Cultivated red raspberry (Rubus idaeus L.) is one of the numerous specialty crops dependent upon bees for fruit production (Klein et al., 2007). Although predominantly self-fertile (Daubeny, 1971), raspberry flowers only partially self-pollinate due to insufficient contact between the anthers and the innermost stigmas within the flower (Free, 1993). Raspberry flowers contain 60 to 90 stamens arranged in an outer ring of whorls around a central receptacle, and a similar number of pistils arising spirally from the receptacle. Pistils must be individually pollinated; each ovary develops into a single-seeded drupelet when fertilized (Jennings, 1988). Cohesion of the aggregate fruit requires that a high percentage of drupelets set. Unmarketable berries result when too few drupelets develop, causing berries to be crumbly and misshapen, often with a terminal tuft of dried unpollinated pistils (Cane, 2005; Free, 1993; McGregor, 1976). Bee visitation is necessary to ensure adequate pollination of these innermost pistils.
Wild bees are often too scarce in larger raspberry fields to satisfy pollination needs (MacKenzie and Winston, 1984; Winston and Graf, 1982). Growers instead rely on rented colonies of European honey bees (Apis mellifera L.) as the primary pollinators of commercial raspberry in Europe and North America (Free, 1993). Honey bees are known to be effective pollinators of raspberry (Bekey, 1985; Cane, 2005, 2008; Chagnon et al., 1991; Sáez et al., 2014; Shanks, 1969; Willmer et al., 1994); however, there is disagreement as to the estimated number of visits required for full drupelet set. Bekey (1985) suggested that every flower may need as many as 68 honey bee visits, whereas other studies estimate around 5 to 10 visits to be sufficient (Chagnon et al., 1991; Sáez et al., 2014). More recent work examining the single-visit efficacy of honey bees for raspberry suggests that as few as two visits are sufficient to maximize yield (Andrikopoulos and Cane, 2018).
The longer a bee is in contact with a flower the more opportunity it has to contact and transfer pollen to stigmas. Drupelet set, therefore, increases with the cumulative duration of a bee’s contact with a flower, up to about 150 s (Chagnon et al., 1991). The first visit to a virgin flower is far longer than the day’s subsequent visits to the same flower (Bekey, 1985). This pattern accords with the daily cycle of nectar volume in a raspberry flower, which is usually greatest in the morning, having accumulated during the previous night (Bekey, 1985; Willmer et al., 1994). The first visitor of the day depletes the flower’s nectar pool. Raspberry flowers remain receptive for at least two days, over which time they continually secrete nectar (Bekey, 1985; Eaton et al., 1968; Redalen, 1976; Willmer et al., 1994). As a consequence, this recharged nectar pool should elicit one prolonged bee visit on each of two days, during which foragers often straddle the receptacle and pivot about to drink nectar, thus ensuring contact with the central pistils. These two prolonged visits could be enough to maximize drupelet set.
An accurate estimate for the number of visits required for full fruit set is necessary to build more refined models of honey bee stocking density on farms. Bekey (1985) estimated that 2 to 5 colonies/ha are needed to pollinate raspberry. However, that calculation was based on the assumption of between 27 and 68 visits per flower for adequate pollination. If flowers only require two visits instead, the estimated honey bee stocking density for raspberry would be considerably less. This discrepancy has major economic implications for growers. Raspberries markets are expanding, evidenced by the doubling of land dedicated to cultivated raspberry between 1984 and 2014 (FAOSTAT, 2017). At the same time, faltering populations of managed honey bees have led to mounting hive rental prices. The price of colonies for raspberry has nearly doubled in Oregon and Washington over the past 20 years, now averaging $40/colony (Burgett, 1997; USDA-NASS, 2016). In California, the rental price is even higher averaging $92/colony in 2016. Growers in California, Oregon, and Washington paid $1.61 million for raspberry pollination in 2016 (USDA-NASS, 2016).
In this study, we compared drupelet set for four honey bee visitation treatments at three red raspberry cultivars: 1) unvisited flowers, 2) a single prolonged bee visit, 3) two prolonged bee visits, one on each of two consecutive days, and 4) openly visited raspberry flowers (open pollination). We posited that two prolonged honey bee visits to a single raspberry flower would be sufficient to achieve full drupelet set.
BekeyR.S.1985Effects of method and timing of pollination and temperature on fruit development in red raspberry. PhD Diss. Oregon State Univ. Corvallis OR
BurgettM.19971996 Pacific Northwest honey bee pollination survey. Oregon State Univ. Corvallis OR
CaneJ.H.2005Pollination potential of the bee Osmia aglaia for cultivated red raspberries and blackberries (Rubus: Rosaceae)HortScience4017051708
ChagnonM.GingrasJ.de OliveiraD.1991Honey bee (Hymenoptera: Apidae) foraging behavior and raspberry pollinationJ. Econ. Entomol.84457460
Food and Agriculture Organization of the United Nations2017FAOSTAT database. FAO-ESS Rome Italy. 12 Sept. 2017. <http://www.fao.org/faostat/en/#data/QC>.
FreeJ.B.1993Insect pollination of crops. 2nd ed. Academic Press London UK
JenningsD.1988Raspberries and blackberries–their breeding diseases and growth. Academic Press London UK
KleinA.M.VaissièreB.E.CaneJ.H.Steffan-DewenterI.CunninghamS.A.KremenC.TscharntkeT.2007Importance of pollinators in changing landscapes for world cropsProc. Biol. Sci.2741608303313
LittellR.C.MillikenG.A.StroupW.W.WolfingerR.D.1996SAS system for mixed models. SAS Inst. Inc. Cary NC
MacKenzieK.E.WinstonM.L.1984Diversity and abundance of native bee pollinators on berry crops and natural vegetation in the lower Fraser Valley, British ColumbiaCan. Entomol.116965974
McGregorS.E.1976Insect pollination of cultivated crop plants. Agricultural Handbook No. 496. USDA-ARS Washington DC
Meso West2017Meso West Data. Univ. of Utah Salt Lake City UT. 10 Oct. 2017. <http://mesowest.utah.edu/>.
SáezA.MoralesC.L.RamosL.Y.AizenM.A.2014Extremely frequent bee visits increase pollen deposition but reduce drupelet set in raspberryJ. Appl. Ecol.51616031612
SAS Institute2006SAS/STAT User’s Guide version 9.1.3. SAS Institute Cary NC
U.S. Department of Agriculture – National Agricultural Statistics ServiceJuly 2016Noncitrus fruits and nuts 2015 summary. U.S. Dept. of Agric. Washington DC
WillmerP.G.BatawA.A.M.HughesJ.P.1994The superiority of bumblebee to honeybees as pollinators-insect visits to raspberry flowersEcol. Entomol.19271284
ŻurawiczE.2016Cross-pollination increases the number of drupelets in the fruits of red raspberry (Rubus idaeus L.)Acta Hort.1133145152