- 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.
- 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.
- 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.
- 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.
- It is known that a bee spends between 3.2 to 6.2 sec per floret and a worker of honey bee works for about five hours per day.
- There are an estimated 86,70,000 florets per hectare.
- For efficiently pollinating all the florets of one hectare of sunflower about 2000 bees per day are needed for about three weeks flowering period.
- The proportion of colony foragers that might visit the sunflower will depend on many factors.
- Strong colony of A.mellifera should be able to supply that much number of forages for the sunflower pollination.
- Thus, one hectare of sunflower requires the service of bees of at least one strong colony for having the maximum set of seeds per flower head.
- In Rumania the recommended number of colonies is 2 per ha. whereas in Russia it is 1 per ha.
- In India, it is suggested to keep 6-8 hives and maintain 3-4 colonies per ha. of sunflower.
- However, it depends on the number of foragers in the hive.
Following measures may be undertaken to encourage activity of pollinators in the sunflower field:
(1) Do not apply insecticide when sunflower is in bloom.
(2) Provide artificial food in the form of sucrose solution (1:1 sugar:water) during lean periods.
(3) Protect bee colonies from natural enemies.
(4) Do not destroy bee flora around the field which is essential to sustain the colonies during different times of the year.
(5) Dusting is more economical than application of EC formulations of insecticides.
- The hybrid seed production involves production of cross pollination of cytoplasmic male sterile lines (CMS or A lines with restorer lines (R lines), also production of CMS lines is done by crossing A lines with maintainer lines (B lines).
- All these cross pollination is being carried out manually by hand which is a costly and tidious task.
- The isolation distance of this plot from other sunflower plot should be at least 2kms.
- By placing bee colonies Rs.6,000/- per ha could be saved which otherwise is needed for manual pollination.
- Since the cross pollination in sunflower is important for better seed setting, hand pollination or pollination by bees is essential.
- When compare to the hand pollination with bee pollination it was observed that, though good seed set was realised with hand pollination it involved so much of labour and time that it is not economically viable to go for hand pollination if the labour availability is scarce and costly.
- It is therefore essential to observe the field of sunflower for activity of bees.
- In the absence of honey bees there is every likelyhood that the yield will go down tremendously.
- Sunflower Management