Many sunflower varieties are open-pollinated with bees usually the
main agents, and to ensure a high seed set there must be a high insect
population.
A small number of empty seeds pops can usually be found in the
centre of most sunflower heads, but a high proportion of such unfilled
seed will drastically reduce oil yield per hectare.
Where large areas of open-pollinated sunflowers are grown in pure
stands, it has been found that the fertilization percentage as measured
by the number of sound seeds per head decreased rapidly from the edge
to the centre of fields.
It was also directly related to the number of bees active in the area.
Since local wild bee populations are seldom adequate to pollinate
extensive sunflower crops and beekeepers state that honey-bees seldom
fly more than a mile or so from their hives or nests, it is strongly
recommended that hives be placed at strategic positions.
In many tropical areas bees may be restricted in their working hours
by high temperature.
At 350 C or above, pollen and nectar gathering tends to
be progressively reduced as workers are diverted to water collection
and hive cooling.
In India, however, bees were most active in the temperature range
37-44 0C.
A minimum of two to four strong colonies per hectare of crop has
been suggested to ensure adequate coverage.
One bee to four sunflower heads is considered the minimum to ensure
complete pollination. In practical terms this means that the number
of bees must be roughly equal to the plant population.
In developed agricultural areas contracts can usually be arranged
with professional beekeepers, but in less-developed regions it may be
necessary to introduced beekeeping as part of normal agricultural operations.
There is usually little difficulty in so doing, for the honey produced
is always readily saleable, or consumed domestically.
Intercropping can also significantly increase the number of pollinating
insects.
In India for instance, by interplanting sunflower with niger seed
yields were nearly doubled. This increase was attributed to the greatly
increased insect population attracted to niger.
Insects as pollinators are less important on many recent hybrids,
which are basically self-compatible, and since these are now becoming
more widely available inadequate pollination may cease to be a major
factor depressing yield.
The presence of bees in commercial crops significantly increases seed
yield.
Hand pollination
Sunflower is a self-incompatible and depends on insects (mainly
bees) for cross-pollination and seed-set, therefore, it is essential
that adequate pollinators are present in the field, for pollen
movement and seed-set.
Otherwise the heads bear chaffy and partially filled seeds resulting
into drastic reduction in yield and quality of the produce.
Bees play a very important role in increasing seed set in sunflower
more particularly in open pollinated populations .
Keeping bee hives in the field increases crop yield.
Hand-pollination gives an increase in the crop yield to the
extent of 18 to 25%.
Hand-pollination could be done by gentle rubbing of the sunflower
heads with palm or with soft muslin clothes during flowing period
between 7 to 11 A M on alternate days for about two weeks.
Ikisan - Role of Honey Bees in the Pollination of Sunflower
Role of honey bees in the pollination of sunflower
The process of pollination involves
three distinct phases viz., the release of pollen from the male
part of the flower, the transfer of pollen from the paternal to
maternal parts and successful placing of pollen on the recipient
surface of the latter followed by germination of the pollen grains.
It is necessary
to know the aspects of flower and about the pollinators before understanding
the role of honey bees in the pollination of sunflower.
Flower structure
The flower head or capitulum of sunflower
is surrounded by an involucre of green bracts.
Next to this is a single row of ray florets.
The large yellow ligules of these florets
make the sunflower head very conspicuous.
The ray florets are otherwise infertile.
Next to the row of ray florets are many
concentric rings of hermaphrodite tubular disc florets the corolla of
which is represented by five united petals.
In each of these florets has a single
inferior ovary with one ovule which after ripening forms an achene.
The number of disc florets per flower
head varies from 1000 to 2000 depending upon the size, variety and number
of flower heads per plant.
Mechanism of flower opening
In sunflower the florets start opening
from the periphery inwards.
Early in the morning two to four consecutive
circles of florets open each day.
Soon after opening the staminal filaments
rapidly elongate and the anther tube consisting of five anthers appears
above the top of corolla.
Immediately the anthers dehisce and the
pollen is shed into the anther tube.
This is followed by the elongation of
style and contraction of the staminal filaments.
Both these acts result in the pollen
being pushed out of the upper end of the anther tube.
There are two distinct stages noticeable
in the sunflower florets opening i.e. the male stage and the female
stage.
Duration of flowering
The duration of flowering in the crop
and within the floral head varies a great deal depending on the environmental
condition.
However, each head flowers for about
six to ten days except in severely cold conditions when this period
is extended.
More the exposure to the external agencies
the greater is the percentage of the seed set.
Although the relaxed stigmatic lobes
are incapable of pecking up their pollen from their style but they readily
accept pollen from the florets of the same flower when insects or other
external agencies help their transfer from one or the other florets
in the head.
Besides the florets of the same flower
head the pollen transported from any other flower head of the same or
from different plots of crop may achieve successful pollination and
fertilization, if it falls on the receptive surfaces of the stigmatic
lobes.
The pollination possibility of this type
by external agencies is equally important and perhaps more desired.
Although inter floral cross pollination
may readily occur, greater seed production is likely to be achieved
following cross pollination between florets of different flower heads.
There is possibility of further increase
in the seed set if there is greater inter mixing of the pollen from
several heads.
The advantages of cross pollination in sunflower are
Higher percentage of seed set.
Exploitation of hybrid vigour which might result in the
seed being more healthy and stout.
More oil content Owing to these advantages cross pollination
in sunflower is naturally favoured.
In India Apis dorsata Fab. is the
most dominating among bees on sunflower. Other pollinator bees include
Apis florea Fab., Trigona fuscobalteata Cam., Certina
sumiuna Smith, Braunsapis mixta Smith, Pithites smarugdula
(F.), Chalicoroma lerma (Cam.), Lasioglossom Sp. And
Xylocopa sp..
The population of pollinators had positive
correlation with relative humidity and negative correlation with temperature
as observed at different places.
The yields of sunflower pollinated by
bees have been much greater than those of sunflower grown in cages which
excluded bees.
It is important in such studies also
to have either cages with bees or cages open to bees, so that the effect
of the cages on sunflower yield can be determined.
Sunflower fields provided with honey bee
colonies @2.5/ha recorded almost double the yield compared to fields
isolated at least 4.8km from the been colonies.
With the increase in distance of sunflower
field from honey bee colonies there will be decrease in seed yield.
This is because when the plots are nearer
to the colonies, the flowers are visited by more bees and thus there
is greater percentage of seed setting per head.
Honey bees visited sunflower head both
for nectar and pollen. However, the main purpose of their visit is the
collection of nectar.
Three types of honey bee activity can be noticed on sunflower
head
Some bees collect only pollen (usually in the morning
hours soon after dehisence).
Some collect only nectar (in the morning and evening).
Some collect both pollen and nectar.
During their visit nectar gatherers not
only confine themselves to the florets in male parent but also move
on to the florets in the female parents as well.
This brings about the transfer of pollen
from the male to the female florets and thus pollinate them.
Comparatively, fewer bees visit the sunflower
for the collection of pollen than nectar.
There is no specific limit to the number
of bee visits on the flowers.
Greater the number better it is. There
is a direct correlation between the amount of seed produced per crop
and the average number of bee visits each floret received.
Longer a bee stays at the flower head
the more number of florets it walks on and pollinates.
Among physical and biological factors around the sunflower head,
only insects play major role in its pollination.
Sunflower during flowering is visited by a large variety of insects
such as moths, butterflies, beetles, house flies, bugs, thrips,
ants and scores of other insect species.
Their visit being irregular, role played by them in the pollination
is minor.
The locally abundant solitary sub-social bees such as Helictus
spp. and Bombus spp. do help in the pollination of sunflower
but the observations made so far on their efficiency are very few and
their number available for pollination is also very low.
The most numerous insects so far recorded as pollinators of sunflower
are the honey bees.
Apis mellifera bees visited sunflower for 261 times, Apis
dorsata 11 times and a butterfly, a beetle, a housefly and a moth
visited only once .
Honey bees are of great value in the pollination of sunflower as reported
in several of the studies.
Sunflower seed production may be limited by the shortage of enough
number of pollinating bees.
The choice of insecticide for sunflower crop in bloom should be determined
by its hazards to pollinators as well as to the seed set of the crop.
It is experimentally proved that fenthion, carbaryl, endosulfan and
parathion reduced the field population of bees immediately after spray
but repopulated with bees within 6 hours.
Fenthion, carbaryl and parathion recorded 100 percent kill of treated
caged bees compared to 33 percent kill in endosulfan.
No significant difference was evident of the seed set and yield due
to different insecticides.
If the application of insecticide is so much essential, the hives
may be closed for a day and the spray may preferably be taken up during
evening hours.
Pollination In Sunflower
