Lucerne must be cross-pollinated in order to produce large amounts of good quality seed, and flower tripping by bees is the only practical method of cross-pollinating lucerne. Tripping is irreversible because the stigma lodges in the groove of the standard petal. Cross-pollina- tion occurs when a bee carrying pollen from previous flowers trips the flower. Subsequent bee visitors to the same flower no longer cause cross-pollination.
Honey bees as lucerne pollinators
Honey bees are the primary lucerne pollinator in California, Arizona, and to some extent southern Nevada. Pollen-collecting honey bees that trip flowers make up 20–100% of the pollinator force in these areas.
Nectar collectors contribute a little to lucerne pollination by accidently tripping the flowers; this happens with about 2% of the flowers.
Lucerne seed yields resulting from honey bee pollination in the south-
134 Chapter 15
Fig. 15.1.Flower of lucerne, Medicago sativa. (Source: Darrell Rainey.)
western US are as high as 1000 lb acre21 (1120 kg ha21) (Robinson et al., 1989).
In the northern US and Canada, pollen-collecting honey bees aver- age only 0–1% of the pollinator force, and nectar collectors accidently trip about 0.2–0.3% of the flowers. Honey bees are more effective in certain isolated valleys of northern Nevada, even at high elevations, where there is less competition from other pollen plants.
Honey bees are not acceptable lucerne pollinators in all areas.
They are not recommended in Canada (Reinhardt, 1953) nor in Washington (Johansen, 1974). Their colonies placed near lucerne may repel other bee visitors (Pesenko and Radchenko, 1993).
Colonies used to pollinate lucerne should be strong with a laying queen and eight or more frames covered with bees in a two-storey hive. Colonies must have plenty of brood, especially young brood that stimulates workers to forage for pollen. Colonies are often placed in fields in two waves – the first colonies go in when the lucerne is about one-third in bloom, and the second wave goes in at or just past one- half bloom. Most seed is set within a 300-ft (92 m) radius of a colony.
Colonies should be deposited in groups of 12–18 per location with about 480 ft (146 m) between locations.
Honey bees visit lucerne most readily when flowering plants are slightly moisture stressed rather than when the plants are freshly irri- gated or too dry. If entire blocks are irrigated on the same schedule, at any one time the entire block may be unattractive to honey bees, that is, either too wet or too dry, and bees will learn to visit competing bloom. This problem can be minimized by irrigating sections of a block in an alternate fashion so that bees always have access to attrac- tive lucerne.
Watering barrels should be placed near hives to keep bees from visiting distant sources of water; if bees must travel far to get water, fewer of them are free to pollinate the crop. Two big barrels of water for each 45 acres (18.2 ha) is recommended, and they must be refilled as needed. Water barrels should have some kind of float on the surface to keep bees from drowning.
Honey bees often learn to avoid visiting lucerne, apparently in response to the experience of receiving a blow to the head when the flower is tripped. Such learned avoidance does not occur with alkali bees or leafcutting bees. However, honey bees respond well to breed- ing programmes aimed at overcoming this liability. Nye and Mackenson (1968, 1970) selected a strain of honey bee that preferen- tially collected lucerne pollen; however, the programme was aban- doned. In later work, three generations of selection yielded a ‘high pollen-hoarding strain’ of honey bee that when placed near lucerne stored 2.4 times more pollen than did a ‘low-strain’ (Gordon et al., 1995). Although the ‘high-strain’ did not prefer lucerne pollen, its
Alfalfa (Lucerne) Seed 135
overall higher pollen collection rate probably enhanced lucerne polli- nation because pollen-collecting bees are generally better pollinators.
Unfortunately, today there are no commercially-available stocks of honey bee selected for high pollen-hoarding behaviour.
Alkali bees as lucerne pollinators
Alkali bees (Chapter 9) are widely used in the Touchet area of Washington and in Nevada. A 1 acre (0.4 ha) alkali bee bed with an average of 1 million nesting females (2.5 million ha21), provides excellent pollination for 200 acres (81 ha) of lucerne grown for seed.
Although these native bees are excellent pollinators of commercial lucerne in western North America, in recent years they have been largely supplanted by alfalfa leafcutting bees.
Leafcutting bees as lucerne pollinators
Alfalfa leafcutting bees (Chapter 11) are the most important lucerne pollinators in the northwestern US, Canada, and to some extent California. Each female can pollinate enough lucerne blossoms to set
!flb (0.1 kg) of seed. Most growers aim for 20,000 bees (7000 female bees) acre21 or 50,000 bees (18,000 females) ha21. Seven thousand females per acre can set a field in 10–14 days under ideal conditions.
Bees are sold while they are dormant in cells, either by the gallon or in wooden nest boards. In spite of its name, the alfalfa leafcutting bee is often highly attracted to other crop plants, including white sweet clover and purple loosestrife (Small et al., 1997).
Non-managed bees as lucerne pollinators
Lucerne is attractive to a large number of bee species, many of which doubtless provide valuable pollination services. In 20 lucerne planta- tions in Canada (16 near Ottawa, Ontario; 4 in the Peace River region), the largest number of bee visitors were leafcutting bees (Megachile spp.), followed by bumble bees. Honey bees were the fifth-most fre- quent lucerne visitors, and no alkali bees were found (Brookes et al., 1994). The researchers avoided sampling managed leafcutting bees (M.
rotundata) by sampling bees no nearer than ~300 ft (90 m) from artifi- cial bee shelters; managed honey bees were never nearer than 1.2 miles (2 km) at every sampling site. Lucerne was not the most attrac- tive bee plant in the sampled areas; there were three other plant species more heavily visited. However, lucerne seemed to benefit from
136 Chapter 15
the presence of other blooming plants that attracted pollinators. Bee visitation was much heavier in lucerne growing within 33 ft (10 m) of blooming wildflowers than it was in the middle of large lucerne fields. Seed yield at one site pollinated only by wild bees was 214 lb acre21 (240 kg ha21) which is within the range expected for non-irri- gated fields.
Alfalfa (Lucerne) Seed 137
Table 15.1. Recommended bee densities for lucerne.
No. of honey bee hives acre21(ha21) Reference
3–6 (7.4–14.8) Todd and Vansell (1952) 2–4 (4.9–9.9) Vansell and Todd (1946);
Hobbs and Lilly (1955); Bohart (1957);
McGregor (1976, 1981); Crane and Walker (1984); Levin (1986);
Berg (1991)
1–3.2 (2.5–8) Williams (1994)
3.2 (7.9) Literature average
Other measures and bees
2–7 honey bee nectar foragers yd22
(2.4–8.4 m22) Jones (1958)
3000 alkali bees acre21(7410 ha21) Mayer and Lunden (1993) 1 leafcutting bee 5 yd22(1.2 5 m22) Bohart (1967)
20,000–50,000 leafcutting bees acre21
(50,000–123,500 ha21) Strickler et al. (1996) 20,000 leafcutting bee cells acre21
(50,000 ha21) Kevan (1988)
6000–16,000 leafcutting bee cells acre21
(15,000–40,000 ha21) Scott-Dupree et al. (1995)
Flowering
The almond (Prunus dulcis) flower has one pistil with one ovary containing two ovules, 10–30 stamens, and five pinkish-white petals. Usually only one ovule develops into a fruit. Nectar is ex- creted into a floral cup at the base of the pistil (Fig. 16.1). The plant blooms from late January to late March, and bees readily visit the flowers for pollen and nectar. Spain is the principal almond pro- ducer in Europe. In the US, practically all commercial almond is grown in California.