple generation leafcutting bees would be useful for pollinating alfalfa in California where the season is long (Mueller and Bitner 1991).
An improvement in the proportion of females (e.g. >40%) would be welcome in these bees. Various modifications of nesting materials have failed to improve the sex ratio (Stephen and Osgood 1965, Gerber and Klostermeyer 1972, Rothschild 1979), and breeding for higher percent females has apparently not been successful.
The most recent threat to the alfalfa leafcutting bee is a congener, M. apicalis Spinola. This exotic bee has been found in California, and it appears to be displacing M. rotundata in nest materials there (W.P. Stephen, unpublished data). Unfortunately, this species prefers to collect pollen from the Compositae rather than alfalfa. It is unknown how far this bee will spread and to what extent it will displace M. rotundata. An attempt to breed M. apicalis to improve its preference for alfalfa is being made. Perhaps these selected bees can be established in the Great Basin before the wild type bees reach the region.
The Future
Efficient pollination and the relative ease of managing the alfalfa leafcutting bee has made this bee the preferred pollinator of alfalfa seed in the Pacific Northwest and Canada. In addition, current work suggests the potential for using leafcutting bees in combination with honeybees in California to improve pollination (Mueller and Bitner 1991). Such an expansion in leafcutting bee use would further increase its importance and demand as a pollinator.
In the United States, chalkbrood levels may be kept at low levels (i.e. 10% or less) if growers fastidiously follow current recommendations. This will probably not be enough for growers to avoid buying at least some new bees each year. Of course there is a cost in labor and equipment associated with hygienic management of leafcutting bees. The question remains whether this cost is less than simply buying new Canadian bees each year. In our opinion, seed growers are better off managing their own bees for maximum reproduction so that a minimum of new bees need to be bought each year. Future research may yield innovative management practices to further reduce chalkbrood incidence and selective breeding may produce chalkbrood resistant strains of bees.
The use of an effective, short residual, pre-bloom insecticide only when pests are a problem will keep bee poisoning cases to a minimum. The development of new, more specific insecticides that are safe for bees will also reduce the bee losses. With increased emphasis on sustainable agriculture and integrated pest management, bee poisoning by insecticides should become less frequent.
Finally, research on improved leafcutting bee management practices may yield new chemical, biological or cultural controls for the insect enemies of alfalfa leafcutting bees. Past research has provided continual improvements in leafcutting bee management practices and greatly enhanced pollination success. While the future of the leafcutting bee in alfalfa seed appears sound, it will depend on continued support for research and development for this relatively new pollinator.
Acknowledgments
We appreciate the invitation by Lawrence Connor to write an article summarizing leafcutting bee biology and management. We thank Darrell Bolz for providing information on the economics of alfalfa seed production. We are grateful for the comments of John Vandenberg and Hugh Homan. Scientific paper no. 92711, University of Idaho Experiment Station.
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Figure Captions
Fig. 1. Certified alfalfa seed acreage in 1990 (AOSCA 1990).
Fig. 2. The alfalfa leafcutting bee, Megachile rotundata.
Fig. 3. A completed leafcutting bee cell, approximately 8 mm in length.
Fig. 4. Cocoons of the leafcutting bee cut open to show larvae.
Fig. 5. Estimated alfalfa seed production costs in Idaho for 1991. Custom pollination cost based on 25% of production receipts. (Production receipts estimated at $1360/ha, assuming 535 kg seed/ha at $2.54/kg seed).
Fig. 6. A healthy leafcutting bee larva (left) and one that has developed chalkbrood disease (right).
Fig. 7. Tetrastichus megachilidis, one of the eight species of parasitoids that attack leafcutting bee larvae.