CACAO
Theobroma cacao L., family SterculiaceaeCocoa is the processed product derived from the beans of the cacao plant.
World production of cocoa exceeds a million tons, with Ghana producing 429,000 tons; Nigeria, 201,000 tons; Ivory Coast, 105,000 tons; Cameroon, 73,000 tons; Brazil, 94,000 tons; and Equador, 35,000 tons, with other countries of North and South America, Africa, Asia, and Oceania producing the balance. Of this amount, the United States consumes 25 percent; Germany, 13 percent; United Kingdom, 10 percent; and the Netherlands, 9 percent ( Purseglove 1968*). Europe, as a whole, takes over 50 percent and the American countries, about 40 percent of the entire crop.
Plant:
The evergreen cacao tree grows 15 to 25 feet primarily between latitudes 10 deg N to 10 deg S, usually below 1,000 feet in altitude, and in areas with a monthly average rainfall of about 4 inches. Various cultivars, propagated by seed, are grown. The oblong or oval fruit (fig. 58), commonly called a pod, is 4 to 12 inches long, and green when immature, but may be yellow, red, purple, or green when ripe. It contains afrom 20 to 60 reddish-brown beans 3/4 to 1/2 by 1/2 to 1 inch in size, usually arranged in five rows (fig. 59). Pods are produced throughout the year, but the main harvest usually begins at the end of the wet season and may extend for 3 months. From 7 to 14 pods will produce a pound of dry beans. Yeilds range from 200 to 3,000 pounds dry beans per acre, but 600 lb/acre is considdered a good yield (Purseglove 1968).
Inflorescence:
The cacao flowers arise in groups directly from old wood of the main stem or older branches at points which were originally leaf axils (fig. 60). Each flower has five prominent pink sepals, five smaller yellowish petals, each of which forms a pouch, an outer whorl of five staminodes, and an inner whorl of five double stamens, each stamen bearing up to four anthers. The staminodes are about as tall to twice as tall as the upright style and form a "fence" around the style. The stamens are curled so that the anthers develop inside the petal pouches. The ovary consists of five united carpels each having four to 12 locules, and one style that has several linear stigmatic lobes (van Hall 1932). According to Cheeseman (1932) and Urquhart (1961), the flower produces no nectar and has no discernible scent. However, Stejskal (1969) stated that there are two types of microscopic nectaries, ( 1 ) the cylindrical multicellular ones, 60 to 450 microns in size, on the pedicels, sepals, and ovaries, and (2) the conical unicellar ones 20 to 25 microns in size, located on the "guide lines" of the petals and on the staminodia. He showed that they secrete nectar, which has an odor that attracts male mosquitoes and lepidopterous insects.
The flower opens about dawn, and the anthers dehisce just before sunrise. The stigma is usually pollinated 2 to 3 hours later but is receptive from sunrise to sunset of the day of opening (Cheeseman 1932). The stigma is receptive to pollen along its whole length, and not merely at the apex as in most flowers. If the flower is not pollinated, it usually sheds the following day (Sumner 1962). Pollination before noon is best (Chats 1953).
[gfx]
FIGURE 58.- Maturing cacao fruit on the tree.
FIGURE 59.- Ripe cacao fruit opened to show the beans.
FIGURE 60.- Cacao flower cluster growing on the trunk of the tree, showing the open flower, a flower ready to open, and a small fruit.Pollination Requirements:
Although the full story of cacao pollination is not yet known, there seems little doubt that the flower is not self-pollinating, as flowers bagged to exclude insects invariably shed (Gnanaratnam 1954). Also, some plants are self-incompatible but set fruit well if pollinated with pollen from compatible trees (Chats 1953, Cope 1958, Knight and Rogers 1955). The method of the transfer of the pollen in nature is the somewhat questionable factor. The sticky pollen is not carried by the wind. Furthermore, it is produced and released in the petal pouches where wind is unlikely to disturb it (Cobley 1966*, Gnanaratnam 1954). Glendenning (1962) noted that pollen found on a stigma was usually from more than one flower, but the amount of foreign pollen depended on proximity to other plants. Little pollen seemed to move more than a couple of trees' distance.
Pollinators:
There is general belief that small insects are the primary pollinating agents of cacao, but no general agreement as to which insects are responsible. Numerous authorities credit midges, especially Forcipomyia quasiingrami Macfie and Lasiohela nana Macfie (Barroga 1964, Chatt 1953, Fontanilla-Barroga 1965, Macfie 1944, Saunders 1959, Toxopeus 1969). Others credit ants (Crematogaster spp.), aphids (Aphis gossypii Glover and Toxoptera spp.), thrips (Frankliniella parvula Hood), and unidentified wild bees (Billes 1941; Cope 1940; Harland 1925a, b; Hernandez 1966; Jones 1912; Muntzing 1947; Posnette 1942a, b, 1944, 1950; Posnette and Entwistle 1957; Urquhart 1961; Voelcker 1940).
Thrips and aphids move about but slightly from tree to tree, yet Glendenning (1958) reported, after a study of albino trees, that a considerable proportion of pollination takes place across two intervening trees, though less than over shorter distances. This would indicate an agent with considerable movement between trees.
The ants Wasmannia suropunctata (Roger) and Solenopsis geminata (F.) and the wild bee Trigona jaty Smith were occasional visitors. Glendenning (1958) concluded that the midges (Forcipomyia spp.) were the main pollinators, accounting for twice the pollination service performed by all of the other species combined. This was verified in various experiments with different numbers of insects per cage over cacao flowers. Hernandez (1965) reported pollination percentages ranging from 1 to 52 percent when he used midges, bees, thrips, and ants. However, Hernandez did not, indicate how pollination was accomplished.
Although midges seem to get the most credit as pollinators of cacao, there is clearly a lack of knowledge as to which insects are responsible in the different areas for the commercial set of fruit of this important crop.
Harland (1925a) found that of 5 percent of the flowers on trees not infested by ants and aphids, only 0.3 percent set fruit; whereas, on trees heavily infested by these insects, 35 percent of the flowers were pollinated and 2 percent set. At the same time, 5 percent of the hand pollinated flowers set fruit.
Little has been said about the adequacy of pollination of the individual flower or the minimum number of seed in relation to fruit set or shedding. However, at least as many pollen grains must fall upon the stigma as there are subsequently developed seeds. Thus, a minimum of 60 pollen grains is necessary to set the highest number of seed.
Many of the flowers are never pollinated (Harland 1925b), at least under Trinidad conditions. Apparently, wherever the crop is grown the lack of adequate pollination is a strongly limiting factor in production of the beans. Sumner (1962) stated that most of the pollination occurs 2 to 3 hours after dawn with a second much smaller peak in the afternoon, but only 2 to 5 percent of the flowers ever get pollinated, and these may not set if pollinated too late or with incompatible pollen. Urquhart (1961) stated that only about 5 percent of the stigmas ever get pollinated; Harland (1925b) found only 9 percent to be pollinated. Because some plants are self-incompatible - some are male sterile or sterile (Gnanaratnam 1954) - many of the flowers would appear to be doomed to shed. Knoke and Saunders (1966) tried a mist blower for mechanical transfer of pollen but achieved uneconomical success.
The use of honey bees under saturated pollination conditions has never been tried, probably because the blossom has no aroma and produces no nectar. Quite conceivably, however, honey bee colonies could be concentrated in numbers sufficient to exhaust the supply of pollen and nectar on competing plants and the bees induced to visit the flowers of this important crop for pollen and increase the percentage of cross- pollination and fruit set. A search for a selection of cacao pollen-loving honey bees might produce an acceptable and controllable pollinating agent. One or more of the various species of pollen-foraging wild bees might be found that could be controlled and used as a profitable pollinating agent of cacao.
Pollination Recommendations and Practices:
There are no recommendations on the use or manipulation of insect pollinators of cacao. According to Faegri and van der Pijl (1966*), the Forcipomyia spp in Africa breed mainly in decaying pods. If the pods are removed by too scrupulous cleaning of the plantations, these midges might also be removed. This would result in deficient pollination of the flowers. Otherwise, the presence or numbers of insect pollinators are left entirely to chance on this billion-dollar crop.
LITERATURE CITED:
BARROGA S. F.
1964. PROGRESS REPORT ON THE STUDY OF INSECTS, PARTICULARLY MIDGES ASSOCIATED WITH POLLINATION OF THEOBROMA CACAO, APRIL 1963. Philippine Jour. Plant Indus. 29(3/4): 123 - 133.BILLES, D. J.
1941. POLLINATION OF THEOBROMA CACAO L. IN TRINIDAD, B.W.L Trop. Agr. [Trinidad] 18: 151-156.CHATT, E. M.
1953. COCOA. 302 pp. Interscience Publishers Inc., New York.CHEESEMAN, E. E.
1932. THE ECONOMIC BOTANY OF CACAO. A CRITICAL SURVEY OF THE LITERATURE TO THE END OF 1930. Trop. Agr. [Trinidad] Sup., v. 9, June, 16 pp.COPE, F. W.
1940. AGENTS OF POLLINATION IN CACAO. St. Augustine, Trinidad, Imperial College of Tropical Agr. [Trinidad], Ninth Ann. Rpt. on Cacao Res. 1939: 13-19.______ 1958. INCOMPATIBILITY IN THEOBROMA CACAO. Nature 181: 279.
FONTANILLA-BARROGA, S.
1965. A PROGRESS REPORT ON THE STUDY OF INSECTS ASSOCIATED WITH POLLINATION OF THEOBROMA CACAO WITH SPECIAL EMPHASIS ON MIDGES. Philippine Jour. Agr. 27(3/4): 147-159.GLENDINNING, D. R.
1958. PLANT BREEDING AND SELECTION. Cocoa Res. Inst. Rpt. of West Africa, 1957-58, pp. 50-54.______ 1962. NATURAL POLLINATION OF COCOA. Nature 193(4822): 1305.
GNANARATNAM, J. K.
1954. POLLINATION MECHANISM OF THE CACAO FLOWER. Trop. Agr. [Ceylon] 110: 98 - 104.HALL, C. J. J. VAN.
1932. CACAO Ed. 2, 514 pp. Macmillan, London.HARLAND, S. C.
1925a. STUDIES IN CACAO. THE METHOD OF POLLINATION. Ninth West Indian Agr. Conf. Proc. Kingston, Jamaica, 1924: 61 - 69.______ 1925b. STUDIES IN CACAO. PART I. THE METHOD OF POLLINATION. Ann. Appl. Biol. 12: 403-409.
HERNANDEZ, B. J.
1965. INSECT POLLINATION OF CACAO (THEOBROMA CACAO L.) IN COSTA RICA. 173 pp. Ph.D. thesis and Diss. Abs. 28(1): 2B-3B, 1967, AA-257/71, Wis, Univ., Madison.JONES, G. A.
1912. THE STRUCTURE AND POLLINATION OF THE CACAO FLOWER. West Indian Bull 12: 347 - 350.KNIGHT, R., and ROGERS, H. H.
1955. INCOMPATIBILITY IN THEOBROMA CACAO. Heredity 9: 69 - 77.KNOKE J. K., and SAUNDERS, J. L.
1966. INDUCED FRUIT SET OF THEOBROMA CACAO BY MISTBLOWER APPLICATIONS OF INSECTICIDES. Jour. Econ. Ent. 59: 1427-1430.MACFIE, J. W. S.
1944. CERATOPOGONIDAE COLLECTED IN TRINIDAD FROM CACAO FLOWERS. Bul.:Ent. Res. [England] 35: 297 - 300.MUNTZING, A.
1947. SOME OBSERVATIONS ON POLLINATION AND FRUIT-SETTING IN ECUADORIAN CACAO. Hereditas 33: 397 - 404.POSNETTE, A. F.
1924a. NATURAL POLLINATION OF COCOA, THEOBROMA LEIOCARPA, ON THE GOLD COAST. Trop. Agr. [Trinidad] 19: 12-16.______ 1942b. NATURAL POLLINATION OF COCOA. THEOBROMA LEIOCARPA, BERN., ON THE GOLD COAST II. Trop. Agr. [Trinidad] 19(10): 188-191.
______ 1944. POLLINATION OF CACAO IN TRINIDAD. Trop. Agr. [Trinidad] 21(6): 115-118.
______ 1950. THE POLLINATION OF CACAO IN THE GOLD COAST. Jour. Hort. Sci. 25: 155 - 168.
______and ENTWISTLE, H. M. 1957. THE POLLINATION OF COCOA FLOWERS. Rpt. Cocoa Conf. Grosvenor House, London, Sept. 10-12, pp. 66-69. (Abs.) Plant Breeding 28(4): 4550. Oct. 1958.
SAUNDERS, L. G.
1959. METHODS FOR STUDYING FORCIPOMYIA MIDGES, WITH SPECIAL REFERENCE TO CACAO-POLLINATING SPECIES (DIPTERA, CERATOPOGONIDAE). Canad. Jour. Zool. 37: 33-51.STEJSKAL, M.
1969. [NECTAR AND AROMA OF THE CACAO FLOWER.] Oriente Agropecuario 1(2): 75-92. [In Spanish, English summary.]SUMNER, H. M.
1962. [COCOA] POLLINATION. In Wills, J. B., ed., Agriculture and Land Use in Ghana, pp. 260 - 261. Oxford University Press, London, Accra, New York.TOXOPEUS, H.
1969. CACAO. In Ferwerda, F. P., and Wit, F., eds., Outlines of Perennial Crop Breeding in the Tropics, pp. 79-109. H. Veenman and Zonen, N. V. Wageningen, The Netherlands.URQUHART. D. H.
1961. COCOA. Ed. 2, 293 pp. Longmans, Green & Co., Ltd., London.VOELCKER, O. J.
1940. THE DEGREE OF CROSS-POLLINATION IN CACAO IN NIGERIA. Trop. Agr. [Trinidad] 17: 184-186.
