Onion Pollination As Affected by Different Levels of Pollinator Activity
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Utah State University DigitalCommons@USU All PIRU Publications Pollinating Insects Research Unit 10-1970 Onion Pollination as Affected by Different Levels of Pollinator Activity George E. Bohart Utah State University William P. Nye Utah State University L. R. Hawthorn Follow this and additional works at: https://digitalcommons.usu.edu/piru_pubs Part of the Entomology Commons Recommended Citation Bohart, G. E., W. P. Nye, and L. R. Hawthorn. 1970. Onion Pollination as Affected by Different Levels of Pollinator Activity. Utah Agr. Exp. Sta. Bull. 482, figs., tables. This Bulletin is brought to you for free and open access by the Pollinating Insects Research Unit at DigitalCommons@USU. It has been accepted for inclusion in All PIRU Publications by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. ONION POLLINATION- AS AFFECTED Utan State University BY DIFFERENT LEVELS 'OF POLLINA1:0R ACTIV G. E. Bohart, W. P. Nye and L. R. Hawthorn Bulletin 482 - October, 1970 Utah Agricultural Experiment G. E. BOHART and W. P. NYE are with the Entomology Research Division, Agricultural Research Service, USDA, logan, Utah 84321. 1. R. HAWTHORN was formerly with the Crops Research Division, Agricul tural Research Service, USDA, Logan. He is presently at 113 Wiggins Street, West Lafayette, Indiana 47906. / - ACKNOWLEDGl\t!ENTS Weare indebted to Mr. John R. Pease now at the University of Utah Medical School in Salt Lake City; Dr. Charles 1. Graham, now at the U. S. Army Biological Laboratories at Fort Dietrick, Maryland; and Mr. Ross A. Nielsen, now at the Southeastern Bee Research Laboratory at Baton Rouge, Louisiana, for making many field counts and measurements during the course of these studies. Dr. Lynn Kerr, now at the Charter Seed Company at Twin Falls, Idaho provided most of the plot management and could be counted on when needed. Many of the insects collected on the plots were identified by specialists, including the following: Hymenoptera Aculeata (except Apoidea) - R. M. Bohart Apoidea - G. E. Bohart Braconidae - P. March Chalcidiodta - B. D. Burks Diptera Bombiliidae - J. C. Hall Stratiomyidae - W. J. Hanson Syrphidae - H. S. Telford Therevidae - 1. Knutson Muscoidea, Chloropidae - C. Sabrosky Otitidae, Tephritidae - G. Steyskal 1 CONTENTS ACKNOWLEDGEMENTS 1 INTRODUCTION 3 1958 4 Methods and Materials 4 Conditions Affecting the Experiment . 6 Results · 11 1959 • 14 Methods and Materials · 14 Conditions Affecting the Experiment . · 15 Results • 17 1960 • 21 Methods and Materials · 21 Conditions Affecting the Experiment . · 22 Results • 28 1961 • 29 Methods and Materials • 29 Conditions Affecting the Experiment . · 30 Results · 34 DISCUSSION AND CONCLUSIONS · 39 SUMMARY · 45 LITERATURE CITED .46 APPENDIX A . • 48 2 ONION POIJJNATION AS AFFECTED BY DIFFERENT LEVELS OF POIJJNATOR ACTMTY G. E. Bohart, W. P. Nye, and L. R. Hawthorn According to Jones (1937), the pollination of onion (Allium cepa Linnaeus) is effected mainly by insects that visit the nectaries at the base of the three inner stamens. Muller (1833) and Jones (1937) stated that cross-pollination is the rule, although self-pollination between flowers of the same umbel is undoubtedly of frequent occurrence. Muller (1883) and Hayward (1938) reported that the flowers are imperfectly proterandrous the anthers of the inner whorl of stamens dehisce before those of the outer. Both whorls dehisce before the stigma is receptive. Moll (1953) stated that the normal receptive period of the stigma is 3 days. Several investigators have noted that onion flowers are visited freely by many kinds of insects, including honey bees. (Apis melli/era Linnaeus) (Muller, 1883; Jones and Emsweller, 1934; Shaw et. at. 1935; Kordakova, 1956). For this reason, it has often been taken for granted that pollination is not a limiting factor in onion seed production in spite of the tremendous variation known to occur in yields from different fields, seasons, and cul tural practices (Morse, 1923; Hawthorn, 1951). The recent development of hybrid seed grown on male-sterile inbreds pollinated by male-fertile inbreds has intensified the pollination problem. Pollinating insects are required to disseminate pollen over 6 to 12 or more rows from two pollen parent rows Complicating problems are the supposed relative unattractiveness of male sterile plants to pollen-collecting insects and the possibility that their receptive period will not coincide with the pollen dehiscence period of the pollen parents. Another problem, and one particular to the Treasure Valley of Idaho and Oregon (the most important onion seed producing area in the U. S.) has been the great increase in alfalfa seed production, resulting in thousands of acres of alfalfa bloom which drain off populations of honey bees and other pollinators. Finally, increased use of insecticides has reduced the populations of pollinators. In Treasure Valley, the wide spread use of Dylox (sometimes called Dipterex) for alfalfa insect control has probably had a particularly disastrous impact on dipterous pollinators. 3 In 1958, we started a 4-year program to learn more about the pollin ation factor as it affects onion seed production. In the following discus sion, the methods, conditions, and results for each year will be treated separately, and a general discussion will follow. Handling of the plots and methods of measurement were the same from year to year unless otherwise stated. 1958 Methods and Materials Location White farm, northeast bench, Logan, Utah. Treatments The three treatments (randomized within the row of plots) were as follows: (1) Plots caged to admit only tiny insects; (2) plots without cages (open pollination); and (3) plots caged to enclose a small (four-frame) colony of honey bees. The cages (Pedersen et aI., 1950) were 21.5 feet long, 11 feet wide, 6 feet high, and covered with 12-mesh clear lumite screen. As soon as flowering began in late June, the cages were erected and the hives of bees installed. Plot layout In 1958, there were 12 plots that measured 10 x 20 feet arranged in a single row. Each plot contained five rows, each planted with 40 onion bulbs. The rows were 2 feet apart, and the plants were spaced ab()ut 6 inches apart in the rows. The center row was plante:! with U-16-3-10- 2B, a male-fertile inbred line. The other four rows were planted with the following three male-sterile inbreds: B-1900A, B-2147A, and B-2267A. Two rows of the B-2267 A were planted in each plot and the positions of all the male-sterile rows were randomized. More than one male-sterile line was used merely because we were unable to obtain enough bulbs of a single line. When the bulbs were obtained, we anticipated that male steril ity would be complete, but at least a small amount of pollen was present in all of the lines. Irrigation and insect control The plots were irrigated once each week throughout the growing season. 4 The onion maggot [(Hylemya antiqua (Meigen)] was controlled by the application of Dieldrin in the furrow at the time the bulbs were plantl~d. Onion thrips (Thrips tabaci Lindeman) were controlled with a foliage ap plication of 10-percent DDT dust just prior to spfitting of the sheath on the umbel. No insecticides were applied after blossoming started. Eriophyid mites were not recognized as a potential problem, and, hence, no attempt was made to determine their presence or to control them. Measurements 1. In 1957, in the course of a series of experiments on carrot pollination (Hawthorn et aI., 1960), the effect of the cages on air movement, air tem perature, and light intensity at the flower level, and on temperature at the soil mrface, was studied in detail. In 1958, we made another series of readings in our onion pollination studies to find the effect of the cages on relative humidity and soil surface temperature. 2. Nectar sugar concentration in the flowers was measured on July 18 on all plots (except those excluding large insects) by reading the sugar concentration of the nectar in the honey stomachs of bees taken directly from the flowers. A Bausch Lomb'*' low rang.e (0-60 percent) hand re fractometer was used for this purpose. 3. Flowering was measured 12 times throughout the season on the basis of the number of flower heads on which at least half of the flowers had opened. The duration of bloom was not measured. 4. On the mornings and afternoons of July 18 and 21, the relative numbers of nectar-collecting and pollen-collecting honey bees were meas ured in the cages with bees enclosed. 5. Counts of each species of pollinator were made on each row in the open plots. Four counts were made on each of 12 days th:o:.I.shout the season. The ob~erver 's route from plot to plot and row to row was changed for each observation. Tiny flies were difficult to see and their p~pulations were undoubtedly understated by the counts. Populations of tiny flies in the cages excluding bees were cnly evaluated in general terms. 6. A pollination efficiency rating for each pollinator was assigned by estimating the quantity of loose pollen grains fro.:n a representative sample and combining this factor with the size, hairiness, and a:tivity pattern of the insect. This type of rating, used previously br carrot pollin ators (Bohart and Nye, 1960), is admittedly subjective, but whea it is mul tiplied by the populations to give a pollination index, it should give a truer 5 picture than population figures alone. In the case of honey bees, separate ratings were given to nectar and pollen collectors, and the overall rating for the species was based upon the proportion of the two activity patterns taking place. 7. The seed heads were dried in September and threshed on a small rubbing board.