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Managing megachilid bees often involves establishing small genetically uniform populations within agricultural settings very different from the bees’ native habitat. Among select populations of Osmia ribifloris used in our blueberry pollinator program, mtDNA (COI) markers show two very distant populations from different subspecies actually shared the closest phylogenetic kinship. In fact, the fittest adult females originated from Texas and California. These bees, when kept in captivity, provisioned the largest and healthiest broods using pollen from southern blueberry cultivars. In contrast, Osmia ribifloris from northern Utah, their Mississippi progeny, along with a floral generalist from east Texas (O. lignaria) produced comparatively fewer brood. Overall, fitness of O. ribifloris also depends on a female’s reproductive status. Namely, mated bees out-produced unmated bees by nesting 2 weeks earlier and producing broods twice as heavy. Unmated bees in fact could be counter-productive to nest growth, as 22% of them reduce the fitness of conspecifics through aggressive nest parasitism, which killed 67% of brood in affected nests, a huge loss of potential blueberry pollinators. Such high rates of infanticide as well as a steepening rate of male production indicate declining growth for such a small captive bee population. Thus, prompt releases of wild O. ribifloris into blueberry fields with ample forage and nests are preferable to long-term captivity and its risk of unstable secondary sex ratios and facultative nest parasitism.
Life History, Population Structure, Management,
& Commercial Status of a Promising Blueberry Pollinator,
Osmia ribifloris
(juvenile mite)
male female
(anterior end)
Overwintering: Oct - Feb
Poplarville MS
Logan UT
Calaveras Co. CA
Portland OR
Tucson AZ
Austin TX
Life History and Population Structure:
Species (subspecies): Osmia ribifloris ribifloris (TX)
Osmia ribifloris biedermannii (AZ CA), see map inset
Population Structure: TX and CA populations are more closely related,
suggesting a north-south subspecific split occurred
between 0.9 1.4 mya during the early Pleistocene
(Fig. 1)
Description: iridescent green or blue megachilid bee (Fig. A)
Distribution: Upland sites & foothills in TX, AR, CA, OR, UT (see map)
Niche: solitary, univoltine, protandrous & xylophilic oligolege of
Ericaceae & Berbidaceae
Floral Host Records: Vaccinium (Fig. B), Arctostaphylos, Diospyros, Rosa,
Cercis, Sophora, Berberis, Mahonia
Brood Size per female: 16 22 progeny in 2 -3 straws (Fig. D):
Female production: 1 per 2.7 males in captivity (Figs. D & E)
Cocoon Weight: male 60 90 mg, female 120 180 mg (Fig. F)
Diapausal requirements: 90 120 days @ 6oC (43oF). Bees
from higher latitudes that we tested in MS required
artificial cooling. Pre-emergence mortality plus post-
emergence mortality was low (<8%).
Incubation-emergence interval: 4 8 days depending on length of diapause
and incubation temperature (recommended: 21 29oC
or 70 85oF)
Incubation to nesting interval: 10 days
Courtship and post-copulatory bonding: lasts 30 90 minutes
Emergence Period: February March, synchronous with bloom
Pollination period (when bees are active): 28 35 days
Thermal threshold for flight: 9oC (48oF)
Pollination efficiency (single visit fruit set): 50 % 75%, comparable to a benchmark pollinator,
the southeastern blueberry bee
Management Considerations:
Preferred nesting habitat: cavities in wood, cardboard, particleboard, other fibers (Figs. E)
Reliable nest block designs: solid wood (pine), wood laminates, hollow boxes (Fig. D), bundled reeds.
Acceptable tolerances for nest cavity size: 6 – 9 mm (1/4” – 3/8”) interior diameter (preferred: 6 – 7 mm) 7.5 15.0
cm (3 – 6”) depth (preferred: 15.0 cm)
Recommended leaf sources for bees to fashion partitions and entrance plugs: ranked by preference: Rosa (Fig.
C), Vaccinium, Quercus, Rubus, Fragaria and perhaps other leaf sources with shiny waxy surfaces.
Color of nest blocks: white (acrylic or latex paint) that contrast with dark colored nest entrances.
Factor that will most limit population growth on-farm: Pre-nesting female dispersal, high male sex ratios due to likely
matched matings and homozygosity at the sex-determining locus (Figure 2).
Natural enemies of concern: Chaetodactylus mites (Fig. F), birds, lizards, ants, chalcid parasitoids, chalkbrood
United States Department of Agriculture - Agricultural Research Service
Thad Cochran Southern Horticultural Laboratory, Southern Research Horticultural Unit, Poplarville MS 39470
Pollinating Insects - Biology, Management, Systematics, and Research Unit, Logan UT 84322
Pollinator Status for Osmia ribifloris:
-Native US Bee
-Not an endangered species, but wild populations are
limited by host and nest site availability. Wildfires
can destroy trap-nesting stations established in chaparral
-Evaluated as a blueberry pollinator in
California on highbush types (1990)
Maine on lowbush clones (1994)
Alabama on rabbiteye cultivars (1995 - 1997)
Mississippi on wild, rabbiteye
& southern highbush types (1998 2006)
-Not commercially available to US blueberry producers.
-Release in blueberry growing areas outside the bees
native range unlikely in the foreseeable future.
-California and Oregon farmers have a unique opportunity
to easily manage this locally available and docile bee for
blueberry pollination, in tandem with Osmia lignaria in
almond orchards. Both of these Osmia spp. have similar
management needs.
-Other native US Osmia species are being evaluated as
commercial fruit pollinators (O. chalybea and O. lignaria).
Courtship: Feb - Mar Nesting: Mar - April Nesting: Mar - Apr Brood Development: Apr - Aug
Blair J. Sampson, James H. Cane, Christopher T. Werle, Timothy A. Rinehart & John J. Adamczyk
Figure 1. Parsimony cladogram of aligned (by Clustal W method) paternal mtDNA (COI) nucleotide sequences for 14 adult individuals from
four distinct subgroups of Osmia ribifloris (C,UT, UT, MS, CA,TX). Fully resolved branches are depicted with bootstrap support. Population
designations are as follows: California: (CA), Mississippi (MS), Texas (TX), Utah (UT), and the outgroup Osmia lignaria (T2, not shown).
years in isolation (n = 5)
2000 2001 2002 2003 2004 2005
Adult M/F
Sex Ratio
y = 2.58 + 0.04e0.59x
Figure 2. Increasing male-bias in
post-emergence sex ratios for
captive O. ribifloris. Sex ratios of
newly arriving bees (parental
stock) shipped in 2000 and 2001
were excluded from this analysis.
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