UNIVERSITY OF CALI FORNI A PUBLICATION S COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE COMMON HONEY BEE AS AN AGENT IN PRUNE POLLINATION BY A. H. HENDRICKSON Instructor in Pomology BULLETIN No. 274 December, 1916 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1916 Benjamin Ide Wheeler, President of the University. EXPERIMENT STATION STAFF HEADS of divisions Thomas Forsyth Hunt, Director. Edward J. Wickson, Horticulture (Emeritus). Herbert J. Webber, Director Citrus Experiment Station; Plant Breeding. Hubert E. Van Norman, Vice-Director; Dairy Management. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Robert H. Loughridge, Soil Chemistry and Physics (Emeritus). Charles W. Woodworth, Entomology. Ralph E. Smith, Plant Pathology. J. Eliot Coit, Citriculture. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Enology. Warren T. Clarke, Agricultural Extension. John S. Burd, Agricultural Chemistry. Charles B. Lipman, Soil Chemistry and Bacteriology. Clarence M. Haring, Veterinary Science and Bacteriology. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. James T. Barrett, Plant Pathology. Fritz W. Woll, Animal Nutrition. A. V. Stubenrauch, Pomology. Walter Mulford, Forestry. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. D. T. Mason, Forestry. J. B. Davidson, Agricultural Engineering. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. William G. Hummel, Agricultural Education. John E. Dougherty, Poultry Husbandry. S. S. Rogers, Olericulture. Frank Adams, Irrigation Practice. H. S. Baird, Dairy Industry. David N. Morgan, Assistant to the Director. Mrs. D. L. Bunnell, Librarian. Division of Pomology A. V. Stubenrauch A. H. Hendrickson W. L. Howard W. P. Tufts R. H. Taylor Richard Schmidt W. L. Sweet THE COMMON HONEY BEE AS AN AGENT IN PRUNE POLLINATION* BY A. H. HENDRICKSON During the past three years the Division of Pomology of the University of California, and the Horticultural Commissioner of Santa Clara County have been conducting a number of experiments in an attempt to solve some of the problems connected with the pollination of plums and prunes. A part of this work was done at the University Farm at Davis and a part in the Santa Clara Valley. These experi- ments were undertaken to endeavor to find out why some plums bore abundant crops under certain conditions, while the same varieties under different conditions were either sterile or produced light crops. The question of what effect the interplanting of certain varieties had upon the size of the resulting crops also seemed to be of utmost im- portance, especially in view of the repeated inquiries concerning this factor which have been received at this office. As a result of these investigations, the Division of Pomology has accumulated a mass of data which may help to elucidate some of the problems now confronting the plum and prune growers of this state. Among the most interesting results obtained during the season of 1916 were those relating to the common honey bee as a pollen- carrying agent. The experiment demonstrated to all who saw it that the honey bee is one of the most important agencies in transferring pollen from tree to tree. In the Santa Clara Valley, where this part of the experiment was carried on, there are a large number of orchards of mixed varieties. Wherever the French prune is planted next to either the Sugar or Imperial prunes, there seemed to be a difference of opinion as to whether or not the size of the crop was in any way affected by the * The following bulletin is a report on part of the pollination project which has been carried on by the University of California for the past three years. It is presented with the idea of bringing before the prune growers of the state the importance of the common honey bee as a factor, not to be over- looked, in the problems involved in prune pollination. Other reports will be presented from time to time as data warrant. 128 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION interplanting. Some growers maintain that larger crops of French prunes are produced on trees planted in close proximity to the Imperial or Sugar varieties, and vice versa. Others state that no difference was noticeable, and no benefits could be seen as a result of interplanting varieties for purposes of cross-pollination. It was noticed that at the height of the blossoming period there seemed to be a scarcity of insects, especially bees, which are generally credited with being responsible for the transfer of pollen from one flower to another. In some cases the writer worked for days among the blossoms in several large prune orchards without seeing or hearing a bee. The scarcity of other insects which might have worked in the flowers was also very noticeable. In most orchard sections bees can be found working upon fruit trees during the blossoming season, as it is at this time that the supply of nectar is most plentiful. The opposite seemed to be true in certain sections of the Santa Clara Valley. Although the weather was favorable for bees to work among the blossoms, being warm and with but little wind, some of the orchards in the vicinity of Meridian seemed to attract practically no bees at all. The Santa Clara Valley is largely planted to prunes, principally the French or Agen variety, with smaller interplantings here and there of the Sugar and Imperial varieties. These three varieties blossom at nearly the same time and should theoretically pollenize one another effectively. Because of the conflicting reports of the value of inter- planting varieties and the noticeable lack of bees, it was decided to find out in an experimental way what effect a large supply of bees would have upon the setting of fruit on some typical French and Imperial prune trees. The experiment was planned in such a way as to have the bees as nearly under the control of the experimenter as possible. Two pairs of adjoining French and Imperial prune trees, as nearly as possible of the same age and size, were chosen for the experiment. Over each pair was constructed a tent of white mosquito bar. The tents were approximately 40 feet long, 25 feet wide, and 16 feet high, and constructed as nearly insect-proof as possible. Aside from the fact that the force of the wind was broken by this arrangement, the trees under the tents were under approximately the same conditions as the remainder of the trees in the orchard. Both tents were com- pleted before any of the blossoms opened. They were taken down after the blossoms had reached a stage where there was no further danger of being pollinated from outside sources. As soon as 25 per cent of the blossoms had opened, a hive of bees was placed under one tent and kept there throughout the blossoming period (about five days PRUNE POLLINATION STUDIES 129 in the season of 1916). All insects were excluded from the second tent, and i careful watch maintained to see that none entered. The bees under tent number 1 started to work almost immediately and did not seem to object to being imprisoned in the tent or try to escape. They seemed to prefer the flowers of the French prune to those of the Imperial, and seemed to devote most of their attention to the former variety. The hive was kept under the tent until it appeared that all of the blossoms had opened and it could reasonably be sup- posed that nearly every blossom had been visited by one or more of the bees. The bees were then removed. The tents were taken down after the petals of the blossoms had fallen and there was no danger of pollen being brought from outside sources to the trees in the experi- ment. As practically all of the trees in the orchard blossomed approx- imately at the same time as the trees under the tents, it is reasonable to suppose that no pollen reached the trees under experiment, after the tents had been removed. Results were apparent as soon as it was possible to tell how much fruit had set. It was evident that the French prune tree under the tent with the bees had set a much heavier crop than that of the best trees exposed to normal conditions. The French prune tree under the tent from which bees and all other insects were excluded, had set a very light crop (fig. 1). For some unaccountable reason, the Imperial tree under the tent with the bees set only a light crop of fruit. 1 The Imperial tree under the tent from which all insects were excluded set but a few scattering fruits. All trees were carefully watched throughout the season. At an early date it became necessary to prop the French prune tree which had been under the tent with the bees, to prevent serious damage to the tree by breaking the limbs (fig. 2). The difference in the amount of fruit set by the French prune which had been under the tent with the bees and the amount borne on the adjoining trees, all in the same state of thrift and having been given the same care, was easily appar- ent to even the casual observer. Approximately one thousand blossoms were counted and tagged on each tree. The record of the fruit resulting from these blossoms was kept, and from these results the percentage of fruit to set and mature was calculated. The following table gives the percentages of fruit that actually matured : i This part of the experiment will be repeated in order to determine, if possible, why the Imperial prune did not respond to cross-pollination in a similar way to the French prune. 130 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION No. of blossoms in count. April 11, '16 French prune tree under tent with bees 1069 French prune tree under tent from which bees were excluded 1058 French prunes, average orchard set 9991 Imperial prune tree under tent with bees 1060 Imperial prune tree under tent from which bees were excluded 1050 Imperial prune tree, average orchard set 2180 No. of fruits that matured. Aug. 1, '16 Per cent 193 18.05 11 1.04 359 3.59 18 1.69 157 7.20 Fig. 1. — French prune tree which had been enclosed in a tent of mosquito bar from which insects were excluded during the blossoming period, graph taken August 1, 1916. Compare with Fig. 4. Photo- A glance at the table shows plainly that the amount of fruit set per one hundred blossoms on the French prune was much greater on the tree under the tent with the bees than on the trees under normal conditions. Only a very small percentage of fruit matured on the tree which had been covered by the tent from which all insects were excluded. In the case of the Imperial prunes, the tree under the tent with the bees set only a light percentage of fruit for reasons which are not as yet understood. The tree which was enclosed in the tent from which all insects were excluded set no fruit. PRUNE POLLINATION STUDIES 131 The results would seem to warrant the following conclusions : 1. That the French prune may be aided in setting a satisfactory crop by the presence of a large number of bees in the orchard during the blossoming period. 2. Without the aid of bees or other insects, the set of fruit on the French prune is often light. 3. The Imperial prune does not seem to be able to set fruit unless pollinated by insects with pollen from other trees. Fig. 2. — French prune tree which had been enclosed in a tent of mosquito bar in which a hive of bees was kept during the blossoming period. Photograph taken August 1, 1916. It should be stated that, although the crop of prunes on the tree which was under the tent with the bees was much heavier than that on the tree under the tent from which bees were excluded, the size of the prunes was smaller. The prunes were also smaller than the average on the trees not in the experiment. This smaller size was due, no doubt, to the exceptionally heavy crop that the tree was compelled to mature. The introduction of a number of hives of bees into an orchard would not necessarily mean that as heavy a crop of prunes would be produced as resulted with the tree in the experiment under 132 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION the tent. It is only reasonable to suppose that the bees flying some distance from the hives would have less effect on the setting of fruit in the orchard as a whole, than did those whose efforts were confined to but two trees. Since the results of the experiment have become known, consider- able discussion has arisen as to the best method of testing out the work of bees in the various sections. Some growers are preparing to purchase a number of swarms of bees to have them permanently in their orchards; others expect to try to keep the bees in the foothill sections for most of the year, bringing them into the orchards during the blossoming period only ; while others expect to try the plan adopted in parts of southern California, where bee-men having large numbers of swarms are hired to place their bees for a certain length of time in the orange groves. It seems to be the consensus of opinion that the best work would result if the bees were brought in from some outside district and scattered about the orchards, about one hive to the acre, during the blossoming period, and then removed. Some of the growers are at present giving the latter plan serious consideration. The results of this experiment, while interesting, do not necessarily solve all of the problems involved in pollination, but serve to throw some light on this one side of the question. It is the purpose of this division to repeat this experiment in whole or in part and in con- nection with other experiments now in progress, the station may be able to give further light on the question from time to time.