ALFALFA
Medicago sativa L., family LeguminosaeAlfalfa, also known as lucerne in many other countries, is the most important forage crop in the United States, accounting for about half of all the hay produced. More than 27 million acres, mostly in the North Central States, produced 3 tons of hay per acre with a farm value approaching $2 billion in 1969. One-half million acres were also devoted to the production of over 100 millions pounds of alfalfa seed. The seed crop, valued at $40 million, was produced in many States but about three- fourths of it came from 11 Western States. California led in seed production with 96,000 acres and 33 million pounds of seed. Washington and Idaho each produced about 14 million pounds.
Because of the worldwide importance of alfalfa and the unique relationship of its flower structure to its pollination requirements, hundreds of papers have been written that deal with its pollination, probably more than for any other crop. Citing all of these papers here is both unnecessary and impractical. For that reason, the majority of the citations in this publication are written in English, although excellent research on alfalfa has been conducted in many foreign countries, and the results have been published in German, Japanese, Russian, Swedish, and other languages. Some of the key papers that deal with the history, culture, and development of alfalfa, listed chronologically, include: Brand and Westgate (1909), Oakley and Westover (1922), Stewart (1926), Carlson (1932), Westover (1946), Tysdal and Westover (1949), Graber (1950), Pedersen et al. (1959), Taylor et al. (1959), Bolton (1962), and Jones and Pomeroy (1962).
Plant:
Alfalfa is a perennial herbaceous legume that grows from a semiwoody base or crown. The crown sends up many thin, but succulent, leafy multibranched stems 2 to 4 feet high; each stem terminates in a raceme or cluster of 10 to 100 purple, white, or greenish-yellow florets.
When alfalfa is grown for hay the seed is usually drilled or broadcast at the rate of about 10 to 20 lb/acre, and the plants are clipped when the field is in about one-tenth bloom. When grown for seed it is usually planted in rows at the rate of only about 1 lb/acre. Jones and Pomeroy (1962) stated that highest seed yields are obtained from alfalfa planted in 36-inch rows, the plants 12 inches apart in the row. This spacing would require only a few ounces of seed per acre. Frequently, the crop is planted for hay, then climatic, agronomic, or economic conditions cause the grower to leave the crop uncut to develop seed, which usually results in low seed yield. Stands thicker than 100,000 plants per acre are excessive for seed production (Pedersen et al. 1959). The highest yields can be expected from sparse stands that flower during the warmest part of the season, but other factors also affect seed production. Proper agronomic care, sufficient pollination, freedom from harmful insects and diseases, and proper seed-harvesting methods are equally important.
The great demand for alfalfa and other legume seed in the early 1940's, encouraged by a Congressional subsidy for such seed (Enlow 1944), stimulated interest in both increased production of seed and in new cultivars adapted to particular areas and conditions. This interest led to studies on insect pollination and plant breeding and culminated in the development of improved cultivars (Kehr 1959, Hanson et al. 1964b) and stabilized methods of seed production. Search continues for early high-yielding (hay) cultivars resistant to the alfalfa weevil and other pests and diseases.
There are many hardy, semihardy, and nonhardy cultivars of alfalfa; some are certified ( by State agencies), others are proprietary (owned exclusively by private firms), and still others are uncertified. The major cultivars, their history and qualities, were reviewed in detail by Hanson et al. (1960). The breeding that goes into a synthetic cultivar (typical of many crops) is illustrated in the sketch of the wilt-resistant cultivar A- 136 by Kehr (1959) shown in figure 31.
[gfx] FIGURE 31. - Origin, history, and breeding methods used in developing Ranger (A 136), a wilt-resistant alfalfa cultivar. (Reproduced from Kehr 1959.)
Inflorescence:
The l/2-inch-long florets begin opening at the base of the 1-to 4- inch-long alfalfa raceme. A week is usually required for the opening to proceed from the base to the tip of a raceme. A floret may open at any time of day and remains open for about a week if not pollinated, but wilts within a few hours after pollination.
The corolla consists of the standard petal, sometimes considered to be the landing support for bees, two smaller wing petals, and two fused petals called the keel (fig. 32). The keel encloses, under considerable tension, the sexual column, which terminates in the stigma and 10 anthers. The details of the floral characteristics and their modifications were discussed by Graumann and Hanson (1954), Larkin and Graumann (1954), Nielsen (1962), and Pankiw and Bolton (1965).
The color of the corolla varies from purple or violet through various shades of blue, green, yellow, or cream, to white. A scale for visually classifying alfalfa flower color was proposed by Barnes (1972).
The sexual column is normally nonfunctional, unless it is released from the keel. Once released ("tripped"), it does not return to its former position within the keel like the column in most other legumes. After release, if successful fertilization occurs, the ovules in the ovary begin to develop, and a tightly curled pod results. The number of curls, varying from one to five, is determined by the number of ovules that develop into mature seeds. A pod may have a dozen seeds but usually it has fewer, the number depending at least partly on the degree of pollen compatibility. The pod matures and is ready for harvest about a month after pollination.
Tripping:
The release of the sexual column is a phenomenon that has been known for many years. Henslow (1867) described the tripping process, but Cockerell (1899) was apparently the first to use the term "tripping." After much study and observation (Piper et al. 1914, Brink and Cooper 1936), and also much controversy (Carlson 1928, Coffman 1922, Whornham 1936, Pengelly 1953), tripping was proven necessary for profitable seed production (Armstrong and White 1935; Tysdal 1940, 1946; Zaleski 1956). The column is released when the bee, in searching for nectar or pollen, inserts its proboscis into the flower throat and exerts pressure upon the keel petal, causing it to separate (fig. 33). Upon release, the column strikes the standard petal, sometimes striking the underside of the head of the bee first, at times with such force that the bee can extricate its head only after a struggle. When the flower is tripped, the pollen is dusted upon the bee and is then carried to another alfalfa flower. At the same time, pollen brought from another flower is accidentally rubbed upon the stigma and cross- pollination results. In the field, less than 1 percent of the self-tripped flowers produce seed, and most nontripped flowers fail to do so (Cooper and Brink 1940, Tysdal 1946), although from time to time workers - for example, Carlson (1930) - have reported seed set from nontripped flowers.
[gfx] FIGURE 32. - Alfalfa flower longitudinal section, x 20.
FIGURE 33.- Tripped Alfalfa floret and pollen- ladden honey bee.Field Appearance And Seed Set:
When the flowers are tripped as rapidly as they open, racemes can be found with developing seed pods on the lower part, a circlet of one to four open florets in the middle and unopen buds toward the tip. Growers sometimes refer to this as the crop going "from bud to curl," and associate it with the likelihood of a big seed crop. This condition was incorrectly interpreted by Whornham (1936) who believed that the flowers were self- pollinated without coming into flower. Such a field has a brownish cast, in contrast to fields with a "flower-garden" appearance, where each raceme has a large number of open florets but few if any seed pods.
Honey Yield, Nectar Secretion, And Pollen Production:
Vansell (1941 ) showed that some alfalfa cultivars yield more honey than others. Loper and Waller (1970) showed that when several clonal lines of alfalfa were presented in bouquets to honey bees, the bees consistently showed preference for certain ones. Several terpenoid compounds have been identified in alfalfa varieties (Loper et al. 1971, Loper 1972). The significance of these compounds in honey bee behavior is under investigation. Loper et al. (1971) identified one of the aromatic compounds as ocimene. Its true significance in bee attractiveness has not been determined. If an attractant factor can be isolated, its use in the breeding and selection for cultivars with greater attractiveness to pollinators could become quite important.
Alfalfa produces a large amount of nectar, which is highly attractive to many species of bees, and from which honey bees produce excellent crops of high quality honey. Kropacova (1963) estimated that alfalfa produces 416 to 1,933 pounds of nectar per acre. McGregor and Todd (1952*) estimated that 54 to 238 pounds of nectar per acre were produced during a peak flowering day.
When alfalfa is cut for hay just as flowering starts, as is normally practiced, the beekeeper gets little or no alfalfa honey. If the crop is left to produce seed, the amount of nectar available to a colony depends upon the plant density, the competition from other bees, and other environmental and agronomic factors. As a general rule, one strong colony per acre of seed alfalfa should store 50 to 100 pounds honey. When the colonies are in the area at the rate of three per acre they may store little or no surplus honey.
Alfalfa is a poor source of pollen for honey bees. Usually they will collect it only when no other source is available. When honey bees have only alfalfa upon which to forage, the colony strength diminishes rapidly. Alfalfa pollen is relished by many other species of bees including the genera of Bombus, Halictus, Megachile, Melissodes, and Nomia. Numerous observers have reported that honey bees collect alfalfa pollen more freely in the Southwestern and Western States than in the Northeastern States. But whether the higher visitation rate is due to condition of the alfalfa plants, lack of pollen producing competing plants, or both conditions has never been resolved.
Tysdal (1946) estimated that 2 billion flowers per acre of alfalfa were produced in Nebraska. Lesins (1950) calculated that about 200 million flowers per acre were capable of setting pods. At five seeds per pod and 220,000 seeds per pound, this indicates a potential of 5,000 pounds of seed per acre. Pedersen et al. (1956) showed that 46.7 percent of the flowers can produce pods, indicating that a ton of seed per acre is possible.
Pollination Requiremtents:
As previously stated, the alfalfa flower must be tripped if seed is produced. Furthermore, if cross-pollination occurs, the stigma must come into contact with pollen from another plant during the fraction of a second after the stigma is released from the keel, and before it imbeds itself against the standard petal. Tysdal et al. (1942) and Jones and Olson (1943) showed that cross-pollinated flowers not only set more pods than selfed flowers, but they also set more seeds per pod. Moriya et al. (1956) showed that the highest percentage of pods developed from flowers that were pollinated the first day after they opened.
When the rays of the sun are focused through a magnifying glass into a flower, it will trip almost instantly. Also, rough treatment of the flower, for example by a strong wind, will cause some flowers to trip during the warmer part of the day. Knowing this, various growers and researchers have tried heat and other mechanical devices including the dragging of a rope, wire, chain, brush, or roller across the plants to increase the number of flowers tripped (Carlson 1930, Goff 1953, Koperzinskii 1949, Pharis and Unrau 1953). One grower employed a helicopter to fly, a few feet above the plants each afternoon, dragging a broad cloth behind. He hoped the downdraft would cause the flowers to trip and the cloth would hold the pollen in the air around the plants so that when tripping occurred the stigma would come in contact with the pollen. None of these methods proved to be of practical value in increasing seed production, even though Lejeune and Olson (1940) had shown that artificially tripped flowers set a few more seed than nontripped ones.
Of particular significance pertaining to selfed plants was the test by Tysdal et al. (1942) that showed that production of forage from self- pollinated plants decreased rapidly in a few generations to about a third of the former capability. This was further verified by Wilsie (1958). This information means that even if self-pollinated seed could be produced in large amounts, such seed is undesirable for planting use, either for forage or seed production.
Busbice and Wilsie (1966) and numerous others have looked for self- tripping or easily tripped strains, but because of the rapid degeneration of such lines none have been or are likely to become acceptable cultivars. Stevenson and Bolton (1947) left little doubt that self-tripping or self- fertile alfalfa plants are undesirable as a source of breeding material for improving the yield of alfalfa seed. The grower should, therefore, always obtain his planting seed from fields in which every effort possible was made to produce only cross-pollinated seed. Lovell (1924) then prophetically stated: "They can be disproven only by statistical investigations in which it shall be shown that the honey bee trips a large number of flowers . . . in regions where alfalfa produces a large seed crop, and is freely visited by bees for nectar." This test was conducted more than two decades later (Utah Agr. Expt. Sta. 1950).
Even with the need for tripping and cross-pollination established, lack of agreement continues as to the best pollinating agent. Hunter (1899) covered blossoms with cheesecloth and found that no seed were produced. He examined pods one-half mile from an apiary and found 5.6 plump seeds per pod as compared to 3.3 shriveled seeds per pod in a field 25 miles away where there were no honey bees. Cockerell (1899) stated that an alfalfa field in Kansas, supplied with honey bees, produced twice as much seed as a similar field without bees, and the pods were larger. Aicher (1917) gave some credit to wind and various bees, but Hay (1925) concluded that the honey bee was of no practical value in alfalfa seed production. Carlson (1935, 1946) and Carlson and Stewart (1931) associated good seed crops only with low populations of harmful insects. Gray (1925), Engelbert (1931), and SIaden (1918) considered the leafcutter bee or bumble bee beneficial but honey bees of no value in tripping alfalfa flowers. Lovell (1924) agreed with Sladen (1918), stating: "These facts [that honey bees are ineffective] cannot be controverted by hasty assertions of over-ardent defenders of the honey bee who think that because they are often numerous in alfalfa fields they must be valuable pollinators."
Gray (1925) was apparently the first to study the effect of caging flowering alfalfa plants to exclude pollinating insects, and he showed that doing so reduced seed yields. In a limited way, Megee and Kelty (1932) and Dwyer and Allman (1933), also using cages, showed that honey bees are effective pollinators. An editorial note (Bowman 1934) stated, without supporting data, that good seed crops usually result when honey bees work alfalfa freely. Vansell (1928) stated: "The matter of pollination of alfalfa seed crop [in California] does not bother the alfalfa grower, particularly because bee men are anxious to concentrate their bees about alfalfa fields. The set of seed seems satisfactory generally." Jackman (1940) discounted the honey bee, but Pellett (1941) suggested that five colonies of honey bees per acre might produce a full crop of alfalfa seed. Stephens (1942) also indicated that honey bees were of value, and Rudnev (1941) showed that stimulative feeding of colonies caused some increase in storage of pollen by colonies in the vicinity of alfalfa. Stimulative feeding has since been largely abandoned as impractical. Knowles (1943) discounted the value of honey bees but gave credit to leafcutter bees; however, the same year, Hollowell (1943) concluded that increasing honey bees in the alfalfa field "may be of considerable value."
Eventually, wind, self-tripping, or the setting of seed without tripping were less frequently mentioned as pollinating agents of alfalfa, and the controversy settled down to the relative merits of honey bees and wild bees.
POLLINATORS
Honey Bees:
Before 1946, honey bees were attributed a minor role in the production of alfalfa seed, however, studies by means of pollen traps (Hare and Vansell 1946) established that under certain conditions honey bees collect large quantities of alfalfa pollen. Vansell and Todd (1946, 1947) showed that honey bees have an essential role in seed production. The flowers on plants they caged to exclude bees failed to trip or set seed, whereas flowers in cages with bees or in the open set seed abundantly. These men concluded that in Utah the most important alfalfa pollinating bees were honey bees, alkali bees (Nomia spp.), and leafcutter bees (Megachile spp.). Honey bees collecting pollen from alfalfa were differentiated from nectar-collecting bees, which frequently take nectar from the flower without tripping it. Tucker (1956) showed that bees "learn" to avoid tripping flowers but trip 7 to 85 percent of them during the learning process. This points up the importance of having a preponderance of new foragers in the colonies used for alfalfa pollination.
Bohart et al. (Utah Agr. Expt. Sta. 1950) stated: "Alfalfa under most conditions is an attractive source of nectar and suffers little from competition with other plants for visits from nectar collectors. It is not an attractive source of pollen, however, and pollen collectors are apt to neglect it in favor of better sources. Consequently in alfalfa fields nectar collectors nearly always outnumber pollen collectors, in some areas by more than 100 to 1." Pedersen (1953a, b; 1958) showed that nectar secretion of alfalfa influenced its seed production. When large numbers of honey bees are concentrated on alfalfa fields, however, the competing pollen in the area may be exhausted so the bees resort to alfalfa pollen from lack of choice. This was proven in a seed production test on alfalfa grown in replicated open plots and cages of the type designed by Pedersen et al. (1950). In some of the cages, bees were excluded; in others, a colony of honey bees was present (Utah Agr. Expt. Sta. 1950). In this test, with harmful insects controlled by use of DDT, the cages without bees produced only 14 lb/acre, whereas similar cages with bees produced a maximum of 1,018 lb/acre. This, incidentally, was the experiment to prove the value of honey bees that was specifically called for decades earlier by Lovell (1924) after his review of the literature failed to support claims of ardent beekeepers that honey bees increase production of alfalfa seed.
This experiment (Utah Agr. Expt. Sta. 1950) also presented data showing that colonies transported from California to Utah alfalfa fields for honey production affected Utah seed production. A correlation (that was highly significant statistically) was calculated between the number of colonies of honey bees transported into Utah and the alfalfa seed yields per acre in that State. It showed that high seed yields occurred in years when large numbers of colonies were moved in and low seed yields when few colonies arrived.
Before 1947, the beekeeper placed colonies near alfalfa fields to obtain honey crops. Reports on the value of such honey bees to alfalfa were generally unfavorable. Pellett (1941 ) hinted that there was a difference in operating colonies for honey production and for seed production and that probably more seed could be obtained if as many as five colonies per acre were used, but no data were given to support the statement. He also recognized that such a colony concentration would produce no surplus honey for the beekeeper.
Vansell (1951 ) showed the value of a high concentration of honey bees in fields. In 1947, a 95-acre field at Knights Landing, Calif., had 275 colonies distributed in small groups within the field (2.9 colonies per acre), and the grower harvested 560 pounds of recleaned seed. A 200-acre field at Ryer Island, Calif., had three colonies of honey bees per acre placed around the field, and the yield of seed was 550 pounds per acre. In 1949, six colonies per acre were distributed throughout a 132-acre field at Davis, Calif., and the grower harvested 1,120 pounds per acre of thresher-run seed.
According to Whitcombe (1955), in 1948 an alfalfa seed grower at Hemet, Calif., paid a beekeeper $1.40 per colony to place 275 colonies in small groups on three roadways across a 95-acre field. The grower harvested 540 pounds of recleaned seed per acre. The colonies showed no gain in weight while in the alfalfa fields. Previously, beekeepers had paid alfalfa growers (usually with a 60-pound can of honey) for the privilege of setting an apiary near the alfalfa field. The grower at Hemet paid the beekeeper to place the colonies in the field, a gamble that made history in legume seed production although it caused a financial loss to the beekeeper.
These and other convincing data presented by Vansell (1951) proved that alfalfa seed production could be stabilized by using honey bees distributed within the field. He stated, "An especially heavy set was obtained from plants within 100 yards of the colonies." With the grower obtaining 150 or more pounds of alfalfa seed per acre from the service of each bee colony and with the colonies producing little honey for the beekeeper under such conditions, Vansell (1951) concluded: "For pollination service requiring a large number of colonies the seed grower should pay the beekeeper [an amount] at least equal to that [obtainable] from a good honey crop." Todd (1951) urged similar compensation for the services of the bees.
With this basic information, growers and beekeepers cooperated in the rental and use of bees for alfalfa seed production, and the seed industry was stabilized to the benefit of both. Also, dependable use of honey bees made possible the production of various selections and cultivars, which were confined to the breeder's shelf before the insect pollination requirements of alfalfa were understood. It also opened the door to the development of hybrid alfalfa, and in this regard it shows the importance of attempts to find cultivars attractive to bees or special alfalfa-pollinating strains of bees (Boren et al. 1962; Cale 1970, 1971; Clement 1965; Hanson et al. 1964a, Pedersen and Todd 1949; Nye and Mackensen 1965,1968a, b; 1970; Mackensen and Nye 1966,1969).
The rental and placing of many thousands of colonies of honey bees in alfalfa fields became an accepted practice in the early 1950's (Townsend et al. 1956) and has continued to the present. This practice is responsible for producing the bulk of the alfalfa seed (Doull 1967).
Jones (1958) reported that about 75,000 colonies were used per year on legumes in California from 1942 to 1947, but by 1956 the number had risen to 400,000 colonies. Experience and experiments proved that large numbers of colonies distributed uniformly throughout the field produced satisfactory seed crops even though only nectar collecting bees were active in the field (Akerberg and Lesins 1947, 1949; Bieberdorf 1949; Bohart 1957; Linsley and MacSwain 1947; McMahon 1954; Pedersen 1962; and many others). The maximum economic number of bees was never established.
That nectar-collecting bees were contributing to pollination was attested to by the fact that alfalfa pollen was found in the proboscis fossae of such honey bees (Vansell 1955, Grinfeld 1956, Furgala et al. 1960, Kropacova 1964). Levin and Glowska-Konopacka (1963) showed that increasing the numbers of colonies in the groups in the field caused the bees to forage closer to their own hives. Todd (1957*) urged that this type of behavior be exploited by uniformly distributing groups of colonies at 1/l0-mile intervals in the field and creating the competition necessary to force the bees to "shop around" within their foraging area (fig. 34).
The alfalfa pollination fees established by beekeepers in the early 1950's and carried over into the 1970's were not too different from fees for pollination of other crops paid almost half a century ago. At that time, the beekeeper expected his colonies to improve in population or stores while pollinating the crop. Under present agricultural conditions, colonies frequently deteriorate to such an extent that no surplus honey is stored, the population of the colonies is reduced, and some colonies fail to survive the winter.
Unless beekeepers establish fees commensurate with their operating costs, or unless the use of pesticides on or near alfalfa seed fields is replaced by some form of biological control, the prospects of a continued supply of an adequate number of strong honey bee colonies for maximum alfalfa seed set are gloomy to say the least. See "Pesticides and Beekeeping."
[gfx] FIGURE 34. - One of many groups of honey bee colonies placed in large fields of alfalfa grown for seed.
Wild Bees:
The value of wild bees - numerous species in numerous locations - as pollinators of alfalfa has also been reported by scores of researchers (Bohart 1947, 1952*,1958b; Bohart and Knowlton 1952a, b; Burton et al. 1964; Crandall and Tate 1947; Hobbs 1956; Hobbs and Lilly 1954; Medler 1957; Menke 1952a, b, 1954; Pengelly 1958; Stephen 1955, 1959; Tysdal and Westover 1937; Utah Agr. Expt. Sta. 1950; Wilson 1968). (Also see "Wild Bees and Wild Bee Culture.") An advantage suggested for honey bees over the various wild bees, so far as planned pollination is concerned, is that honey bee colonies can be transported when desired, and in appropriate numbers, to the alfalfa fields. Recent studies, however (Bohart 1958a, 1962b), have shown that at least two species of wild bees, the alkali bee and the leafcutter bee, can also be transported and manipulated for the pollination of alfalfa on a commercial scale, and they do an excellent pollination job in some areas. Much credit for our knowledge of these two bees must be attributed to Bohart (1947, 1950, 1952*, 1958b,1962b,1967,1970), Bohart et al. (1955), Hobbs (1956, 1962,1964, 1965,1967), Hobbs and Lilly (1954), Menke (1952a, b, 1954), Stephen (1955, 1961, 1962, 1965), and Utah Agricultural Experiment Station (1950). Bohart (1962a) stated that there might also be other pollinating insects, in foreign countries, superior to any indigenous species and that they might warrant our importing.
Both alkali bees and leafcutter bees are far more efficient, on a bee for bee basis, than honey bees in pollinating alfalfa. Their primary motive in visiting the flowers is to collect pollen to provision the nest for their young, and they show a preference for alfalfa pollen. By contrast, the honey bee, if given a choice, visits the alfalfa flower to collect nectar, which it must have in great abundance to survive, but will visit some other flowers for pollen.
The alkali bee will nest in highly alkaline areas on which little or no plant growth occurs. Growers can prepare such areas for nesting sites (Frick et al. 1960). Once established, a favorable site may produce enormous populations - as many as 20O,000 nests. Although each female builds her own nest in which she may rear about 5 to 20 offspring, the bees are instinctively gregarious; that is, they nest close together, sometimes with as many as 100 nest entrances per square foot. The foraging range of the alkali bee is similar to that of the honey bee. This bee is much less likely to sting people or domestic animals than is the honey bee. It overwinters in the immature stage.
The disadvantage of the alkali bee is that the nesting sites require a year or so to become established, and they cannot be transported from field to field. A special area must be maintained for them (Bohart 195&). Also, because they are affected by the elements, they may not emerge at the right time to pollinate a desired crop. They may be destroyed by flooding, cultivation, pesticides, parasites, predators, or diseases.
The leafcutter bee is also gregarious, but prefers to nest above ground in holes about three-sixteenths inch wide by 2 to 4 inches deep. To utilize this bee, the grower prepares such holes in boards (Stephen 1961, 1962) and places the boards where these bees are abundant and active. The holes are soon filled with nests. The immature bees can then be transported in the boards to other areas as desired. The leafcutter bees do not forage as far afield as honey bees, so the boards must be distributed at close intervals in the alfalfa field. The bees are not aggressive and can be handled without protection from stings.
Leafcutter bees, like honey bees, can be transported and established wherever desired and are quite effective as pollinators of alfalfa as long as the weather conditions are favorable during their active period. After this short active period, the adults die. The immature stages can be stored under refrigeration, then placed in incubation to permit the adults to emerge when desired. Leafcutter bees, again like alkali bees, forage freely on alfalfa pollen, with which they provision their nests. They require nesting holes of a rather specific size and depth and, because these insects are gregarious, many hundreds of nesting holes are more conducive than a few to their nesting in an area. Current methods utilize "nesting boards," timbers about 4 inches by 4 inches by 4 feet, with about 2,000 holes, 1/4 inch by 3 1/2 inches deep, although some boards are fabricated with grooves (Nye and Bohart 1964) that, properly placed, form holes. When these boards are disassembled, the individual leafcutter bee nests can be removed, handled in bulk, and placed in containers in the field where the adults can emerge at the nesting site when desired. Leafcutter bees forage primarily within a few hundred feet of the nest, therefore, are more likely to be of service not only in the field but in the part of the field where they emerge. Their use is quite likely to increase because of their ease of handling, safety from the standpoint of stings, and efficiency as pollinators of alfalfa.
In the pollination of alfalfa, honey bees, leafcutter bees, and alkali bees, alone or in any combination, are of great value. The grower who desires maximum seed production should utilize the best combination of these bees and the best information available concerning them. He should keep in mind, however, that the location of his particular field and general area may determine the proper bee or combination of bees most suitable for him. This decision can only be made if he has a thorough knowledge of the bees, the crop, and the environment.
Pollination Recommendations And Practices:
The alfalfa flower must be tripped and cross-pollinated by insects for maximum production of high-quality seed. The majority of the western alfalfa seed producers now use either honey bees, leafcutter bees, alkali bees, or some combination of the three. Honey bees are usually rented from beekeepers. Leafcutter bees are usually purchased in the pupal stage, either in bulk (1 U.S. gallon contains about 10,000 pupae in cells) or with the cells intact in the prepared holes in boards. The grower usually prepares his own alkali bee bed and cares for it as a perennial holding.
Recommended rates for usage of honey bees vary from 1 to 10 colonies per acre. Jones 9 recommended two colonies per acre, plus one colony for each additional 100 pounds of seed expected in excess of 250 to 500 pounds. Later, Jones (1958) recommended a colony concentration that would provide two to seven nectar collectors per square yard. Todd and Crawford (1962) recommended that they be distributed about 0.1 mile apart in the field. Most growers use two to four colonies. From 2,000 to 3,000 leafcutter bee nests, or 10,000 individual leafcutter bees have been recommended, with a bee shelter and nests on each 4 acres. A well- populated alkali bee bed, 30 by 50 feet for each 40 acres of alfalfa, or 2,000 female alkali bee visitors per acre is recommended. The data supporting these recommendations are surprisingly meager.
Many factors influence the degree to which the grower follows these recommendations. Also, many variables influence the effectiveness of the pollinators in the field. As a result, one field may be adequately pollinated while another, in which the grower tried to follow the same recommended treatment, may suffer from lack of adequate pollinator activity. Such factors as competing plants, pesticides, adverse weather, bee diseases, strength of colony (of honey bees), and agronomic manipulations can alter effectiveness of the pollinators.
When the grower elects to use honey bees, each colony should have a minimum of 800 in2 of healthy brood in all stages and sufficient bees to blanket 15 to 20 combs (Todd and Reed 1970). There should be three to six honey bees per square yard of flowering alfalfa during the more active part of the day, to provide maximum pollination to every bloom. This may mean some colonies should be moved into the field at the beginning of flowering and augment their numbers as flowering progresses. Water for the bees should be within one-quarter mile of any colony, and shade should be provided in warmer areas.
When alkali bees are used, an equivalent of about 40 ft2 of a well- populated nesting site should be provided per acre of alfalfa. The nesting site should be protected from flooding, exposure to pesticides, trampling by livestock, or damage by predators and parasites. In the field, there should be about one bee for each square yard of blooming alfalfa.
When leafcutter bees are used, from one to five boards, bearing about 2,000 nest-filled holes, or 1 to 5 gallons of pupae should be placed for emergence, and nesting holes should be supplied on each 4 acres of alfalfa. The nesting areas should be protected from hot sun, rain or irrigation water, parasites, and predators. There should be one female leafcutter per 5 yd2 of alfalfa flowers (Bohart 1967).
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9 JONES, L. G. FACTORS IN ALFALFA SEED PRODUCTION, INCLUDING WEED CONTROL. Div. Agron., Univ. calif., Davis. File 3.21, 3.061, 5 pp. 1949. [Processed.]LITERATURE CITED:
ATCHER, L. C.
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