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central Pennsylvania, often clinging to
flowers during their visits.
Note that one of the most important
pollinators of blueberries in the southeast
region of the U.S. is Habropoda laboriosa,
commonly known as the Southeastern
blueberry bee. While this bee has been
recorded in Pennsylvania, it has not been
observed in any of our study sites in
central Pennsylvania.
Signs and causes of poor
Insufficient pollination may happen
when there are too few pollinators
to visit and pollinate every flower.
In addition to the low abundance
of pollinators, other causes of poor
pollination include that the available
bee species may be unable to perform
sonication (e.g., honeybees) and/or
the weather may be cold or windy and
not conducive to bee foraging. Lack of
sufficient bee-mediated pollination may
result in blueberry bushes relying on
self-pollination, which often develop
into lower-weight fruit. To quantify
crop pollen limitation, berry weights
and numbers are compared between
flowers that were hand-pollinated and
Figure 2. The common wild bees pollinating blueberry crops in central Pennsylvania. Both bumble bee queens (left) and mining
bees (right) are active in early spring and are abundant pollinators of blueberries. Photos: Margarita López-Uribe and Nash Turley/
Penn State.
flowers that received natural pollination
(a.k.a. open pollination). In our study
from sites in central Pennsylvania,
we did not observe severe signs of
pollination limitation.
The number of berries from the
open-pollinated flowers was not different
from those in the hand-pollinated
flowers. However, at one site, we found
a 17% decrease in the average weight
of berries in the open compared to the
hand-pollinated flowers. Reductions
in berry weight are one sign of pollen
limitation, so our results suggest there
is potential to maximize pollination
at these sites in central Pennsylvania.
Other signs of poor pollination in
blueberry crops include: later ripening
berries, decreased berry size and
reduced number of viable seeds in each
berry (Benjamin and Winfree, 2014).
Supporting more and
healthier bee pollinators
Several studies and our own
observations indicate that higher
pollinator abundance is correlated
with increased berry weight. This
highlights the importance of
enhancing wild pollinators to increase
crop yields. Therefore, growers in
Pennsylvania should be aware of the
critical importance of wild bees for
the production of blueberries. In
order to reach maximum crop yield,
growers should follow best practices
to support and enhance a diverse array
of wild pollinators. Some of these
recommendations include:
1. Do not apply insecticides during
bloom. If fungicide applications
are necessary during bloom,
choose effective products with the
lowest toxicity ratings possible, as
indicated on the pesticide label.
2. Keep in mind that flowering
weeds around crops can become
important routes of pesticide
exposure for bees. Therefore,
it is recommended to remove
blooming weeds such as dandelion
or white clover before pesticide
sprays rather than afterward.
3. Help maintain nesting habitat for
wild bees by preserving natural
forest habitat surrounding the
blueberry field. Maintaining
nesting habitat may also include
refraining from tilling the soil
surrounding the blueberry field
if possible. Soil disturbance from
tilling disrupts the habitat for
many ground-nesting bees.
4. While it is common practice
to supplement pollination with
commercial honeybee colonies,
growers can also consider
purchasing commercial bumble
bees to maximize pollination.
However, introducing managed
bees for pollination can facilitate
the transmission of pathogens
from managed to wild bees.
Therefore, it is recommended to
limit the introduction of managed
pollinators when possible. Some
32 |
additional management practices
that can be considered to decrease
pathogen transmission include:
renting honeybee colonies that
have low levels of varroa mites and
placing managed colonies in the
center of the plantations where
wild bee abundance is lower than
at the edge of the fields.
Growers in Pennsylvania
should be aware of
the critical importance
of wild bees for the
production of blueberries.
5. Consider integrating other
plants or crops near blueberry
fields that bloom both before
and after blueberries in order to
provide wild bees with sustained
resources throughout their active
season and incentivize them to
maintain nests nearby. To help
support early-emerging bees like
mining bees try planting native
spring-flowering trees like willow,
red maple, serviceberry, cherry
and redbud. Native perennial
wildflowers such as milkweeds,
black-eyed Susan, mountain
mint, Joe Pye weed, coneflower,
sunflowers and bee balm will help
support bumble bees and a wide
variety of other pollinators.
It is clear that adequate pollination
is necessary for achieving maximum
yield from blueberry crops. While signs
of pollination limitation appear to be
weak at farms in Pennsylvania, growers
should be aware of this possibility
and are encouraged to implement
recommendations to sustain and
enhance wild pollinators in their farms.
If signs of pollen limitation are
present, such as low fruit set, smaller
than average berries, and late-ripening
berries, growers should consider
changing management practices.
Actions such as enhancing habitat
to encourage wild bees near the farms
or supplementing farms with managed
pollinators can promote blueberry
pollination. FGN

March 2022

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