240 YEARBOOK OF AGRICULTURE 196! Varieties developed from such selec- tions tend to remain true to type, and they do not depend very much on ex- ternal agents of pollination. Their Pollination of Seed principal drawback is the loss of hy- brid vigor. The more recent emphasis on inherent yielding capacity has led Crops by Insects to reinvestigation of many self-polli- nated crops to determine the advan- GEORGE E. BOHART AND FRANK E. TODD tages and possibilities of growing hy- brid seed.

WIND, BIRDS, and insects are the IN A PLANT that is adapted for self- main external agents of pollination. pollination, the male spores often are Birds are of little importance to com- transferred from the anther of the plant mercial crops, especially in temperate to its stigma by gravity or movement of regions. parts within the flower. Wind is the principal pollinating If the floral parts are so arranged as agent of corn and other grasses, conif- to prevent or limit sharply the ability erous trees, nut trees (except almonds), of the plant to pollinate itself, the pol- and a scattering of other crops (for len has to be transferred by such ex- example, castorbeans, spinach, and ternal agencies as wdnd and insects in beets). order for fertilization—the union of Most other plants are self-pollinated the sex cells—to take place. or insect pollinated, or both. Peas, Plants adapted for cross-pollina- beans, tomatoes, and lettuce are ex- tion—that is, the transfer of the pollen amples of plants that produce good from a difl'erent plant—depend en- crops of seed without help from ex- tirely on external agencies. ternal agents of pollination. Many plants are adapted for both self- and cross-pollination in diflferent WE GIVE a partial list of seed crops flowers or at difí'ercnt periods in the that require insect pollination or at life cycle of the flower. For such plants, least benefit from it. Not included are cross-pollination usually results in ornamentals, medicináis, and most more seeds of better quality, but auto- spices, many of which are also insect matic self-pollination is insurance that pollinated. The stars indicate crops in some seeds will form even if external which insect pollination usually in- agencies fail. creases yields of seed but is not essen- Plants w^ith specific adaptations for tial for commercial production. automatic self-pollination are almost Leguminous crops: always self-fer tile—that is, the sperms alfalfa *horsebcan (broad- and eggs of the same plant are com- clovers bean) patible. Self-fertility is also present in alsike *lima bean some plants that depend on external crimson Cruciferous crops: white cabbage agencies for pollination. Ladino brussels sprouts Many plants that are adapted pri- red cauliflower marily for cross-pollination are more ^strawberry coUards or less self-sterile. They must have ex- Egyptian broccoli s wee telo vers kohlrabi ternal agents of pollination and ex- yellow kale ternal sources of pollen to produce seeds white turnip in any quantity. Hub am mustard Plant breeders in the past often se- sour rutabaga trefoils radish lected plants for breeding for their self- lespedezas (bush) Chinese cabbage fertile, self-pollinating characteristics. vetch (hairy) rape POLLINATION OF SEED CROPS BY INSECTS 241 Ciicurbitaceous crops: Umbelliferous crops: wide variety of flowers—wider than cucumber carrot any other insect. As an individual on cantaloup parsnip watermelon celery a single foraging trip, it usually con- citron celeriac fines itself to one species of plant. squashes parsley It is large enough and hairy enough pumpkin Solanaceous crops: to accumulate many grains of pollen Malvaceous crops: *eggplant *cotton *pep :)ers and to touch the stigma in most of the *okra Miscellaneous cro{)s: flowers it visits. Because it must store kenaf *flax enough pollen and nectar to carry the Allium crops: sunflower larval and adult bees through the onion buckwheat leek asparagus winter and other dearth periods, it is unusually industrious in its flower- Some plants in the list (onion, for visiting habits. example) produce no seeds without But as one might expect, the honey insects. Others, like cotton, produce bees are not always fully dependable nearly as many seeds without insects pollinators. Because they generally as with them, but varieties within seek the richest sources of pollen and species often vary in this respect. Most nectar, they may neglect the crop are somewhere between these extremes. requiring pollination and go to richer When the production of hybrid seed food sources that are blooming at the is developed for more crops, the list same time. will be increased, because nearly every Honey bees sometimes visit seed crop that is not pollinated by wind crops for nectar alone. On many crops, requires insects for the production of nectar collectors are less efficient pol- hybrid seed. linators than pollen collectors. An Many thousands of kinds of insects individual honey bee usually works visit flowers. Nearly all of them can within a small radius after it starts pollinate at least a few plants. Most foraging. This presents a problem in of them, because of small size, scarcity, the production of hybrid seeds when specialized food habits, or for other a grower wants to use as few pollen reasons, however, are unimportant as parent plants as possible. crop pollinators. Although the honey bee is probably A large variety of insects visit um- the most important pollinator of every belliferous flowers, but even on these crop in our list, other species have a flowers, which are general in their valuable supplementary role. On a pollination adaptations, a small per- few crops, such as alfalfa and cotton, centage of the visitors accomplish most other species of bees may be more of the pollination. efficient individual pollinators. On a The honey bee is by far the most few crops (carrot and squash), other important insect pollinator. Indeed, insects sometimes are more abundant. for commercial crops in temperate regions it is more important than all WILD SPECIES of bees rank next to other species combined. It occupies honey bees as pollinators of seed crops. this preeminent position by virtue of a Relatively few of the many kinds are number of valuable qualifications. sufficiently abundant on seed crops to Because it produces honey and be of great importance. The principal beeswax, the honey bee is maintained advantage of most wild bees is that at a high population level in many they visit flowers primarily for pollen agricultural areas. It can be readily and have no tendency to take the •increased and moved to satisfy polli- nectar without effecting pollination. nation requirements. It is widely Bumble bees usually are excellent adapted and successfully fends for pollinators of the crops they visit^ but itself in most parts of the world. a few of the shorter tongued species As a species, it visits an extremely cut holes in the bases of red clover and 556888°—61- -17 242 YEARBOOK OF AGRICULTURE 1961 vetch flowers and in that way prevent realize the possibilities and take ad- pollination. vantage of them. Wild bees usually have a much It is possible to create artificial nest- narrower range of hosts than honey ing sites when natural soil conditions bees. If the favored host is a crop that are unfavorable. This can be done by requires insect pollination, the reduced installing a plastic film a few feet be- competition from the other plants is low the soil surface to retain the mois- beneficial. ture and placing a layer of gravel Many wild bees ignore cultivated above the film to transport the water crops, preferring a narrow selection of laterally. Water added through verti- native plants. Most wild bees are non- cal pipes leading to the gravel layer social and require large areas of un- rises through the overlying soil to the cultivated land to develop effective ground surface. Finally, enough salt populations. Some solitary bees, like is added to the soil to control vegeta- the alkali bee, however, are gregarious tion and aid in maintaining moisture and can build up immense numbers at the surface. in a few acres of ground. Building and maintaining sites for Insects other than bees are impor- alkali bees is a growing development in tant on only a few of the seed crops in Oregon, Washington, and Idaho. the list. Colonies of bumble bees rarely ex- Flies of various families and sphecid ceed 100 individuals. It is impractical, wasps are important pollinators of um- therefore, to attempt "bumble beekeep- belliferous and AUium crops and to a ing" for pollination, except in enclo- lesser extent of cruciferous crops. Such sures, such as greenhouses. They can insects generally can be considered probably be encouraged best by pro- abundant only when all species are viding them with spring and early fall combined. They are usually most forage (for example, hairy vetch and abundant on small seed fields sur- red clover) and suitable conditions for rounded by uncultivated terrain. nesting. Permanent areas of bunch- grass make good nesting sites for some WILD BEES can be encouraged by species. Rock walls and well-drained providing them with nesting sites, holes in banks are attractive to others. maintaining suitable bloom through- Because bumble bees usually select out their nesting season, and protect- abandoned rodent nests for their domi- ing them from insecticides. ciles, studies should be made to de- In sections where farming is intensive termine the practicability of alter- and wasteland is scarce, it may be im- nately encouraging and controlling possible to increase populations of most certain species of rodents. species of wild bees to a practical level. Leaf-cutting bees are stout, gray- The best opportunities for successful colored bees that often nest in beetle management are with highly gregari- holes in timber or in hollow- or pithy- ous ground-nesting species or ones that stemmed plants. The nurseryman is nest in timber or wooden structures. only too familiar with the circular and Specific studies have been limited to oval pieces these bees cut from the alkali bees, bumble bees, and leaf- leaves of roses and other nursery stock. cutting bees. Although there arc recommended The green-banded alkali bees are the methods for destroying them, the good chief pollinators of alfalfa in the North- they do as pollinators warrants their western States. They concentrate their protection instead. nests in moist, bare, alkaline soils, usu- Leaf-cutting bees in Saskatchewan ally developed from irrigation seepage. have been conserved and increased by In many places where suitable soil growing alfalfa seed in narrow strips conditions can be developed with little through aspen and cottonwood forest difficulty, growers are beginning to land. The bulldozed timber from the POLLINATION OF SEED CROPS BY INSECTS 243 cleared land is piled along the margins ON RED GLOVER, honey bees insert of the surrounding forest so that the their proboscis straight into the flower bees can nest in the burrows made by throat and thus trip the pollination invading beetles. People in Utah and mechanism. They trip the red clover Oregon have observed that any holes flowers readily to obtain pollen, but drilled into the walls of outbuildings they find it difficult to reach the nectar attract many leaf-cutting bees. unless it is high in the corolla tube. The introduction of foreign pollina- Nectar collectors consequently may tors is a step for the future. Most of our avoid a field that is not secreting seed crops were introduced from other heavily or suddenly leave it when the continents. Some of them undoubtedly secretion slows down. are pollinated in their homelands by Studies have been made on the specially adapted species of bees. For value of flowers with short corolla example, none of the many species of tubes and of long-tongued bees, but wild bees that pollinate alfalfa in its without definite results. central Asian homeland occurs on this Various research workers in Russia continent. Some of them would prob- have claimed success with training ably do well here. honey bees to visit the red clover by Investigations have been started in feeding them sirup scented with red Utah and California with a view to clover. Similar attempts in other coun- establishing the feasibility of intro- tries, however, generally have been ducing them into the United States. unsuccessful. The best results with red clover are MOST LEGUME flowers are adapted obtained in the Northwest, where com- for visitation by bees. Honey bees are peting sources of pollen and nectar are the most important pollinators of the not too abundant, and warm, dry con- legumes, but some problems have ditions favor the secretion of nectar. arisen with their use, especially on red Favorable conditions occasionally re- clover, alfalfa, hairy vetch, and Ladino sult in good yields in the Central clover. States, where most of the red clover is For most of the legumes, the pollina- grown. tion problem is simply one of knowing Bumble bees are especially fond of how many bees are needed and then red clover, and most species do an ex- placing enough good colonies in or cellent job of pollinating it. A few next to the fields to achieve the desired short-tongued species cut holes at the density. base of the corolla tube to secure One bee to the square yard appears nectar. Subsequent visitors, including to be enough for most legume crops. At honey bees, learn to search for these that level, the beekeeper can usually holes to the neglect of normal flower make a satisfactory honey crop if nec- visitation. tar secretion is normal. Yields of sweet- When bumble bees were introduced clover seed apparently continue to in- into New Zealand about 60 years ago, crease as bee densities increase to ro satisfactory yields of clover seed were or 15 bees per square yard. Sweet- produced for the first time. One of the clovers yield both pollen and nectar in three species introduced was a hole good amounts, especially in the North- cutter, which partly negated the ad- ern States, and such population den- vantage gained. sities are not incompatible with good crops of honey. HAIRY VETGH presents difißiculties The problem in Ladino clover is one similar to those of red clover. It is at- of low nectar secretion. Beekeepers, tractive to honey bees, but in Texas, therefore, are reluctant to pasture their where much of the seed is grown, bees on it without a substantial nectar secretion is erratic. When the subsidy. nectar is low in the corolla tube, honey 244 YEARBOOK OF AGRICULTURE 1961 becs often use a side approach, thus are usually attractive to honey bees and circumventing pollination. If they use provide good supplies of both nectar the direct approach under the same and pollen. conditions, they usually split the They are visited also by many kinds corolla tube, although this may not be of wild bees and several families of harmful. flies, all of which probably serve as Even if nectar secretion is good, they pollinators. may spend most of their time searching The small domestic acreage of cole for holes previously cut in the corolla crop seed (Brassica olerácea varieties) is bases by bumble bees. Since hairy centered in the Puget Sound area of vetch is highly attractive to many kinds Washington, where honey bees are not of long-tongued wild bees, small fields very abundant but where syrphid and surrounded by wild land are usually muscoid flies abound. well pollinated. Turnip, radish, and mustard seed are grown in sunnier parts of the West- ALFALFA has several unique pollina- ern States and are probably well polli- tion problems. nated by honey bees, although the Honey bees generally do not favor honey produced is not generally fa- alfalfa as a pollen source. When they vored by the beekeepers. In Canada, visit it for nectar, they soon learn to where rape seed is produced, honey visit the flowers from the side to avoid bees are effective pollinators, and rape the pollination mechanism. However, is a good source of nectar for honey nectar collectors accidentally trip a few production. of the blossoms they visit this way. Even nectar collectors can set a good UMBELLIFEROUS CROPS are self-fertile. crop of seed if sufficiently abundant on Automatic self-pollination is not ex- the field for a long time. tensive, however, since it must take Honey bees in North America tend place between rather than within flo- to pollinate alfalfa more efificiently rets. Their umbels attract a wide va- toward the Southwest. In this direc- riety of insects and are almost equally tion, the percentage of pollen collectors attractive to bees, wasps, flies, and and the accidental tripping rate of the beetles. Bees and some of the larger, nectar collectors increases. hairier wasps and flies are the most Two or three bees per square yard valuable, although usually not the can do a fine job of pollination in most abundant, pollinators. southern California, but 8 or lo are Honey bees visit carrots for both pol- required in northern Utah. High yields len and nectar, but alfalfa and sweet- of alfalfa seed from pollination by clover offer strong competition. Carrot honey bees are rare in Washington and honey is not considered desirable for Canada. the market, and some beekeepers try Most species of wild bees that visit to avoid it. alfalfa do so for pollen. They therefore pollinate it efíiciently. They usually ALLIUM CROPS are moderately self- are too scarce to be reliable, however. fertile but require insects for either An exception is the alkali bee of the cross- or self-pollination. Northwest, which in favorable areas Onions attract many insects, but is responsible for exceptional yields fewer than carrots, and many of the year after year. smaller and less hairy visitors transfer almost no pollen. CRUCIFEROUS CROPS vary from self- Pollen-collecting honey bees rank sterile to partly self-fertile, but they are among the most efficient pollinators, nearly incapable of pollinating them- but nectar collectors apparently avoid selves. Thus they require insects for the stamens to a large extent. commercial production of seeds. They For good pollination by honey bees, POLLINATION OF SEED CROPS BY INSECTS 245 onions should not be grown adjacent tors, and native varieties of tomato are to a good pollen source, like yellow structurally adapted for cross-polli- sweetclovcr. nation. Male-sterile lines probably could be GuGURBTTAGEOus GROPS require in- used for the production of first-gen- sect pollination. The squashes are na- eration hybrid seed if they were grown tive to America and are pollinated by in the country of origin or if the several kinds of wild bees, besides natural species of pollinators could be honey bees. The best pollinating spe- established in this country. The prob- cies are restricted to squashes and lems involved may be difficult to gourds, but their cfiforts are largely un- overcome, but when we consider that appreciated since they forage only in hybrid tomato seed produced in Cali- the early morning. fornia by hand pollination cost 200 to Cantaloups, watermelons, and cu- 300 dollars a pound in 1961, the effort cumbers are from the Old World, and seems to be justified. are attractive principally to honey bees in this country. The practice of renting THE HONEY BEE can be considered bees for the pollination of cantaloup as a tool in the grower's hands for in- and watermelon is increasing, par- creasing seed production. Natural pol- ticularly in southern California and lination is like natural soil moisture and Arizona. fertility—it is there for the using, but it usually requires supplementation. COTTON, the principal malvaceous "How many colonies do I need?" crop, is self-fertile and largely self- is the question most often asked by pollinating. Some varieties produce as growers of seed. much seed without insects as with A reasonable answer is almost im- them, but others yield up to 25 percent possible to give for growers who have more when they are insect pollinated. little control over the total acreage in There is considerable interest among bloom or the total number of colonies cottongrowers in developing a hybrid within flight range of their fields. seed industry, which, of course, would A more practical question is, "How make insect pollinators mandatory. many bees per square yard do I Honey bees visit cotton flowers for need?" nectar only, but when the flovv^ers are Before the research worker can give not too wide open, the nectar-gather- an answer for the crop in question, ing bees transfer pollen satisfactorily. he must determine four things: The Many of the bees visit extrafloral nec- number of flowers per day requiring taries, and often it is necessary to pollination; the number of visits re- '^saturate" the field with bees to force quired per flower; the working speed enough of them to visit the blossoms. of honey bees; and the seed-setting capacity of the plants. The efficiency THE TOMATO FLOWER is unattractive and abundance of the wild pollinators to most domestic species of pollinators present should be included in the cal- and has structural adaptations to culation of the number of honey bees insure self-pollination. Consequendy needed for a particular field. seed of first-generation hybrid tomato The grower can work toward the is produced by emasculating and hand calculated optimum population on the pollinating the flowers. field by continually increasing the Suitable male-sterile lines of toma- number of colonies, reducing com- toes have been developed, but pollina- peting sources of bloom, and making tors are not attracted to them in his field more attractive to bees. sufficient numbers to set much seed. Some seed crops give ä clear indi- In their Peruvian homeland, tomatoes cation of the adequacy of pollination are attractive to their natural poUina- by the speed with which the flowers 246 YEARBOOK OF AGRICULTURE 1961 wilt. For crops like alfalfa, in which upon cooperation throughout the com- ' flowers appear progressively along the munity. Honey bees range far from their ' stems and racemes, an appearance of colonies—sometimes several miles. If full bloom is a sign of inadequate the neighbors use insecticides careless- pollination. ly, the bees and their pollination serv- Colonies should be distributed ices suffer, to the detriment of both throughout the field if the acreage is parties in the cooperative agreement. large. Although honey bees sometimes If the neighbors raise seed and do fly long distances to forage, most of not enter into cooperative agreements them range within a few hundred feet with beekeepers, they benefit from of their colonies if plenty of attractive pollination provided by the cooperat- bloom is available within that radius. ing parties and provide nothing in Furthermore, if colonies are placed return. That obviously is economi- within the field, flight to other fields cally unsound. is lessened, and the bees conserve time Growers of alfalfa seed in California and energy. Very likely two-tenths of have formed what amounts to pollina- a mile is a good spacing for groups of tion districts, in which the seedgrowers colonies in large seed fields. in an area deal with a pollination co- Strong colonies should be used for ordinator, who contracts for a specific pollination. The percentage of field number of colonies to be placed in the bees available for pollination and the fields according to a predetermined possibility of obtaining surplus honey time and space schedule. Applications are better with strong colonies than of insecticides follow a careful program weak ones. supervised by entomologists working Progressive movement of colonies for the growers under the guidance of into a field insures that some bees will the California Agricultural Experi- be present to pollinate the first flowers ment Station. and reduces the danger of mass orien- This arrangement, which approach- tation to other sources. If the full es the ideal, seems to be working out to number of colonies is brought to the the satisfaction of all concerned. In field when it begins to bloom, many other areas where only a few members of the bees are likely to locate other of the farming community have par- sources of bloom and remain faithful ticipated, little progress has been made to them for long periods. The intro- toward consistently good pollination duction of colonies to the field should by honey bees. be scheduled to the blossoming cycle of the crop. GEORGE E. BOH ART and FRANK E. ToDD are members of the Entomology Re- COOPERATION among beekeepers and search Division^ Agricultural Research secdgrowers and community action are Service. The former is in charge of the Bee the twin keys to the pollination of seed Culture Laboratory at Logan^ Utah^ and crops by honey bees. the latter of the Bee Culture Laboratory at Cooperation by the grower usually Tucson^ AriZ' The research work of these takes the form of providing locations stations is carried on with the cooperation for apiaries and exercising care in the of the agricidtural experiment stations of use of insecticides. Utah and Arizona^ respectively. If the number of colonies needed exceeds that which the beekeeper con- For further reading: Bohart, G. E.: Insect Pollination of Forage siders best from the standpoint of a Legumes. Bee World, volume 41, number 3, honey crop, he will expect supple- pages 57-64. and number 4, pages 85-017. mental income in the form of rental or i960. a percentage of the seed crop. Hambleton, Jas. I.: The Honey Bee as a Pollinating Agent, The Hive and the Honey Bee, The success of a grower's relation- chapter XVII, pages 4Í23-446, edited by Roy ship with the beekeeper usually hinges A. Grout. 1949.