Ten tips for pollination success

Cross-pollination by honeybees is a prerequisite for setting many pome- and stone-fruit cultivars. How can growers take control of this process? By Anna Mouton.

There’s no recipe for perfect pollination — growers need to fine-tune their practices according to the demands of the site and season. But every grower can improve their results by ticking the ten boxes below.

1.   Cross-pollinators

Pome and most stone fruit are self-incompatible. They generally require pollination by a different cultivar for fertilisation and fruit set. Growers can hormonally assist the development of unfertilised pears, but cross-pollination improves fruit shape and boosts yields.

Cross-pollinating cultivars must be genetically compatible—the details are explained in the article on compatibility in this issue. Fully compatible cross-pollinators must provide at least 10% foreign pollen to pollinate the main cultivar in an orchard effectively. The flowering of cross-pollinating cultivars must overlap.

Bees tend to work down rather than across tree rows. Cross-pollinators are best spaced within rows of the main cultivar rather than planting alternating rows of cross-pollinators and the main cultivar.

Some experts maintain that pollen transfer occurs between bees in the hive and that cross-pollinators’ position in the orchard is less critical — more research is needed.

The topic of cross-pollinators is covered in detail in one of the other articles in this issue.

2.   Sourcing bees

Honeybees are the only significant pollinators in commercial South African orchards, and growers rely on managed hives for pollination services.

Growers are legally required to use registered beekeepers with a DALRRD registration number. All hives must display an identification number.

Not all bee colonies are equal. Professional beekeepers invest considerable resources in optimising hives for pollination, and growers would do well to remember that you get what you pay for. Low-priced hives could be ruinously expensive if they cause poor pollination.

The Western Cape Bee Industry Association advises all their members to sign pollination contracts with growers — growers should likewise insist on written agreements with beekeepers.

The Association has also published minimum standards for pollination beehives on its website. Read more about their standards before diving deep into beekeeping elsewhere in this issue.

3.   How many hives?

There are no hard rules for the number of hives per hectare. Adjust the numbers according to the fruit type and cultivar.

As a rule of thumb, apples need roughly 2–4 hives per hectare, and pears 6–8 hives per hectare. Plums can be anywhere from 2–12 hives per hectare—some cultivars are notoriously tricky to set.

Cherries can likewise have fleeting effective pollination periods, so they need about ten hives per hectare. On average, apricots require four hives per hectare.

Peaches and nectarines are self-compatible and do not need cross-pollination.

More hives are necessary when pollination conditions are suboptimal — very hot and dry or very cold and wet — or if the main crop must compete with more attractive forage. Pollination under nets benefits from extra hives.

Growers must monitor bee activity to ensure they have enough hives — see the sidebar for easy-to-measure metrics.

4.   When to introduce hives

Hives can be brought in at around 35% of full bloom. A second wave can be introduced at around 85% of full bloom if necessary.

Impatient growers should wait until at least 10% of full bloom — 25% may be better — before adding bees or risk the bees straying to alternative forage. Bees can be safely removed at 90% of full bloom.

5.   Timing the delivery of hives

No one wants to wake up in the morning and find themselves trapped in a box while being transported or handled — who needs that? Bees don’t like it either. Daytime deliveries can lead to more losses as agitated bees explode from the hives.

Generally, hives are best delivered at night, so growers should provide beekeepers with easy access and reasonable roads. Bees may benefit from being introduced in colder weather, so they settle in gradually before work starts in earnest.

Some experts propose introducing and switching hives inside completely enclosed nets during the day. They reason that younger foragers are better able than older foragers to learn how to cope inside nets.

Introducing new hives without most of their field force may speed up the recruitment of young foragers. Switching hives while net-adapted foragers are outside potentially forces them to integrate into the replacement hives. It should be noted that daytime hive introduction and switching are controversial.

6.   Inside or outside?

The topic of bees and protective nets is vast — read a whole article about this. The short version is that bees do better outside than inside nets.

Retracting nets to uncover trees during pollination is first prize but could be too risky in bad-weather areas. Retracting a small area of nets directly above hives may be a good compromise between weather protection and pollination performance.

Placing hives outside net structures works well, provided the nets are open-sided, and the netted area is relatively small. Growers should check whether bees are working in the middle of the orchard and place hives inside if necessary.

Hives must be positioned deeper and distributed more evenly under extensive netted areas. Closing the sides of these nets will prevent bees from flying out and getting lost, whereas closing the sides of small areas — less than 15 hectares — will trap and kill more bees in the corners of the nets.

Colonies lose about half their strength within 7–10 days under nets, so don’t leave hives inside netted orchards for more than two weeks.

7.   Happy places for hives

Bees enjoy the same things we do — warmth and shelter — so put hives in a dry spot out of the wind but not in the shadow of windbreaks. The bees will get moving faster every morning if hives face the sun and are lifted off the cold soil. Placing hives on high ground encourages the bees to disperse more widely.

Bees don’t appreciate disturbance. Keep hives away from vehicles and people. Don’t mow during pollination. Bees dislike the dust, noise, and freshly cut vegetation.

When little competing forage is nearby, hives can be as far as 500 metres from the orchard. Beehives can be scattered in small groups around the farm. Clustering hives in groups of 4–8 forces the bees to forage more widely and disperse into more blocks.

It’s unnecessary to have the hives at the heads of tree rows unless they’re close to the orchard — bees tend to stay in a row once they start working in it. Pollination in plum blocks larger than 2.5 hectares is improved if hives are placed in the centre.

Avoid barriers between the bees and the trees. This includes not putting hives against the long side of a tree row or windbreak.

8.   Competing forage

Bees will readily fly 4–5 kilometres to access quality forage, so keeping them focused on unattractive pear and plum blossoms can be challenging. The competing forage should be removed before the bees arrive — think weed control. But sometimes, the competing forage is adjacent wildflowers or another crop.

Introducing colonies in waves exploits the tendency of naïve bees to start foraging near the hive after introduction. The second wave comes in once the first bees have abandoned the target crop. Growers can also delay bees’ discovery of alternative forage by placing hives closer to the target crop.

Beekeepers prefer cover crops and flowering weeds in orchards as these support colony health and reproduction. When hives are inside enclosed nets, alternative forage can stimulate the queen to keep laying, fuelling the hive’s demand for pollen.

However, alternative forage is also viewed as distracting bees from the target crop, necessitating more hives for longer to achieve sufficient pollination. More research is needed on the interaction between pollinators and alternative forage in orchards—hence a new Hortgro-funded project on pollination biology.

9.   Providing water

Always put out water for bees, especially in hot weather and in enclosed nets. Every minute they’re flying around searching for water is a minute they’re not pollinating the orchard.

Water can be provided by opening a dripper line or in deeper containers. Containers must have sticks or floating plant material on which bees can land to access the water.

Don’t expose bees to water contaminated by any form of agrochemical.

10.        Avoiding sprays

All spray applications are bad for bees — even water. A wet bee is a dead bee so avoid spraying during pollination or spray at night if spraying is essential.

Consult the beekeeper before spraying to ensure that the least harmful product is applied in the least damaging way.

Bonus

Checking colonies

The morning after

Growers should visit the hives on the first morning after delivery. They should check that the hives are open and bees are active. There should be no or few dead bees. Bees should not be stressed and clumped under the boxes.

During pollination

Normal foraging usually starts the day after delivery. Given a reasonably still spring day of 20–22 °C, approximately 50 bees should leave the colony per minute. Fewer bees indicate a potential problem with colony strength.

Field strength is the number of bees visiting flowers. This will vary by crop and cultivar, but two new bees arriving per tree per minute is a general starting point. Growers must adjust this figure according to their orchard system, crop type, and conditions.

For plums, two days of pollination with four bees per tree, or even one perfect, sunny day with eight bees per tree during full bloom, should ensure a good fruit set.

Visit hives and monitor colony and field strength twice weekly. Growers should contact their beekeeper immediately if they suspect a problem.

Contributors

The content of this article is largely based on a presentation by bee expert Mike Allsopp of the ARC Plant Protection and Health. View his talk and the rest of the Hortgro Pollination Webinar on the Hortgro YouTube channel.

Thanks to beekeepers Herman Brink and Brendan Ashley-Cooper, and horticultural advisers Nigel Cook and Petru du Plessis for additional inputs.