March 2022 - 31

BLUEBERRY
Take stock of pollination of blueberries in Pennsylvania
By Margarita Lopez-Uribe
and Kathy Demchak
Penn State University
Blueberry (Vaccinium spp.) is a highvalue
and economically important fruit
crop native to Pennsylvania and Eastern
North America. Nationally, the total
value of the blueberry crop was
$797 million in 2018 (USDA NASS).
Demand for blueberries in
Pennsylvania is also high; prices range
from $2-5 a pint (USDA NASS).
There are four native blueberry
species to Pennsylvania: Low sweet
blueberry (Vaccinium angustifolium),
lowbush blueberry (V. pallidum), sourtop
blueberry (V. myrtilloides), and the
highbush blueberry V. corymbosum,
which is the most often cultivated
species on commercial production
farms.
Common varieties of V. corymbosum
in Pennsylvania include Duke, Patriot,
Bluecrop, Jersey and Elliott, among
others. While these different varieties
vary in their capacity to self-pollinate,
sufficient pollination is crucial for
maximum yield. This article provides
information about the pollination biology
and requirements of blueberries and
general recommendations for improving
blueberry pollination in Pennsylvania.
Pollination biology and
requirements
Highbush blueberries produce small
bell-shaped pendulous flowers with short
anthers hidden inside the white or pink
corolla and a stigma that sticks out near
the opening (Figure 1). In order to set
fruit, a sufficient number of viable pollen
grains must be transferred from the
anther to the stigma of the same variety
Figure 1. The shape of blueberry flowers. Before pollination, left, the white or pink corolla covers the anthers and stigma. After
pollination, right, the corolla drops off and the stigma of the flower gets exposed. The presence of corollas on the ground is
generally a sign that pollination has occurred. Photos: Nash Turley/Penn State
(self-pollination) or a different variety
(cross-pollination). After pollination
is complete, corollas often drop off
leaving only the stigma and sepal
attached to the ovary that will develop
into the fruit (Figure 1). Needs for
cross-pollination vary across varieties.
Some such as Duke and Bluejay do not
need cross-pollination, while Elliott,
Bluecrop and Legacy benefit from
cross-pollination but can set fruit with
pollen from the same variety.
Blueberry flowers have poricidal
anthers (i.e., the pollen is released
through pores) that require sonication
or " buzzing. " This behavior is necessary
for the pollen to be released from
the flower and become available to
be picked up by a pollinator and
transported to a different flower.
Blueberry pollen is sticky and is not
easily transported by the wind. Because
of these traits of blueberry flowers,
the most efficient pollinators of this
crop are wild bees, many of which can
perform buzz pollination and transport
the pollen on their hairy bodies.
Honeybees are not able to perform
buzz pollination. As a result, pollination
efficiency by wild bees is three times
higher than pollination by honeybees.
On average, when pollinating blueberries,
honeybees transfer 11 pollen grains to
the stigma per flower visit, compared
to ~45 pollen grains transferred by wild
bees (e.g., bumble bees and mining bees)
(Javorek et. al., 2002).
Despite their inability to buzz
pollinate, honeybees are still crucial
pollinators to commercial blueberry
farms where large acreage of blueberry
bushes require pollination and low
abundance of wild bees may be available.
However, pollination by honeybees alone
is not enough to achieve maximum
yield. Therefore, in all cases wild bees are
helpful, or even necessary, to get the best
results (Reilly et al 2020).
Pollinators of blueberry
Since Pennsylvania lies within the
native range of the blueberry, there
is a great diversity of native bees
that provide pollination services
to blueberry farms. Observations
performed in three small diversified
farms with Bluecrop plantings in
central Pennsylvania revealed that the
smallest non-commercial planting had
the greatest diversity of pollinators.
While managed honeybee colonies
were present at all sites, they were
not commonly observed pollinating
blueberries, compared to other wild bees
such as bumble bees. For about every
honeybee visiting blueberry flowers, we
observed about three bumblebee queens
(Figure 2).
Mining bees in the genus Andrena
were also a major group of wild bees
observed at all sites (Figure 2). Both
bumble bee queens and mining bees
are active in the spring when blueberry
flowers begin to bloom. At least 11
different species of Andrena have
been observed pollinating blueberries in
See POLLINATION, page 31
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March 2022

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