Prior to the discovery and exploration of the New World in 1492 the cherimoya probably had been established in nearby subtropical zones close to it endemic center in the highlands of Ecuador and Peru. Documented accounts of subsequent movement and establishment of the species in Florida in 1838 and later its appearance in California in 1871, in Hawaii somewhat earlier than 1886 in New Zealand and Australia about 1890 and possibly it was in Israel and Egypt about 1900. While the earlier accounts of this attractive fruit were directed primarily to the description of the nature of the fruit and its unusual, appealing flavour, the problem of poor or limited yields possibly was first mentioned by W.J. Wester in 1910. He was investigating the introduction of the cherimoya into American horticulture in Puerto Rico and Florida. It was noted that the plant grew normally and produced flowers, but few fruits resulted from natural pollination. Frequently there was a crop failure. Wester demonstrated possibly for the first time that there was a condition of dichogamy exhibited by the cherimoya flower and that transfer by hand of freshly shed pollen to receptive flowers did indeed induce fruit set. He also reasoned from these early observations that natural insect or other possible pollinating agents were absent from these trees far removed from their endemic center of origin. Somewhat later in 1924 Orange I. Clark in California made extensive observations on a large collection of cherimoya seedling trees in San Diego County. Clark concluded it was the absence of insect visitors which resulted in the failure of fruit to set regularly on otherwise well growing, healthy trees. Clark transferred pollen by hand which resulted in a good fruit set. He then announced the apparent absence of natural pollinating insects in the area, but did not pursue the problem. In 1930, Ahmed in Egypt pointed out the effectiveness of hand pollination to induce fruit set in the sugar apple Annona squamosa in that country.
In 1941, Schroeder's observations on a collection of cherimoya cultivars in California indicated a wide variation in the fruit set each year often among trees of the same cultivar. The problem of dichogamy was noted. Pollen transferred by hand induced and increased fruit production markedly. This hand pollination procedure was rapidly adopted by the growers. The results from these early hand pollinations stimulated the interest of growers and the subsequent expansion of the young developing industry in California. The successful application of hand pollination for cherimoya was noted by visitors from other countries. Subsequently the method was tried by growers in Chile about 1945, in New Zealand about 1970 and in Spain about 1975 where it has become a standard practice to maintain the cherimoya industries in those countries. Hand pollination of the atemoya and the cherimoya in Australia now has been demonstrated to be economically feasible.
While pollen transfer by hand is laborious and expensive, it is nevertheless economically feasible as it does allow almost complete control by the grower of the number of fruits set, the specific location of the fruit on the tree, the time of fruit set and subsequent season of maturity. Moreover control of fruit quality is also controlled by application of appropriate amounts of pollen to limit fruit size in those cultivars which tend to produce very large fruits that tend to split at maturity. Conversely increase in fruit size can be induced in some instances where inadequate otherwise natural pollination produces small, irregularly shaped fruits. Symmetry of fruit form which in part determines fruit quality is also controllable with hand pollination. Cherimoya fruits which develop by natural means are often so small in size and frequently of such irregular form that they must be separated from first class fruit and discarded or sold as culls.
Trees producing seedless cherimoya fruits are reported from time to time, but these observations are seldom if ever verified or repeated. Seedless fruits can be induced by the repeated application of gibberellic acid (Yonemoto-1995), but the investment of time and labor to produce a given fruit and the questionable fruit quality which results makes this method probably not feasible and uneconomical at this time. The idea of mechanical transfer of pollen within a given flower by vibration of the flower at time of pollen shed has been suggested and demonstrated on a limited scale. Extensive field observations in California and Spain and in others where the cherimoya has been introduced indicated that the domesticated honey bee is efficient in gathering pollen, but does not transfer this pollen to other, receptive cherimoya flowers. The bee will collect the entire pollen mass from flowers in their second stage of dichogamy and return directly to the hive. The bee, therefore, is not a pollinator of the cherimoya. The role of the common fruit fly, Drosophila melanogaster, as a pollinating agent for cherimoya has been suspected, but caged branches exposed to great numbers of these flies set very few fruits. Wester and others have suggested that possibly the ubiquitous flower thrips might be responsible for pollen transfer in the cherimoya, but this fact has never been carefully established. The common house fly is seldom seen as a visitor to the cherimoya.
The presence of natural insect pollinators for cherimoya trees in their native habitat has been mentioned and discussed occasionally in the literature, but careful observations and reports on these insects are lacking. Extensive field studies by Gottsberger in South America on several genera and species of the Annonaceae including some species of Annona, indicated that a number of species of (Coleoptera) beetles probably "are the exclusive or most prominent insect group responsible for pollen transfer" in this botanical family. The species Annona cherimola Miller was not included in these investigations. Although specific beetles, covered with pollen, have been observed and photographed within the flowers in these field studies by Gottsberger, there are no data to indicate that the specific flower was observed at a later date to verify that it had subsequently developed into a fruit. Assumptions were made that the presence of the beetle induced fruit set. Control flowers, protected from possible beetle or other insect visitation, were not observed to determine whether they could set fruit by some means other than beetle pollination.
A series of observations and reports during the past several years from several cherimoya growing areas of the world areas have verified that possibly several species of beetles and perhaps other insect groups as well can become adapted to assist in the pollination of the cherimoya and the atemoya. The extensive observations on Annona flowers in Israel (Gazit, et al) of insect visitors and experimentally confined, high density population of Nitidulid beetles on these flowers indicated the important role of insects in fruit set compared to flowers protected from insect visitors. Four species including the confused sap beetle, the dried fruit beetle and the pineapple beetle appeared to be effective pollinators in Israel. Similar observations by George, et al in Australia describe the effective pollination response of the dry fruit beetle in fruit set in both the atemoya and the cherimoya.
During recent years another pollinating insect has been associated with fruit set in the cherimoya in Spain. Again circumstantial evidence indicates that the presence of the pirate bug Orius spp.,(Hemiptera) in cherimoya flowers probably has been responsible for unusually greater fruit set. in cherimoya trees adjacent to rows of maize (Zea mays), on which Orius completes its life cycle. Nearby trees produced more fruits than trees at a distance from the rows of maize. The amount of naturally set fruit per tree declined with the distance from the maize. Orius spp. is a predator on thrips hence thrips might be suspect as the basic pollinator. However it was noted that the Orius beetle was covered with pollen when observed in the cherimoya flower a condition not necessarily associated with the ubiquitous thrips.
The practice of hand pollination in the commercial cherimoya orchards in California has been considered a somewhat bothersome yet economically feasible orchard management procedure to insure good yields of high quality fruit. Occasionally some trees will set a few or sometimes several fruits without the aid of hand pollination but the yields and fruit quality from these sporadic fruits have been undependable. During the past seasons of 1993-94 and 1994-95 and in the present flowering season of 1995-96 it has been noted that an unusually large numbers of fruits have set without hand pollination in certain communities where the cherimoya is grown commercially. This unusual behaviour is especially in evidence by the fact that the fruits have set high in the trees above the line normally reached by the hand pollinator who pollinate only those flowers accessible from the ground. Ladders are not normally used in these pollination procedures. It has been determined that a possible factor in this unusual fruit set is the presence of a "Rove" beetle (Staphylinidae) in many of the flowers. This small dark beetle about 1-1/2 mm long is found in the late afternoon in flowers which are starting to shed their pollen. Upon bending the petal to observe the pollen shedding anthers, the beetle darts rapidly into the air and disappears, a common behavioral response of rove beetles in general. This beetle has been recovered from nearby rotting fruits. Its life cycle and possible alternate hosts in California has not been investigated carefully. Again the effectiveness of the rove beetle in cherimoya pollination is entirely circumstantial at the present time. Controlled exclusion from flowers and forced foraging observations of the rove beetle have not been done.
Anderson, Peter and A. Richardson. 1995. Giving Nature a Hand. Calif. Cherimoya Assoc. Newsletter, Spring 1995:4-5.
Ahmed, M.S. 1936. The Annonas in Egypt. Booklet No. 14. Min. of Agri. Cairo.
George, A.P., R.J. Nissen, D.A. Ironside and P. Anderson. 1989. Effects of Nitidulid Beetles on Pollination and Fruit Set of Annona spp .Hybrids. Scientia Horticulturae 39:289-299.
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Gazit, S., I. Galon and H. Podoler. 1982. The role of Nitidulid beetles in natural pollination of Annona in Israel. Jour. Amer. Soc.Hort.Sci. 107:849-852.
Nadal, Hannah 1990. Beetle pollination of sugar apples and atemoyas. Tropical Fruit News / May 1990. pp. 4-5.
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Yonemoto, J.Y. 1995. Production of seedless cherimoya. Calif. Cherimoya Assoc.Newsletter (Spring 1995).:7-12.