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Maximising pollination

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Dr Gordon Brown reviews a paper on the Foraging behaviour of honeybees in apple trees of the Cripps Pink and Red Chief Varieties by Estela Santos and Cyrus Invernizzi.

Excellent pollination of apple trees is required to ensure high yields of quality fruit.  Apple flowers have 10 ovules and all of them need to be fertilised to ensure large round fruit.  Poor pollination leads to smaller, often deformed fruit which are only suitable for low grade sales.

Although pollination can occur due to the activity of a number of naturally occurring insects, in most orchards, this task is predominantly performed by honeybees which have been brought in specifically for this purpose.  However, not all bees that visit flowers are equally effective in pollination as it has been found that bees that collect nectar often extract it from the side of the flower without making contact with the pollen or stigma.

To evaluate the extent of this honeybee behaviour a Cripps Pink and Red Chief orchard in Uruguay was visited three times throughout the flowering period of each cultivar to record bee behaviour.  On each visit bee behaviour was observed for two hours in the early morning and two hours in the late afternoon.  In total 603 bee flights were documented.

It was found that some bees collect only pollen from flowers while other bees collect only nectar and there are some bees that collect both pollen and nectar.  For bees that collect only pollen almost all approached the flower from above optimising pollination (Figure 1).  The majority of bees that collect only nectar approach the flowers from the side.  In total it was observed that 37 per cent of bees approached the flower from the side with a low chance of pollinating the flower.  For nectar collecting bees, while many came in from above, the number was less than those collecting only pollen confirming the relative importance of pollen collectors for pollination.

 

Figure 1. Percentage of bees that approach flowers from above or from the side for bees carrying pollen or nectar or both.

This observation clearly demonstrates that for optimal pollination bees that are collecting pollen (in this case approaching the flower from above) should be encouraged to maximise the number of bees coming into direct contact with both stigmas and anthers of flowers.

Another character of bee foraging that was observed is a difference in foraging habits between the early morning and late afternoon (Figure 2).  Bees that are foraging only for pollen dominate the early morning period while bees focusing on nectar dominate the afternoon foraging period.  The tendency of honeybees to collect pollen during the morning has been observed in other crops.  Thus the greater collection of pollen in the morning may be due to the needs of the colony or because the resource is easier to extract from the flowers in the early hours of the day.

 

Figure 2. Percentage of bees that collected pollen, nectar and pollen and nectar during the morning and afternoon.

Healthy bee colonies require a supply of both pollen and nectar.  The pollen provides the colony with a source of protein while the nectar provides a source of energy.  Protein requirements of adult bees is rather low, however, it is essential for the growth and development of juveniles.  Pollination of apple trees occurs in early spring when bee colonies are becoming active and require large quantities of pollen to sustain adequate numbers of working bees.

The take-home message:

This study has shown that on average 37 per cent of bees approach flowers from the side, contributing very little to pollination.  Conversely the bees that were collecting pollen almost always approached the flower from above in order to access the resource.  Increasing pollen collecting bees is possible with management, for example, adding extra honeycombs with larvae to hives, setting pollen traps to restrict pollen movement into the hive or feeding the bees with sugar syrup.  Hives positioned in the early morning sunshine will warm up quickly and encourage early morning bee activity, when pollen collection is important to the hive.  These management tools can be used to increase the pollinating capacity of bees, potentially reducing the number of hives that are needed in the orchard.  When organising the supply of bee hives for apple pollination discuss methods that the apiarist would be happy to use to increase the number of pollen collecting bees during apple flowering.

Further reading:

Foraging behaviour of honeybees in apple trees of the Cripps Pink and Red Chief varieties. 

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