How do Bees Pollinate Apple Trees?

Honey Bees as Plant Pollinators in Fruit Trees Orchards

Self-pollination is when the pollen from the anthers (male reproductive part) is deposited into the stigma (female reproductive part) from flowers of the same plant. Apples require cross-pollination, which transfers pollen from one tree (e.g. Crabapples) to the stigma of another apple tree of the same species (e.g. Granny Smith). Insect-mediated pollination is a form of cross-pollination where pollen from the anthers is deposited into the stigma of another.

This article uses honey bees (Apis mellifera) and apple trees (Malus domestica) as models for describing cross-pollination. A variety of apple trees (“cultivars”) are involved in cross-pollination, and the fruit production process is greatly enhanced when pollinators are present. It compares wind and insect pollination, and the potential for apple trees to self-seed, finishing off with an analysis of alternative, native pollinators.

How Apple Tree Pollination Works

A one sentence summary: honey bees forage in the orchard during floral blooms, carrying and transferring pollen between the various apple tree cultivars planted, resulting in fertilisation of the stigma, seed set, and fruit development.

Structure of an Apple Flower

Apples have a five-petaled flower, pink to white in colour. Below the petals are five green sepals, forming the calyx, which provides protection to the developing flower bud. The reproductive parts are the stamen (male) and pistil (female). Multiple stamens surround a central pistil, and consist of a filament topped by an anther, where pollen is produced. The pistil consists of an ovary at its base, a style leading upwards, and a stigma at the top. The stigma is where the flower is receptive to pollen, and upon successful fertilisation, the ovary develops into the apple fruit (reviewed in Ramirez & Davenport, 2013).

The five-petaled flower contains 2-4 ovules each, surrounded by multiple stamens. The apple flowering clusters consists of six flowers formed during the previous summer on small woody shoots. The first flower to open is known as the “King” bud, and produces the largest fruit when pollinated. Blooming takes place for brief period of 1-2 weeks, with the first 9-10 days being critical for king bud development.

Stages of Apple Flowering

Foster and colleagues categorise apple flowering in eight stages, based on the meristem (region of plant tissue containing undifferentiated cells) and apex (the tip of a shoot; Foster et al., 2003):

1. The meristems are flat and narrow – the tree is dormant.
2. The dormant meristems broaden,
3. The meristems broaden into a dome shape,
4. The main domed meristem begins developing 4-6 meristem shoots, separated by a bract (leafy structure),
5. The meristem becomes a “terminal floral meristem”, which contains the floral organs,
6. Sepal formation begins on the terminal flower (the “king bud” listed above), and the lateral meristem shoots from stage three begin forming bracts,
7. Septal formation begins on the lateral terminal floral meristems,
8. Continuation of the sepal formation on the remaining terminal floral meristems until all sepals have been developed.

For many apple cultivars, this process starts at the end of winter as temperatures increase, leading to full blooms by early spring. After the flowering and fruiting is complete, the tree returns to stage zero.

Basics of Cross-Pollination

Cross-pollination means that the pollen has to come from different tree varieties, rather than different flowers, for a successful pollination; this process is highly dependent on pollinators. In apple farming, the cross-pollinated species are various crabapple trees placed in alternate rows within the orchard. Crabapple trees flower in large numbers, for longer periods, and on younger wood (as early as one year), providing more pollen per plant than other apple species (Pusey, 1997).

Most apple cultivars require pollen from a different variety for successful pollination and fruit set due to “gametophytic self-incompatibility”, or being self-incompatible for self-fertilisation; unlike self-fruiting plants (reviewed in Ramirez & Davenport, 2013). This mechanism maintains genetic diversity and prevents the increased expression of recessive traits, which could be detrimental in times of a selective pressure.

Successful cross-pollination is achieved when pollen from one apple cultivar is transferred to the flowers of a different, compatible cultivar. Compatibility between cultivars is essential for successful pollination and is influenced by genetic factors. Weather, bee health and the right apple varieties impact the success of cross pollination. If the blooming season happens during adverse weather conditions, then A. mellifera activity will be minimal, reducing the rate of fruit set.

Floral Overlap is When Different Cultivars Bloom at the Same Time

Having at least two differing apple varieties is required for cross-pollination, and these cultivars must bloom in synchrony to maximise pollen transfer. Second to that, the cultivars should also have a long flowering season, and be annual or biennial to maintain a yearly pollen supply¹. Crabapples fit this criteria, and are a common choice for pollinators (Ramirez & Davenport, 2013). In practice, a combination of early-blooming and late-blooming varieties are used to maintain a supply of pollen to the growing crop.

The blooming periods of apple trees is affected by genetics, temperature (hint: they bloom and wilt quicker with increasing temperatures) and plant physiology. The germination of pollen grains is highly dependent on temperature. The farmer can’t control the weather, so they’re better off focusing on maximising bee and tree health.

Comparing Wind and Insect Pollination

Both forms of pollination evolved in response to the environment and pollinator availability, with wind pollination being the more ancient process, as plants were present prior to pollinators. A table summary is presented below.

Wind-Pollinated Flowers are Smaller and Lack Nectar

Large quantities of pollen are produced by a wind-pollinated plant. Wind pollination is a non-specific process – you never know where the breeze will take a pollen grain, and much of the dispersed pollen may not reach another flower. The flowers of such plants are small and hidden – they do not produce nectar nor scents to attract pollinators.

Evolutionary, animal pollinators are not always be available, so in response to this selective pressure, plants developed the ability to produce large amounts of lightweight pollen that can be carried away in a strong breeze.

Insect Pollination Directly Fertilises Flowers

Insect (or avian or arachnid or mammal) pollination involves the transfer of pollen between flowers from the visitors who collect nectar, pollen or both. Insect-pollinated flowers are usually brightly coloured, large, scent-producing and contain structures to assist pollinators in performing the process (petals, sepals) – basically, the traits listed in the table above.

Since insects carry pollen directly from one flower to another, the fertilisation rates are much higher than wind pollination – combined with a (relatively) higher ratio of female ovules in each bloom, the seed set of insect-pollinated plants is far higher than wind-pollinated species.

While many insect species feed on the apple pollen & nectar, A. mellifera is the ideal pollinator species due to her preference a single flower type during foraging, as well as the large colony numbers. As the worker lands on apple blossoms, she continues to visit other apple flowers until she can forage no more or the floral resources are depleted. Two to three per hives per hectare, or one hive per acre provides sufficient worker bees for apple orchard pollination (Somerville, 1999).

Non-Honey Bee Fertilisers of Apple Trees

A. mellifera is an important apple pollinator, but she is not the most efficient pollinator. Honey bees would sometimes consume the nectar without pollinating it, depending on how they approach the flower (Ramirez & Davenport, 2013). Other visiting bees include:

• Andrena, the mining bees,
• Bombus, the bumble bees,
• Halictus, part of the sweat bee family, and
• Osmia, the mason bees.

These bees show varying degrees of pollination success, with some being more efficient than A. mellifera. Kendall (1973) showed that flies and beetles also visited apple trees, with limited success. Considering that nectar is sought after by many insects, it is unsurprising to see beetles (Coleoptera) and butterflies (Lepidoptera) and true bugs (Hemiptera) visiting apples. Whether these insects also pollinate them in return is a different story.

Can Apples Be Pollinated by Wind?

One small experiment by Free (1964) showed no evidence that wind pollination affects fruit set in apple trees. When a fresh boutique (as in, “flower boutique”) containing different apple tree species was placed on the branches of the target tree, no wind pollination was observed. This was not attributed to a drastic reduction in wind speed, as this bag only reduced top speeds by ~30%. The boutique flowers were required for cross-pollination- there were no other cultivars in this experiment. The trees in the group containing the boutique alone did not bear fruit.

The biology of apple flowers rewards insect pollination rather than wind. Blossoms contain scents and markings that attract insect pollinators, as well as a nectar reward for the visiting insect (Wang et al., 2002).

Self-fruitfing Apple Trees

Many pollinator-dependent crops can still produce a lesser level of fruits with self-pollination. Fruits from self-pollinated apples are smaller in size and number (De Witte et al., 1996). This varies across cultivars, with De Witte and colleagues reporting that higher levels of self-pollination were reported in “Elstar” (7%) and “Idared” (12.3%). By contrast, cultivars like “Cox’s Orange Pippin'”, have a variable self-fertilised fruit set, between 0.7% – 17% of a total harvest.

In the review of Ramirez & Davenport (2013), they reference a Japanese study by Yuda et al., (1991) that reports higher levels of self-fertile apple trees in the tropical region of East Java, Indonesia, perhaps driven by the higher temperatures, or cultivar type (“Rome Beauty” in this study). I can’t read Japanese, so I’ll take their word for it. Another (Japanese) study by Satio & colleagues showed that the self-pollination of “Megumi” and “Orin” apples had a success rate around 40 to 48% in the second year and from 16.3 to 38% in the third year of their four-year study (Saito et al., 2007)².

How Harmful are Honey Bees to Pollination?

We conclude by looking at the downsides of honey bees for commercial fruit tree pollination. Honey bees are not the “the biggest problem” threatening native bees, just one of them. The arguments that man depends on A. mellifera for food crops are balanced out by how they out-compete native and solitary species, requiring controlled management to avoid environmental imbalance.

There are also plenty of crops only pollinated (or only “efficiently pollinated”) by non-Apis species, like blueberries and tomatoes being more receptive to the buzz pollination of bumble bees (reviewed in Cooley & Vallejo-Marin, 2021). In the absence of A. mellifera, apple pollination is ensured by wild species like Bombus or Osmia. A variety of solitary soil- or stem-nesting insects also pollinate apple blossoms, nesting directly within the orchard or in the surrounding vegetation.

If farmers are interested in building up their populations of wild pollinators, then they can start with providing non-crop (native) flowering plants in adjacent areas. Over time, the insect numbers will increase and then likely visit the farming area during flowering season, providing an alternative pollination source that could save the farmer money from trucking in bees for commercial pollination.

Habitat Loss is More Harmful to Wild Pollinators than Honey Bees

Endnote: I think that loss of habitat due to urbanisation is far more harmful to native pollinators than honey bees. Multiple studies show that other insect pollinators can survive alongside managed honey bee colonies, but they cannot survive the loss of their habitat, or specific flowering plants. Even if governments were to focus on “rewilding their areas”, the presence of pesticide use in farms and gardens, as well as the invasion of other plant species does not revert the area to its pre-settlement stage.

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Footnotes

1. Apple flowers are either annual (flowering once a year) or biennial (Flowering once every two years), not perennial (flowering multiple times a year)… yet. But, would a perennial apple fruit taste as good as an annual? ↩︎
2. Could this year-on-year decrease in fruit set be a result of increased recessive genes as a result of inbreeding? Probably. ↩︎