Insect Pollinators Frequenting Strawberry Blossoms and the Effect of Honey Bees on Yeild and Fruit Quality

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1 Utah State University All PIRU Publications Pollinating Insects Research Unit -97 Insect Pollinators Frequenting Strawberry Blossoms and the Effect of Honey Bees on Yeild and Fruit Quality William P. Nye Utah State University J. LaMar Anderson Utah State University Follow this and additional works at: Recommended Citation Nye, W. P., and J. L. Anderson. 97. Insect Pollinators Frequenting Strawberry Blossoms and the Effect of Honey Bees on Yeild and Fruit Quality. J. Am. Soc. Hort. Sci. 99():0-. This Article is brought to you for free and open access by the Pollinating Insects Research Unit at It has been accepted for inclusion in All PIRU Publications by an authorized administrator of For more information, please contact

2 Reprinted from Journal of the American Society for Horticultural Science Vol. 99, No., January 97 Insect Pollinator~ Frequenting Strawberry Blossoms and the Effect of Honey Bees on Yield and Fruit Quality! William P. Nye and J. LaMar Anderson Agricultural Research Service, USDA Utah -State University, Logan Abstract. Open plots of strawbery (Fragaria sp.) or plots caged with colonies of honey bees (Apis melilfera L.) produced less malformed fruit than plots screened to exclude large insects. Bees and large Diptera, mostly drone flies (Eristalis spp.), were the most numerous visitors to the strawberry blossoms. A list of insects including 08 species representing families frequenting strawberry blossoms in Utah was compiled. The most efficient pollinators were Apis mellifera, Halictus ligatus Say, and Eristalis spp. Many original strawberry (Fragaria sp.) cultivars had either pistillate blossoms or nonfunctional pollen and required cross pollination for fruit set (), but the self-fertile cultivars apparently set fruit well without insects (7), and little attention was given to insect pollination. Nevertheless, Knuth (9) and Darrow () suggested that insect visits were essential for a complete strawberry set, though some pollination occurred when pollen fell on the stigma. Recently, several investigators have studied the role of insects in strawberry pollination. Free (, 6) determined that pollination by the honey bee increased fruit set, berry wt, and percentage of well-formed berries. Moore (0) found that the exclusion of pollinating insects delayed fruit maturity and reduced yields. In Michigan, pollination by the honey bee increased yields 9-% (). Also, Jaycox (8) reported that bees and flies were the most important agents of strawberry pollination. They not only carried pollen between flowers but also agitated the blossoms, thereby aiding the release of pollen. lreceived for publication September, 97. Research Entomologist, Bee biology and Systematics Laboratory and Associate Professor, Plant Science. 0 The present study was made to determine the effect of pollination by the honey bee on the yield of strawberry cultivars in Utah and to compile a list of insects that aid in strawberry pollination, no such list being found in the available literature. Materials and Methods Double row beds of 'Fresno', 'Shasta', and 'Tioga' were planted at the Horticultural Field Station, Farmington, Utah, in August 970. At the onset of flowering, May, 97, replications of each of the following treatments were established; a) open checks, b) screen cages, and c) screen cages enclosing a -frame colony of honey bees. Each colony was provided with supplemental pollen and 0% sugar syrup (). The cages (6 x 6 x.8m) were covered with -mesh clear lumite screen that excluded all but tiny insects and were set up so they covered beds of each of the strawberry cultivars (). Insects trapped in the cages when the cages were being erected were captured and killed. Cages were removed when all tertiary bloom was complete. Visual counts of insects were taken 7 times during the flowering period. 'Shasta' and 'Tioga' were harvested times, and the 'Fresno' times. The following data were recorded on each harvest date: ) number of berries per cup ; ) percentage J. Amer. Soc. Hort. Sci. 99() :

3 of malformed fruit; and ) total fruit wt per double row per plot. The species and numbers of insects identified from Farmington in 97 were added to similar data collected by G. E. Bohart, formerly of this laboratory, in 970 from a strawberry field in Logan, Utah. The pollinating efficiency of the more abundant species of insects was compared by Nye on the basis of the amount of loose pollen carried on their bodies, their size, flightiness, and contact with stamens and stigmas as they move about the flower. This type of rating was used previously for carrot and onion pollination (, ). It is admittedly subjective, but when the ratings are multiplied by the populations to produce a pollination index, it can give a truer picture than population figures alone. Results Insect Populations. Insects observed in the caged plots were primarily tiny flies and ants and few of those examined carried pollen grains. An average of 0.6 insects were present per 6 m of strawberry bed per observation. The uncaged plots contained.7 insects per 6 m of strawberry bed per visual observation. Insects of the order Hymenoptera were most abundant (.8); most others were Diptera (0.79). The most abundant species was the honey bee (.89); the next most abundant was Fig.. Strawberry fruit from open pollinated plots (above) and plots caged to exclude insects (below). Malformed fruit due to incomplete pollination was typical of the plots with insects excluded. Syrphidae. The number of insect visitations to caged plots with honey bees was 6 times greater than the number to the open plots. Nectar collecting bees outnumbered pollen collectors about Ie to. Strawberry production. Little difference in yield of strawberries was observed between the caged bees and open plots, 7.8 and 6.6 kg, respectively. Yield, however, was reduced in caged plots without bees (.9 kg), primarily because the berries were smaller. Also, all cultivars in the plots caged without bees produced significantly more malformed fruit than other plots because of incomplete fertilization (Fig., Table ). Percentage produced by plots caged with bees and open plots did not differ significantly. The high degree of malformation in plots without bees was expected and verified earlier work (,, 6). ' Cultivar and efficiency of po':inators. Insects of 08 species representing families were collected on strawberry blossoms in 970 at Logan and in 97 at Farmington (Table ). More species of syrphid flies were collected at Farmington than at Logan, but bees were more numerous at Logan. Most species recovered were scarce or transient and individually contributed little pollination. The honey bee was the predominant species (6.8%) in the open plots, probably because 0 colonies of bees were located 0. km from the plots and another 0 colonies were within. km. Most insects followed no discernable seasonal pattern in visiting the flowers, but the density of bees and drone flies did fluctuate with the density of flowers. These insects were most numerous during the middle of the blossoming period when pollen was the most abundant and the weather was favorable for insect activity (Table ). The abundance of the various insect species on flowers is a poor measure of their relative importance as strawberry pollinators. For example, bees are much more efficient thane small flies in transferring pollen to the stigma of a strawberry flower. The efficiency of the pollinators was rated on the basis of the amount of loose pollen carried on their bodies, the size, hairiness, type of pulvilli, and contact with stamens and stigmas as they move across the blossom (Table ). The more flighty insects are more likely to accomplish cross-pollination than those that spend more of their time on one blossom. Pollen collecting honey bees literally wade across the blossom, swing their abdomens back and forth, and scrape the pollen from stamens with their forelegs. Nectar collecting honey bees stand higher on the flowers, move about less, and lap up droplets from the exposed nectaries. The females of other bee species usually behave like pollen-collecting honey bees, but the males act more like nectar-collectiflg honey bees. Taking both efficiency and abundance into consideration, the most important pollinators on the open plots were (in order) as follows: Apis mellifera, Osmia trevoris, Eristalis tenax, E. brousii, Halictus rubicundus, and Osmia nanula. Discussion In our test, absence of bees decreased berry size, but the major effect was an increase in the percentage of malformed fruit. Thus, some pollination of strawberries probably occurs when pollen falls from the anthers to the stigmas during wind Table. Effects of insect pollination on strawberry yield and quality. Avg. yield (kg/6m) Treatment Fresno z Shasta Open pollinated 9.BY 0.9AB Caged with bees 9.B.8B Caged without bees 7.9A 9.7A Tioga.0AB.B.A Percentage malformed fruit FresnoY Shasta Tioga.0A.DA B.DA.0A.B.0A.A.B ZA killing frost on May, 97, reduced the yields of all cultivars but especially the earlier blossoming F resno; consequently, varietal yield differences were not compared statistically. YMean separation, within columns, by Duncan's multiple range test at the % level. J. Amer. SOC. Hart. Sci. 99( ):

4 Table. Insect visitors to strawberries, Logan 970 and Farmington, Utah 97. Insects Z Hymenoptera Sphecidae Xylocelia sp. Ammophila sp. Podalonia luc tuosa (Smith) Ectemnius sp. Proctotrupidae Proctotrupes sp. Braconidae Bracon sp. Ichneumonidae Vespidae Polistes fuscatus (F.) Odynerus dilectus Saussure Ancistrocerus sp. Formicidae Formica sp. Colletidae Hy/aeus stevensi Crawford 0 C.( Andrenidae Andrena cressonii Robertson C.( (P) Andrena andrenoides Cresson C.( Andrena miserabilis Cresson C.( Andrena crataegi Robertson C.( (P) Andrena salicif/oris Cockerell C.( Andrena sp. <;> Andrena (Biareolina) neglecta Dours <;> (P) Nomadopsis scutellaris Fowler 0 Halictidae Halictus confusus arqpahonum Cockerell <;> (P) Halictus ligatus Say 'f (P).. Halictus tripartitus Cockerell <;> (P) Halictus rubicundus Christ<;> (P) Dialictus sp. <;> Sphecodes sp. <;> Evy/aeus sp. <;> Evy/aeus sp. # <;> (P) Agapostemon texanus Cresson <;> (P) Agapostemon virescens F. <;> Anthophoridae Nomada articlata Smith 0 C.( Nomada mera Cockerell <;> Tetralonia actuosa Cresson 0 Tetralonia edwardsii Cresson 0 Ceratina acantha Provancher C.( Megachilidae Anthidium sp. <;> Hoplitis fulgida Cresson C.( Hoplitis producta interior Michener <;> Megachile relativa Cresson 0 Osmia lignaria Say <;> Osmia sec/usa Sandhouse <;> Osmia simillima Smith <;> Osmia jux ta Cresson C.( Osmia trevoris Cockerell <;> Osmia nanula Cockerell C.( Osmia indeprensa Sandhouse <;> Osmia kinkaidi Cockerell C.( Osmia spp. 0 Apidae Bombus huntii Greene C.( Bombus rufocinctus Cresson C.( (P) Bombus bifarius Cresson <;> Bombus centralis Cresson <;> Apis mellifera L. C.( (P) Diptera Bombyliidae Villa utahensis Maughan Villa sp. Bombylius sp. Stratiomyidae Odontomyia pubescens Day Conopidae Thecophora luteipes (Camras) Anthomyiidae Hy/emya platura (Meigen). J. Arner. Soc. Hart. Sci. 99( ) :

5 Table. (continued) Muscidae Coenosia tigrina (F.) Calliphoridae Pollenia rudis (F.) Bufolucilia silvarum (Meigen) Phaenicia sericata (Meigen) Phormia regina (Meigen) Calliphora sp. Syrphidae Xylota (Syritta) pipiens (L.) Xylota fiavitibia Bigot Eumerus strigatus (Fallen) Chrysogaster parva Shannon Chrysogaster bellula Williston Eristalis tenax (L.) Eristalis anthophorinus (Fallen) Eristalis brousii Williston Eristalis latifrons Loew Eristalis sp. Eristalis sp. # H elophilus latifrons Loew Helophilus stipatus Walker Helophilus lunulatus Meigen Helophilus sp. Eupeodes volucris Osten Sacken Merodon equestris (F.) Asemosyrphlls polygrammus (Loew) Sphaerophora sp. Otitidae Tetanops myopaeformis (Radder) Sarcophagidae Wohlfahrtia vigil (Wal ker) Sarcophaga spp. Tachinidae Peleteria iterans (Walker) Gonia spp. Lepidoptera Noctuidae Anagrapha falcifera (Kirby) Pieridae Pieris pro todice Boisduval & Le Con te Pieris rapae (L.) Colias sp. Nymphalidae Phyciodes mylitta (Edwards) Lycaenidae Lycaena helloides (Boisduval) Lycaena spp. Hesperiidae Pholisora cattulus (F.) POlites sabuleti (Boisduval) Hesperia juba (Scudder) Satyridae Coenonympha sp. Coleoptera Melyridae Collopssp. Cerambycidae Callidium antennatum Newman Curculionidae Rhynchites bicolor (F.) Homoptera Cicadellidae Gen. & Sp. Hemiptera Pentatomidae Cosmopepla conspicillaris (Dallas) Miridae Gen. & Sp. Trichoptera Family I Z(P) after the insect name indicates individual insects were observed collecting pollen. YRated 0, 0 being least efficient. XRated -, being most abundant, and rare or observed only once. J. Arner. SOC. Hart. Sci_ 99():