I wanted to write this article on bees; honey bees specifically, as they have always fascinated me and I wanted to learn more about them. Whilst accumulating and refining the content I remembered that this platform is not about me 😉, it’s about the mac farmers and how they can improve through exposure to TropicalBytes content. The article then morphed in to one on Pollination. But, as I got into it, I discovered that this, also, is not an end in itself; farmers are interested in whatever results in high quality nuts in large quantities. So, this article is going to look at that goal, and how much pollination even has to do with it?

Photo credit: Australian Macadamia Society

First off, I learnt that pollination is essential! Without it, there will be NO nuts. Below, I will explain why this is so and how can we support this process in line with our goal – LOTS of QUALITY nuts.

Let’s start by examining where and how pollination happens (in the flower) – understanding the mechanics of the process will help us identify where, when and how we can support it.

The timeline below indicates the critical phases of flower development, which are pollination and nut set:

And here is a close-up examination of each step:

BUD DEVELOPMENT

FLOWER DEVELOPMENT

Six to seven days before flowering, the style (tube-like part of the female portion of the flower) begins to bend. The anthers (male bits, containing the pollen) burst open several days before the flower opens (while they’re still enclosed in the perianth tube) and the stigma (tip of the female part of the flower – the bit that receives the pollen) only becomes viable after the flower opens. Effectively making the process of self-pollination (a flower pollenating itself) impossible.

This characteristic is known as protandrous which means that the male reproductive organs come in to maturity before the female. So, even though the female stigma brushes right past the male anthers with all that ripe pollen, she’s too immature to do anything with it. I’m guessing that God was forcing a measure of cross-pollination with this design feature? Throughout nature we see that genetic variation produces healthier progeny.

 FLOWER ANTHESIS (opening)

Up to 500 (some sources claim 800) flowers are carried on each raceme. Each flower is perfect in that it has both male and female parts however, it is incomplete as it does not have petals. Instead of petals, there are four petaloid sepals. A sepal is that green leaf/petal that encloses a bud (as seen on the rose below).

The pistil (female organ of the flower: stigma, style and ovary) is in the centre of the macadamia flower. The ovary and the lower part of the style are densely pubescent (hairy), while the upper part of the style is glabrous (hairless). The ovary is egg shaped and tapers into the slender club shaped style, which becomes thicker towards the apex where the stigma starts.

The male parts of the flower are made up of four perigynous (arising beside the ovary) stallions, each with two anther sacs about 2mm long. They adhere, about two-thirds of the way up the floral tube, by short filaments to the petal-like sepals. The anther sacs contain all the pollen.

Another factor substantiating Nature’s intention that macs to be pollinated is that the flowers have nectar, thereby attracting insects. If there was no need for “outside assistance”, would there be nectar?

Anthesis (flowering time from opening of the flower until withering) is at its peak from August to September.

The flowers that don’t fall off are mostly fertilised and represent the “initial set”.

After anthesis is completed (and the flowers wither) an exponential burst in growth is noticeable for about four to seven weeks (October to November).

Up until 60 days after anthesis, young fruit will drop continuously, with the remaining fruit being fairly well attached due to the thickening and lignification of the rachis. The drop of young fruit is greater on racemes with a high initial set than on racemes with a low initial set. It has been suggested that the cause of this young fruit drop may be as a result of competition between individual nuts for food, water and mineral nutrients which is enough incentive for us, as farmers, to supply whatever the trees need prior to this phase. In some cultivars, a heavy set may also impact the following year’s crop.

Depending on the nut set, the rachis becomes thicker in relation to the size of the cluster of nuts on the rachis.

So many flowers to so few nuts. Note the thickening of the rachi up to the single nut.

So, that’s the basic science of it. Now, how can we support it?

Macadamia trees don’t seem to have any issues generating an abundance of buds or in developing those buds to flowers. The issues, from a mac farmer’s perspective, seem to lie in optimising nut set and limiting nut drop. What factors affect those developments?

• Pollination – undoubtedly the fundamental requirement for nut set. Factors affecting the quality of pollination are:
  • Cross pollen availability, which is affected by:
    • Proximity of different cultivars
    • Timing of the flowering
  • Pollinator (insect) efficacy, which is affected by:
    • Numbers, which are affected by:
      • Spray programmes in the orchard
      • Lure of your orchard vs other food sources
      • Supplementation (hiring in hives or establishing your own hives)
      • Predators
    • Health, which is affected by
      • Natural environment (shelter options, diseases, food)
      • Spray programmes in the orchard
    • Weather – insect activity is reduced by cold, wet and windy conditions.
  • Quality of that nut set – is this a healthy embryo? Will it last to full term? Just as in the fauna world, weak, poorly-bred and diseased embryos will abort. Factors affecting this are:
    • Genetic quality. Cross pollination will surely improve this.
    • Pathogens present during flowering (fungi etc)
    • Health of the host (mother tree)
      • This is greatly influenced by the environment and nutritional resources.
    • Physical integrity of the embryo (no mechanical damage by insects or the environment – weather etc.)
  • Tree capacity – Once the ovary is fertilised (pollination complete) a nut is set. But even the strongest, healthiest macadamia tree is not designed to carry all the nuts that set, especially if fertilisation was highly effective. We can see this by the mere fact that a raceme is not big enough to carry 500 nuts. It is impossible for all flowers (even if they’re all effectively pollinated) to make it to final set. November dump is when the tree aligns its load to its capacity. By increasing the tree’s capacity, we can increase the final nut set (final load), limited only by the size of the raceme. Factors affecting capacity are:
    • Tree health. In assessing its health, a tree does not take only the current condition, it uses recent historical knowledge as well (experts suggest 2 to 3 years back), which is a result of:
      • Genetic characteristics
      • Water availability
      • Nutrition availability
      • Disease pressure
      • Environmental conditions (climate)

A tree may re-evaluate its capacity if the environment changes ie: inclement weather (moisture or temperature-related) that threatens tree health.

• Mechanical damage
  • Flowers (thrips). This is another topic I plan to explore – what are those little critters doing in the flowers – eating them or pollinating them, or both?
  • Weather (hail etc)

More on pollination …

A three-year study of macadamia pollination in Queensland, Australia found pollination to be a limiting factor. They selected racemes and compared natural pollination to supplemented pollination (where the scientists actively assisted the pollination). The results suggested that natural levels of pollination, due to the activities of flower visitors, were suboptimal. If whole trees respond to pollination in the same way as racemes then improved pollination may increase commercial yields of Macadamia. Several factors may cause inadequate natural pollination, including: (1) low pollinator populations and/or pollinator activity in the orchard, resulting in insufficient visits by pollinators to racemes; (2) insufficient supply of cross pollen in the orchard; (3) bee behaviour resulting in transfer of self-pollen but not cross-pollen.

More on cross pollination …

Hand pollination studies indicate that inadequate cross pollination contributes to low nut set and thus low yield. Typically, just 0.3% of flowers develop into harvestable nuts. Lack of nut development after flowering and early abscission are indicators of poor fertilisation of the ovule. Honey bees (Apis mellifera Linnaeus, 1758) are routinely reported as the most common insect visitor to macadamia flowers, and are widely regarded as the most important agent for transferring pollen between flowers. Stingless bees, beetles, flies and birds also visit flowers and have been considered as pollinators.

Cross pollination has been reported to increase nut weight as well as the number of nuts. I found this presentation, by Prof Stephen Trueman, to the Green Farms Nut Company suppliers, absolutely fascinating. Thank you, Barry Christie, for allowing me to share it on TropicalBytes.

https://youtu.be/co4Dme4lQs0

On my recent trip to the Cape, where I interviewed five top macadamia farmers, I learnt that cross-pollination is of particular importance in that region. One Jaff (an experienced and successful ex-Levubu farmer) explained how he almost lost everything because he hadn’t understood the specific peculiarities of that area; once he started top-working precocious, effective cross-pollinators into his Beaumont blocks, yield went up 7 times!! He can not emphasise enough how important cross pollinating is to profitable mac farming in the Cape.

Why can’t macadamias be pollinated by wind?

I have heard some people say that macadamias can be wind-pollinated. But they do not have any of the typical features of a wind pollinated plant. Wind pollinated flowers typically have a very large area to receive pollen, have pollen that disperses easily with the wind, and do not produce nectar. Macadamia has none of these characteristics. The flowers produce nectar in order to attract pollinators. The pollen is sticky and forms clumps, and is not likely to be dispersed easily by wind (Schroeder 1959). The area that receives the pollen, the stigma, is very small, for example, the stigma of 246 is only 0.012 mm2 and A4 is even smaller (0.008mm2) (Wallace 1994). When the flower opens, the stigma is almost completely obscured by large self-pollen clumps (Wallace et al. 1992). These got on to the stigma when it passed by the anthers as the flower was bursting open. These would need to be dislodged before a cross pollen grain could land on the stigma.

Some work has been done in Hawaii on wind pollination using pollen traps (Urata 1954). In perfect conditions for wind pollination, only one pollen grain landed on the trap every 2.18 mm2 in 24 hours. This would equate to less than 2 flowers per raceme for 246 pollinated by wind and 1 flower per raceme in 24 hours for A4 (assuming 300 flowers per raceme).

Summary

Obviously, as the industry grows, so will research into all factors affecting yield and quality. Pollination is already a key focus area and I look forward to bringing you further updates as they develop.

And now, some fascinating information on bees, the creatures we primarily rely on for the essential activity of quality pollination.

BEES

Bees collect nectar to make honey and pollen to make bread.

Honey: The bees use their probiscus to suck nectar into special ‘honey tummies’ where it is mixed with a unique enzyme. When they return to the hive, they transfer (regurgitate) this nectar into the ‘honey tummy’ of another bee who passes it on to another bee … eventually, when it has been through enough bees and gotten enough enzymes, it is spewed into a honeycomb cell. At this point it is 70% water. The bees flap their wings to generate airflow through the hive and this aids in evaporating the additional water. When it is down to about 17%, they cap the cell with wax and the honey continues to brew in this sealed environment. Because it has an acidic pH and low water content, it is not a good environment for bacteria or fungi – hence the extensive shelf-life. This honey is used, by the hive, when their bread stocks are low. Think of it as the pantry shelf, where Gran stores her preserves. And, just like Gran, the bees also make too much; there are always predators breaking into the pantry (including humans) and they cater for this. Responsible bee-keepers know how much the bees need to get through their lean seasons and never rob a hive completely.

Pollen: A bee’s body is hairy! 3 million hairs hairy! They even have hairs on their eyes! The purpose of all these hairs is to collect pollen. The hairs on their front legs are actually combs that collect and move the pollen grains to the back legs where they store the pollen in corbicula (bags). They squish it in these bags like a vain woman forces too many clothes into a small overnight bag. Then they return to the hive and deposit their loot into a cell, usually around the perimeter of the honeycomb. Here it marinates with a bit of honey and rises, just like human bread! It is used to feed the hive (except the queen).

Bees are surprisingly hairy!

These honeycombs have been cut through to expose the bee bread. Each layer is a deposit. So it probably takes about 10 deposits to fill a single cell.

To determine colony condition, work your way to a centre brood comb, and pay most attention to the interface between the honey above, and the brood below. In this dynamic interface, you will be able to tell whether the colony is hungry or storing honey, and how good their protein reserves are. You always want to see a nice band of beebread around the brood.  Should this band disappear, suspect that the colony is suffering from nutritional stress

Some interesting bee facts:

• Communication dance: Bees are the only insects to communicate. When they get back to the hive with a good haul, they will indicate the direction and distance of the source by ‘waggling’ around. The other forager bees take the advice and zoot off to collect the plunder.
• The Queen bee only eats royal jelly which is made in the heads of the nursing bees. Initially all ‘baby bees’ get royal jelly but then they go onto a diet of bee bread. Only the future queens get royal jelly exclusively. We now know that the bee bread has something in it that renders all the bees infertile.
• Drones are the result of unfertilised eggs so they only have one set of chromosomes. They do no work in the hive and exist only to mate.

Bees and Macs

The primary pollinators in macadamia are honeybees.

Beekeepers recommend between 5 and 8 hives per hectare. The work of bees (nectar collecting vs pollen foraging) can be affected by ‘managing colonies’ through high brood to bee ratios, sugar syrup feeding and pollen trapping or stripping.

Each flower produces relatively small amounts of nectar. The pollen gatherers are reported to be better pollinators than nectar foragers as they are more likely to make contact with the stigma.

I found some interesting points in a local study which indicated that pollination and indeed, cross pollination are perhaps more substantial limiting factors than previously thought:

• Growing demand for pollination services in agricultural production contrasts with declines of wild and managed pollinator populations. (I hear a warning bell …)
• When pollinators were excluded (racemes were bagged), the initial nut set (3 weeks after pollination) was reduced by 80% and the final nut set (15 weeks after pollination) by 54%. I am not sure how the flowers were pollinated with the exclusion of pollinators … perhaps the bags mobilised the pollen when blown in the wind.)
• When flowers were pollinated by hand, initial and final nut set increased by 66% and 44%, respectively.
• The scientists observed almost no wild pollinator species. Instead, honeybees constituted 99% of all visits. Honeybee visitation rates increased when apiaries were a focus on the farm.
• However, (and this one was very interesting) neither initial nor final nut set was related to visitation rates, and the final nut set was actually reduced where honeybee colony density was high, with a predicted 50% reduction in final nut set between the lowest and highest colony densities. The study demonstrated a strong pollination limitation in South African macadamia orchards, where managed honeybees fail at delivering the increasing need for pollination services. Indeed, increasing their colony densities may further limit their pollination efficiency. A pollination management that also includes non-Apis managed pollinators and wild pollinators is possibly needed to increase nut set and provide solutions for increasing pollination service demands. In intensive macadamia orchards, this can also necessitate the need for more pollinator-friendly management practices, including habitat restoration and reduced pesticide application.

Learning from the findings of that study, we need to do more than just refine Integrated Pest Management strategies, we need to develop Integrated Pollinator Encouragement strategies.

WHAT CONDITIONS DO BEES NEED TO FORAGE?

Warm, dry and still. Pollen that is too wet will burst rather than germinate. Honeybees will work on macadamia racemes at temperatures above 7°C, but the warmer it is the more they work (Wallace 1994). Also, bees do not like windy conditions – the more wind, the less work they do on macadamia.

These two factors (wet and wind) are common to the Cape and may be why they have to focus more intentionally on pollinating and, to increase nut set, cross-pollination.

RECOMMENDATIONS FOR GROWERS

• Bees are important for efficient pollination of macadamia. Orchards with low bee populations should consider introducing bees.
• Bees do not pollinate in rainy or cold weather. In these conditions, pollination may be a problem, especially if pollen becomes wet and bursts.
• A shortage of cross pollen supplies may limit results. Orchards need to be designed to allow close proximity of cross pollenating options.

In all the research, I came to this conclusion: there are many factors affecting final nut set. Among them is most certainly pollination and indeed, cross-pollination. We have established that insects are required for this job. Although there are many insects in our orchards, we are constantly killing them with pest management practices. Only those we purposely nurture will be able to serve us. Honeybees seem to be the obvious solution as we know so much about them.

But I also learnt that, as with so many things, it’s not as simple as hiring hives. Maximising your farm’s results requires delicate choreography of an integrated plan; one that supports environmental role-players that we may not be aware of.

Bibliography:

https://www.youtube.com/watch?v=sAKkjD3nEv0

https://www.youtube.com/watch?v=nZlEjDLJCmg

https://www.youtube.com/watch?v=I6C9th9rO0U

www.africanhoneybee.co.za

https://courses.lumenlearning.com/wm-biology2/chapter/self-pollination-and-cross-pollination/#:~:text=Self%2Dpollination%20occurs%20when%20the,flower%20on%20the%20same%20plant.&text=Self%2Dpollination%20occurs%20in%20flowers,land%20on%20the%20flower’s%20stigma.

https://www.researchgate.net/publication/284131639_Pollination_of_macadamia_Review_and_opportunities_for_improving_yields#:~:text=Macadamia%20flowers%20are%20pollinated%20predominantly,2015)%20.%20…&text=Macadamia%20is%20a%20long%2Dlived,commercially%20for%20its%20edible%20nuts.

https://beeaware.org.au/pollination/pollinator-reliant-crops/macadamias/

https://themacadamia.co.za/2020/07/28/macadamia-flowers-explained/

https://watermark.silverchair.com/

https://www.colegiobolivar.edu.co/garden/wp-content/uploads/2019/03/Antonio-Bellini-Macadamia-integrifolia.pdf

https://beeaware.org.au/wp-content/uploads/2014/06/Bees-and-the-pollination-of-macadamia.pdf

https://www.sciencedirect.com/science/article/abs/pii/S0167880918301257#:~:text=Stingless%20bees%20(Apidae%3A%20Meliponinae),of%20naturally%20occurring%20Macadamia%20spp.

https://scientificbeekeeping.com/tag/beginning-beekeeping/