The net effect on bees

How plum and pome-fruit growers can optimise pollinator performance in netted orchards. By Anna Mouton.

“The pollination of plums is super-important,” says stone-fruit grower and technical adviser Petru du Plessis. “If you have correct pollination in September, you can look forward to a fantastic and profitable harvest. But if your pollination failed, you have the terrible prospect of staring at an unprofitable orchard for the rest of the season.”

Du Plessis champions the erection of nets to improve plum yields and pack-outs. He began netting his orchards at Bon Esperance in Stellenbosch six years ago with a fixed structure. Unfortunately, he soon encountered a problem. “I think I sacrificed about 15 tonnes per hectare during the first two years,” he says, “because the nets limit the bees and slow down pollination.”

Nets baffle bees

“Generally, when bees want to return to their hives, they fly up and out of the orchard,” says pome-fruit grower Mike Leslie. “But under nets, they bump against the tops and get stuck as they try to get out. So you lose some bees like that.”

Leslie has been producing apples under nets for nearly three decades at Clan Leslie in the Free State. Their nets mainly serve as hail protection and are open at the sides. He notes that bees also get stuck when they fly out of the nets and try to return to hives inside the nets.

“If bees are inside the net, they can see gaps to get out because the sun shines from above,” says third-generation beekeeper Trevor Pye. “Once they fly out and go up, they can’t see the box below, and they can’t find the gaps to get back inside the net again, so they die on the outside of the net.”

Du Plessis adds that bees tend to become stuck in the corners of closed net structures, especially in smaller netted areas, reducing the number of working bees.

“I reckon bees work nearer the hives under fixed nets,” observes Du Plessis. “They spend more time on orientation. So I feel that you need more hives and a longer pollination period to get the same fruit set under fixed nets.”

But growers don’t necessarily have the luxury of time when it comes to pollination. Already brief effective pollination periods in many cultivars can be further condensed by unfavourable weather. Weather also influences pollinator activity — honeybees down tools when it’s too hot, cold, wet, or windy.

Furthermore, in many fruit types, the first flowers to open often develop into the largest fruit due to superior flower-bud quality and a longer fruit growth period. Growers don’t want to waste these flowers due to floundering bees.

The open option

Du Plessis has solved his pollination woes by switching from fixed to retractable nets. “As soon as we finish picking the fruit, we roll the nets up, and we roll them open in spring as soon as the bee hives have been removed.”

His nets are fastened to cables that run above every tree row. Each section of netting is permanently attached to a cable above every second tree row and clipped to the cables on either side. Du Plessis uses clips bent from number 4 steel wire. Plastic clips are also available.

Two small teams atop tractor-drawn platforms retract the nets. One team unclips the nets from the side cables while the other rolls them up and secures them to the central cables. The process is reversed to unfurl the nets in spring.

“Opening and closing the nets costs about R5 000 per hectare,” says Du Plessis. “Your plum orchard can potentially generate R600 000 or R700 000 per hectare in income — it doesn’t make sense not to spend R5 000 per hectare to protect that income.”

Du Plessis’s nets are primarily used to prevent wind damage. He explains that plum fruitlets become vulnerable to wind about three weeks after set. Since not all plum cultivars flower simultaneously and his teams can close more than a hectare of nets in a day, there is plenty of time to cover the trees after pollination.

Leslie has adopted a similar strategy for his apples. “We used to cover the trees as they started to flower because the nets kept the orchards slightly warm. But in the last couple of years, we have held back until the bees have finished pollinating, and we think we’re getting better pollination and fruit set.”

An L-shaped alternative

At Sandrivier in Wellington, estate manager Stephan Strauss is experimenting with L-shaped nets over plums. “Our biggest reasons for netting are to prevent sunburn and wind damage,” he says. “And our problem sun is afternoon sun — from the west — which is also the direction of our problem wind.”

When looking down the tree row, the L-shaped nets have a short section over the top and a longer section running from the top toward the orchard floor on the western side of the tree row. The eastern side and the work row are open. Cultivars that don’t develop sunburn have L-shaped nets on every second tree row.

L-shaped nets don’t require an elaborate support system — a standard trellis system with slightly longer upright posts is sufficient. This translates into material and labour savings compared with full-surface nets.

Although it’s early days, Strauss reports promising results with the L-shaped nets. While improved pollination wasn’t the reason behind them, the open work rows free bees to orient naturally and work efficiently.

Bees under fixed nets

The optimal strategy for managing bees under fixed nets depends on the size of the netted area and whether the sides are open or closed. Du Plessis thinks that retractable nets are the best option for plums, especially for smaller orchards.

“I would argue that a block needs to be at least 15–20 hectares before you can enclose it completely with a fixed structure,” says Du Plessis. “Otherwise, the bees become trapped in the corners or on the outside of the nets.”

In Strauss’s experience, complete enclosure — top and sides — of larger net structures is not only possible but preferable to prevent bees from being lost or getting stuck trying to push through nets from the outside. Conversely, Pye would rather see an unnetted section directly above the hives so bees can find their way home.

Hives are best placed outside for open-sided netted areas of less than 10–15 hectares, as bees do fly into netted orchards.

“We put our hives on the northern side of the block, and we put them on tyres, so they’re off the ground because it can still be quite cold at that time of year,” says Leslie. “The bees then work earlier and have full sunlight to navigate.”

According to Leslie, pollination under the nets was fine even when the trees were covered during flowering, but Clan Leslie mostly has netted areas of around two hectares. Growers who put hives outside more extensive netted areas may find pollination drops off towards the centre, as bees are reluctant to venture too far under the nets.

Another problem with putting hives outside is alternative forage. Being in the Free State, Clan Leslie’s orchards offer hungry bees the only flowers — and the only water source — during August and September. However, growers in other areas may find bees turning up their noses at fruit trees in favour of other crops or wildflowers.

In the Boland, at Sandrivier, unattractive plum blossoms sometimes compete with canola. Strauss positions hives at least 50 metres into open-sided netted orchards, hoping to delay the bees’ discovery of tastier forage until their pollination work is done.

Tips to assist bees

“Bees are like people,” remarks Du Plessis. “They need food and water. And if you don’t give them water, they can’t function.”

He suggests opening a dripper line from around 11h00 to 15h00 on hot days to ensure bees can access moisture. Bees will drink from the wet soil.

Clan Leslie has micro-irrigation, and the Free State tends to be parched during spring, so Leslie provides buckets or other containers of water. “We put some sticks or floating material in the water so they can land on that to drink and not fall in and drown,” he says. “If the water is close, they don’t have to spend a lot of energy getting it.”

Leslie thinks wildflowers such as dandelions help draw bees into his orchards at the end of winter when the surrounding countryside has nothing to offer. He is considering planting cover crops or other flowers to further capitalise on this effect. Western Cape research suggests his plan will work.

When bringing in hives, Du Plessis emphasises enabling bees to orientate themselves before diving into their work. He recommends introducing hives at night rather than in the morning or even introducing hives on colder days so bees can ease into things.

If agitated bees are likely to explode from a newly placed hive and get lost, he partially blocks the hive entrance with a small branch to slow them down. Small branches or other markers on hives also assist bees in identifying their own box. Arranging hives so they don’t all face the same way helps, too.

Not everyone agrees on the benefits of cover crops or wildflowers in the orchard, or the best time to introduce hives under nets — read about other views in Ten tips for pollination success.

Numbers and timing

“I think we probably use more hives per hectare — 4–12 — than the average plum producer,” says Strauss. “If you know a cultivar is good, but you need 12 hives to set fruit, that’s part of its production cost.”

Du Plessis similarly adjusts the number of hives according to the cultivar and weather conditions. But he recommends roughly doubling the number of hives if bees are expected to work under nets.

“As a rule of thumb, we try to put about four hives per hectare,” says Leslie. “We often have a quite narrow window for pollination due to high winds and low humidity that dry the apple flowers out very quickly. So when conditions are right, we want many hives to get the pollination done.”

Apple blossoms are more attractive to bees than pears or plums. And some plums are more attractive than others. At Sandrivier, which has up to 45 hectares under continuous netting, Strauss has noticed bees preferentially working on some plum cultivars while neglecting others.

“When a bee is first introduced, it will initially work close to the hive before moving further away,” he says. “So we follow the same strategy as when we had hives outside the nets and were competing with canola — we use waves.”

In plums, the first wave of hives is typically introduced at 35% and the second at 85% of full bloom. Waves can, of course, be used in other fruit types if there’s a risk of bees being lured away by better food sources.

According to Pye, bees operating from hives outside fixed nets remain strong. “In fact, the hives even come up a bit. What I’m against is putting them inside the nets.”

The consensus is that bee colonies decline when hives are under nets, regardless of whether the sides are open. Two weeks seems to be the maximum time the colonies can stay under nets before suffering significant damage.

“Remember, it’s not about me losing bees,” stresses Pye. “The grower is paying for the service and not getting pollination because the bees aren’t working — they’re dead.”

Although Sandrivier mostly has fixed nets, Strauss doesn’t feel this compromises pollination. “I don’t think we get worse set, but we definitely still have a lot to learn.”

As more and more growers discover the benefits of protective nets, the pressure to optimise bee performance under nets will only increase. “In my role as technical adviser, I’ve learnt that, given good weather during flowering, poor fruit set is a direct result of poor cross-pollination or bee management,” says Du Plessis. “Everything comes down to pollination.”