Attractiveness to honey bees

Unfortunately, apple flowers are not as attractive to honey bees as several other crops. Pear flowers and the flowers of oil seed rape and several weed species prove much more attractive.

Apple varieties may also differ in their attractiveness to bees. Work at Long Ashton Research Station showed that flowers of Golden Delicious were much more attractive to honey bees than those of Cox’s Orange Pippin (Jefferies, et al., 1980). It was suggested that the Golden Delicious flowers had more perfume and that this may have accounted for them being preferred. However, they would almost certainly have also offered more pollen per flower and more flowers per tree than the Cox.

When introducing honey bees to an orchard, it is important not to do this too early.
If the introduction is before a significant number of flowers have opened, the bees will forage for better pollen and nectar sources and become habituated (exhibit what is called ‘flower constancy’) to another more attractive crop nearby. It can then prove difficult to encourage the bees back into the orchard.
Often bees show constancy to flowers of one colour; the reasons for this are not understood.
It can occasionally have implications for growers using ornamental Malus (Crab apples) as pollinators in orchards.
Several of these have dark pink flowers and bees may focus either on these or on the white apple flowers, so failing to move pollen between the two efficiently.
Czech studies have indicated that honey bees tend to prefer trees with abundant blossoms.
This has serious consequences for varieties which frequently produce only sparse flowering, either as a consistent and inherent trait, or as a result of bienniality.
In extreme cases, bees could choose to concentrate their foraging on the variety with abundant flowers and rarely visit the one with low flower abundance.

Introduce hive or bumble bees to orchards only when 15 to 20% of the flowers are open.

Introduction earlier may lead to the bees seeking food supplies on other crops growing nearby.
Once habituated to another crop it is often very difficult to attract the bees back into the apple orchard.
Remove (by mowing or use of herbicides) weeds or other species that are flowering in the orchard at the same time as the apples.
These may prove more attractive to the bees than the apple flowers.

Introduction of hive bees to aid natural pollination

When considering the introduction of hive bees (honey-bees) to apple orchards there are several important considerations:

Are hive bees necessary for pollination of apples?
Timing of the introduction of bees to the orchard.
Avoiding competition with other flowering crops.
‘Strength’ of the bee colonies.
Provision of shelter and favourable habitat for bee activity.
Aids to transfer pollen between bees.

Are hives of honey bees necessary for pollination of apples?

There is some dispute amongst experts as to whether bees, especially honey bees, are important for pollination of apple orchards. Whilst UK research (Free, 1970) shows their importance, research elsewhere has often questioned this.

It can be argued that the number of pollen grains delivered by pollinating insects from the pollinating variety (pollinizer) to the main variety is a measure of the pollination efficiency of those insects.
Most researchers have endeavoured to compare the quality of different insect species as pollinators, by assessing their performance in isolation i.e. without any interaction with other potential insect species.
When different insect species visit flowers, however, interactions are possible making it hard to determine their relative efficiency in pollination.

It is argued that effective pollinating insects should:

collect copious amounts of pollen from the pollinizer variety,
deliver sufficient amounts to the stigmas of the main variety,
make multiple visits to flowers depositing viable pollen at each visit and,
be attracted to flowers of both the pollen donor and pollen receptor varieties.

Preliminary studies in New York State, USA (Goodell and Thompson, 1997) compared the efficiency of honey bees (Apis) with bumblebees (Bombus) in pollen removal and deposition within orchards of apple varieties.

The studies indicated that both species appeared to be equally effective in the collection of pollen from the donor pollinating variety.
Honey bees seeking nectar may differ from bumblebees in their efficiency of pollen deposition, however.
This is because with certain varieties of apple with specific floral anatomy (e.g. the ‘Delicious’ variety) the honey bees ‘sidework’ the flowers and fail to deposit pollen on the stigmas.
Honey bees also carry far less pollen than bumblebees.
However, if no ‘sideworking’ takes place then honey bees can make more visits and be just as effective if not more effective than bumblebees.
Further research is needed to extend this USA study.

Recent experiments in Hungary have shown that the intensity of bee visits to an orchard can have a significant influence on fruit numbers set and the seed numbers in the fruits (Benedek and Nyéki, 1997).

This suggests that on sites where weather conditions are expected to be consistently unfavourable for bee activity, it will be essential to maximise the number of strong bee colonies per acre, if good fruit set and retention is to be achieved.

It has been estimated that approximately 50 pollen grains are transferred to a stigma by insect pollination. This contrasts with hand pollination, which results in the transfer of many hundreds of pollen grains to each stigma (Stosser et al., 1996).

Research conducted in Canada in the late 1920s showed that a strong colony of bees (e.g. >15,000) would, given favourable weather conditions, visit approximately 21 million flowers in a single day.

At the tree spacings and sizes in vogue at that time, this equated with the flowers open on 20 acres of fruit.
However, later studies suggested that if only 25% of a similar bee colony carried viable pollen, then they would pollinate all the flowers on 2.7 acres in a day.
More recent studies suggest that 60,000 to 80,000 bees are needed to pollinate one hectare of apples grown in the UK at commercial spacings (Gettingby, 2001).
A study in the 1960s went on to suggest that very few bees were needed for effective pollination (Horticultural Education Association, 1962).
To pollinate 5% of the flowers on an acre of mature apple trees, a population of only 37 insects was needed for 5 hours.

It has been shown that cropping of fruit trees diminishes the further they are located from suitable pollinators, even when bees are present (Free, 1962).

The explanation for this is that bees have limited foraging areas, they are attracted to other nearby plant species and they tend to work down one row of trees rather than across rows.

It has been suggested that wind borne pollen may also have a role in apple pollination (Burchill, 1963) and experiments have shown that wind borne pollen can collect on apple stigmas (Fulford, 1965).

However, research conducted by Free (1964) suggested that the importance of wind pollination in apple trees was negligible.
Free’s conclusion can be questioned, however, as the insect-proof cages used in this work also inhibited wind movement and the flowers studied were emasculated, which is known to reduce potential fruit set significantly.
It is possible, therefore, that in certain favourable circumstances, wind pollination may play a contributory role in apple pollination.

Some wild bee species, including bumble bees are known to work in less favourable weather conditions than honey bees and may also be useful pollen vectors in orchards.

The importance of bees in the pollination of apple orchards is still disputed by different authorities.
In ideal conditions, one strong hive of bees (>15,000) should be capable of pollinating all the flowers open on a hectare of apples in one day.
In less than ideal conditions, two to three hives per hectare, 60,000 to 80,000 bees, may be necessary.
Wind pollination is likely to have only a small contribution to apple fruit set.
Bumble bees and some other wild bees forage at slightly lower temperatures than honey bees and may prove better in pollen transfer between flowers in poor weather conditions.
However, bumble bees are expensive (currently more than £100 for a ‘triple’ with three queens and a total of 210 bees). For £50 a strong hive with many thousands of honey bees should be available for hire.

Timing of the introduction of bees to the orchard

It is vital that hives of honey bees are not introduced to the apple orchard too soon.
Work conducted at Long Ashton Research Station in the 1970s indicated that the optimum time was when 20% of the blossoms were open (Williams, 1976 see Further reading).
Do not introduce bees to the orchard until 20% of the blossoms have opened.

Avoiding competition with other flowering crops

It is essential that all potential pollen vectors, whether natural or introduced to the orchard, are encouraged to focus on transferring pollen within the apple trees.

The presence of flowers of other crops or weed species in close proximity or within the orchard may detract from this necessary focus.
Hive bees tend to be constant to a particular species on an individual trip from the hive when foraging for pollen and nectar (Free, 1963).
This is called ‘flower constancy’ and is an important factor in the success of insect pollination of apples, as hive or honey bees have been shown to form the majority of pollinating insects in European fruit plantations (Free, 1970).

Studies on the behaviour of hive bees undertaken in Hungary (Benedek and Nagy, 1996) show that flower constancy for pears was very high but was much smaller for apples.

This was supported by the findings also showing that bees working pear orchards carried only 2‑11% of pollen other than pear, whilst for apple the values were 30%-43%.
Bees are attracted away from apple flowers by rape, plum (if still in flower) and weed species such as dandelion and groundsel (Benedek and Nagy, 1996).
Mow the orchard grass and remove flowering weeds from the orchard, as some of these could prove more attractive to the bees than the apple flowers.
Bees prefer the flowers of rape, plum, dandelion and groundsel to those of apple.

‘Strength’ of the bee colonies

There is no benefit in paying for the introduction of honey bees to an orchard unless the hives contain strong colonies (e.g. 15,000 or more bees).

Pre-pollination feeding of hives with sugar may be necessary if colony strength is to be optimised and it may be necessary for growers to help pay for this.
Ensure that the hives of bees hired are strong, well-fed colonies with ideally, 15,000 or more bees.

Provision of shelter and favourable habitat for bee activity

Observations have shown that honey bee activity is limited when cloud cover is seven tenths or more, when wind speeds reach 10 mph or more and/or when the temperature is less than 15^oC (Brittain, 1933).

Anything that can be done in the orchard to lower wind speeds and increase temperatures must aid the pollinating activity of bees.
Endeavour to create sufficient shelter within the orchard to encourage bee foraging.
The ideal is wind speed no higher than 10 mph and temperatures of 15^oC or more.

Aids to transfer of pollen between bees: Use of novel aids to pollen dispersal

One of the potential problems with using honey bees as pollen vectors within apple orchards is also often quoted as one of their strengths; namely flower constancy.

Not only do honey bees focus their foraging for nectar and pollen on a single species, they frequently tend to work only in one variety or down one row of trees (Van den Eijnde and Van der Velde, 1996).
If we accept that bees do aid pollination, then there must be some exchange of pollen between bees within the hive.
The extent that this occurs is at present not known.

Pollen dispensers attached to bee hives

Research by Williams and colleagues at Long Ashton Research Station in the 1970s first showed the value of ‘hive inserts’, which dispensed pollen onto bees as they left their hives.

Bees need to be preconditioned to use these hive inserts. The pollen in these inserts was diluted with pollen of Alnus (Alder) species.
The results of some of the earliest trials were variable with the dispensers improving yields slightly on some sites but having little effect on others (Williams et al., 1978).
Further work by the Long Ashton team in 1979 led to disappointing results with the pollen dispensers.
Only 7% of the seeds in Cox fruits were attributable to pollen delivered by the dispenser, even when no other pollinator was present (Jefferies et al., 1980).
When suitable pollinating varieties were planted nearby the contribution of the pollen dispenser dropped to only 2.5%.
These disappointing results caused research on this strategy in the UK to be abandoned at that time.

Research conducted in the 1990s began to investigate whether the use of soft bristles placed at the entrance to the hive could facilitate bee to bee exchange of pollen (Free et al., 1991; Hatjina et al., 1993).

More recent trials conducted in the Netherlands (Van den Eijnde and Van der Velde, 1996), compared different types of bristles used at hive entrances.
Paintbrushes made with pig’s hair and a paint roller covered with cotton wool (‘soft angora roller’) were compared.
A perspex and wood device was erected at the hive entrance, which forced the bees to crawl towards the light between two paintbrushes or paint rollers.
Bees emerging from hives on which the bristle/roller devices were placed carried pollen of significantly more plant species than hives with no attachment.
By inference, one would expect the same to be true when comparing pollen of different varieties, although this was not checked in these experiments.
The rollers and brushes both achieved the same objective but the rollers were easier to assemble.
Following inconsistent and sometimes disappointing results using hive inserts to aid pollen dispensing, trials at Long Ashton were abandoned and the technique never gained favour in the UK.
More recent trials conducted in the Netherlands may warrant a reconsideration of this strategy.

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Attractiveness to honey bees