Alfalfa leafcutting bees readily accept artificial nesting tunnels made from a variety of materials. Taking advantage of this behaviour, growers and researchers have developed practical methods for mass- rearing leafcutting bees. Management is based upon providing nesting holes and shelters in seed fields, protecting bees during their dormancy, and activating dormant bees in time to pollinate the crop.
Appendix 1 includes a list of vendors that sell leafcutting bees and rearing supplies.
Nesting materials and shelters
Bees must be provided nesting materials and some kind of artificial tunnel in which to nest. Nesting materials and the bees must, in turn, be provided a field shelter to protect bees from weather and provide them an orientation landmark while they provision their nests.
110 Chapter 11
Fig. 11.3. Solid nesting board for alfalfa leafcutting bees. (Source: Daniel F.
Mayer.)
Here follows a summary of some of the most common nesting materials (Peterson et al., 1992):
• Solid board (Fig. 11.3) – a solid wooden board, 47!f 3 6 3 2#f in (120 3 15 3 7 cm), with ~2000 drilled holes per board, each hole
&dsin diameter and 2%kin deep (5 mm diameter 365 mm deep) (see
page 113).
• Removable-back solid board – similar to above except the back can be removed to punch out cells (see page 115).
• Laminated grooved board (Fig. 11.4) – a pair of wood or plastic boards with opposing grooves that form tunnels when strapped together. These nests can be taken apart to remove cells and sanitize the boards (see page 115).
• Polystyrene nest board (Fig. 11.4) – similar to laminated grooved boards, but lighter and less expensive (see page 115).
• Paper nest board – like laminates or polystyrene nest boards, but designed to be used only once.
Nesting materials and bees must be housed in proper field shelters to ensure good bee activity and propagation. Most shelters have three sides, a roof, and a floor (Fig. 11.5), but designs are variable and include modified tractor trailers and school buses. The roof should have a 12–18 in (30–46 cm) overhang to protect nest boards from direct sunlight. Some growers use an awning to further protect nest boards from sun. There should be a 4–6 in (10–15 cm) gap between
Alfalfa Leafcutting Bees 111
Fig. 11.4. White polystyrene laminate nesting boards (rear left), polystyrene block (rear right), wood laminate nesting boards (front). (Source: Rhéal Lafrenière.)
the top of the sides and the roof to let hot air escape from inside the shelter. Shelters should be painted yellow, blue, or green and marked with various geometric symbols to help bees orientate to the shelter (Richards, 1996). Bees orientate best to large objects in a field and work most efficiently when they are housed in large shelters; Stephen (1981) recommends shelters 20 ft long and 10 ft high (6 3 3 m). It is convenient to build shelters on trailers so that they can be moved to other late-blooming fields or away from insecticide applications. If there is a problem with birds preying on bees, the fronts of shelters can be covered with a 2 in (5 cm) mesh wire screen. A smaller mesh size may injure the bees as they fly through the screen.
Shelters should be large enough to accommodate at least 60,000–80,000 nest holes (Stephen, 1981). Shelters with 30,000–90,000 holes can provide enough female bees for 2–6 acres of seed (0.8–2.4 ha), but their small size increases the tendency for bees to drift, especially if there are larger shelters nearby. Large shelters with 150,000–750,000 nesting holes can provide for 10–50 acres (4–20 ha), but they are costly to build and may increase problems with bee parasites and diseases.
Nest boards are placed inside the shelter, stacked back-to-back in rows. There should be at least 24 in (60 cm) between facing nest boards; leafcutting bees must be able to fly freely within the shelter.
The faces of nest boards are painted with various shapes and symbols about 4 in (10 cm) in diameter to help bees orientate to their own particular nest holes. Nest boards should not touch the shelter’s siding or ceiling as these surfaces tend to get very hot.
112 Chapter 11
Fig. 11.5. Field shelter for alfalfa leafcutting bee nesting boards. (Source: Rhéal Lafrenière.)
Shelters in most regions should face slightly north of east so that the nests are not in direct sunlight after 10 a.m. Some growers attach an awning to the front of the shelter so it can be faced south or southeast to increase interior ventilation by prevailing winds. Small shelters should not be placed next to large, trailer-size shelters because the bees tend to drift to the larger shelters.
Cold storage and incubation
Cells of immature leafcutting bees should be kept in cold storage for most of the year and then carefully incubated to bring about bee emergence in time for the crop bloom.
Nesting materials should be removed from fields in mid-August to September and held at ambient temperatures for 2–3 weeks. This gives the bee larvae time to moult into prepupae and spin cocoons.
Cocoons are then stored at 41°F (5°C) and 40–60% relative humidity until the following spring.
In spring, the cells are switched from cold storage to incubation at 86°F (30°C); this process must begin about 21 days before the anticipated beginning of crop bloom. If the weather turns cool and crop bloom is delayed, the emergence of bees can be delayed by lowering the incubation temperature to 59–68°F (15–20°C) during the 15th to 19th day of incubation (Rank and Goerzen, 1982). Bees are then put in field shelters, and it is important to do this only on warm, still days. If the temperature is cold the bees will be sluggish and birds may eat them, and if it is windy the bees will have trouble orientating to their nests.
Solid wood/phaseout rearing system
This is the most common method used in the western US. Diapausing larvae are kept in their solid wood nest boards during cold storage, and nest boards can be reused over and over (see 111). The biggest disadvantage is that reused nesting material can accumulate diseased spores and insect enemies over time. Hence, old nest boards should be phased out every other year; this happens when bees are returned to field shelters at the start of crop bloom.
To phase out old boards, the boards are first switched from cold storage into incubation 21 days before bloom as usual. When the boards are taken to the field, they are placed in phaseout boxesnext to the field shelters. A phaseout box can be any kind of large enclosure (a tractor trailer works well) with numerous slits ~2 3 6 in (5 3 15 cm) in the walls. The field shelters are fitted out with newly- sanitized, empty nest boards. As bees emerge from their nest boards in
Alfalfa Leafcutting Bees 113
the phaseout box, they are attracted to light, fly out through the slits, and begin nesting in the clean boards in the nearby shelter. The old nest boards are then reamed out with a mechanical corkscrew device to remove nest debris. Boards are then sanitized either by kilning them at 260–300°F (127–149°C) for 24 h (Stephen, 1982), by dipping them in calcium hypochlorite (bleach) (Mayer et al., 1988a), or by fumigating them with paraformaldehyde (Mayer et al., 1991).
Boards full of bees but not yet scheduled for phaseout are placed directly in the field shelters from incubation. The grower must provide 1–1!sempty sanitized boards in the shelter for every full board to allow for population expansion.
Because polystyrene or paper nest boards are inexpensive (see page 111), many growers use them in phaseout systems as they would solid wood nest boards. However, polystyrene or paper boards were designed for loose-cell systems, and bee emergence is delayed when they are incubated in these nest materials. Therefore, polystyrene or paper nests are not advisable for phaseout systems (Peterson et al., 1994).
114 Chapter 11
Fig. 11.6. Loose cells are incubated in enclosed screen trays. (Source: Daniel F.
Mayer.)
Loose-cell rearing system
Cells are removed from nest boards with this system and stored and incubated loosely in trays. Emerged bees or mature cells are released in field shelters in spring and provided with clean, empty nest boards for nesting. This method allows for better control of bee parasites and nest destroyers, reduces space needs during cold storage, and reduces spread of disease. However, it is labour intensive.
The loose-cell method requires a removable-back solid nest board or a grooved laminate board (see page 111). The nest boards are opened and the cells punched or stripped out after the larvae have moulted to prepupae at the end of the nesting season. Cells are then sifted through a screened tumbler in order to remove loose leaf pieces, chalkbrood-infected cells, and many of the insect enemies. Cells are put in large, covered containers and placed in cold storage.
In spring the loose cells are incubated in enclosed screen trays (Fig. 11.6) at 86°F (30°C). Each tray is 2 32 or 2 33 ft (61 361 or 61 3 91 cm) and holds about 2 gal (7.6 l) of cells. Incubation rooms can be fitted with ‘bread racks’ to hold large numbers of trays. There should be about 1!s in (3.8 cm) between trays to allow for air circulation. It is important to maintain temperatures carefully because loose cells are more vulnerable to temperature extremes than are cells insulated in nest boards. By days 21–24 the first males, and maybe some females, are starting to emerge and the trays are ready to go to the field shelters.
But first, one must determine how many gallons of bees to put in each field shelter. One gallon (3.8 l) holds about 10,500 leafcutting bee cells, one-third of which will contain females (3465 females gal21; 915 females l21). There need to be about three vacant nest holes for every female bee (it is not necessary to provide nest holes for males). Using the formulae below, we find that a shelter that holds 80,000 nest holes (~40 boards of 2000 holes each) can accommodate ~8 gal or 30 l of cells.
This equates to about four trays. So, a good guideline is to provide five empty nest boards per gallon of cells (1.3 boards per litre of cells).
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Required gallons of cells = no. holes per shelter
3 ¸3465
Required litres of cells =no. holes per shelter
3 ¸915
or
The 80,000-hole shelter we used in the example with ~27,720 females (8 gal of cells 3 3465 females gal21) and a lower target population of 5000 females acre21 (see page 109) is good for pollinating about 5 acres.
Before trays and empty nest boards are put in the field shelters, the empty nest boards must be sanitized by kilning them at 260–300°F (127–149°C) for 24 h. It is also a good idea to spray the field shelters with a 3–6% hypochlorite (bleach) solution to help control chalkbrood disease. Trays are placed inside shelters in the shade, and the bees are released by removing the top screen. Bees will mate and females will begin nesting in the nest boards, and trays will be empty of live bees within a week or so. If the nesting rate is high the grower may need to add more nest boards to the shelter. It may be necessary to move the shelter to another blooming field if the field finishes blooming before the last bee cells are provisioned.