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Pollination under cover with honey bees

Dave Black

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The number of kiwifruit blocks covered by a canopy is increasing. These canopies consist of a hail netting supported on rammed posts, and can cover a considerable area, thousands of square meters. Many, but not all, are fully enclosed with netting down to ground level along the sides. From a grower's perspective these provide some substantial benefits. Obviously, given the name, one is protection from hail. Even unnoticed hail damage can cause a significant fall in the return a grower gets for their fruit. Another benefit is an almost total reduction in bird damage to buds and fruit, and any waste due to bird lime. The benefit that may have pushed these constructions 'over the line' is PSA protection. While the canopy may increase winter chilling (a good thing!), it certainly does protect the plants from wind damage. Broken shoots are an important point of entry for the bacteria, so this is why full enclosures are becoming more popular. There is an opinion that, having reduced the level of inoculum inside the enclosure, the canopy helps to maintain a phytosanitary environment within.

 

Beekeepers are more reluctant to come onto orchards and place hives these days. Access to an orchard at night (when most hives are delivered) is often problematic, and the current phytosanitary requirements do not make it easy for a beekeeper on a tight schedule operating in the dark. It is getting more common for hives to be delivered to a 'dump' site from which they are distributed by the grower, and for the grower to undertake stimulant feeding. This reduces the amount of foreign traffic in the orchard. Beekeepers are particularly unwilling to enter a canopy, either to deliver or feed hives.

 

But what no one really gave much thought to is how to get the flowers pollinated; at the time; racked with fear about PSA it was perhaps optimistic to think orchardists would figure out honeybees might not fare to well in a cage. In the last three or four years people have realised that pollinating under a canopy isn't, after all, a simple thing, and there is a move to look at how it might be done effectively by resolving the problems honey bees face, or by using some alternative.


The optimum placing of hives inside a canopy has not been determined, but may well be different compared with an uncovered block. There are clues that suggest things are not quite the same. There can be a drift of bees from hives in the centre to the hives on the margins near the netting. The distance between male and female vines may be more significant, within a decline in successful pollination with increasing distance from a male, and some anecdotal doubt about effective cross-pollination. With little information about the density of bees, or the density and distribution of males, it’s difficult to know if these are problems that would have existed anyway and are unrelated to an enclosure. It is also difficult to make a judgement about the stocking rates required. Logically, in a confined space without competing forage, less hives per hectare would be assumed.


Often large numbers of bees are seen flying at the canopy mesh; those that find a way through are unable to return. A bee’s eyes probably only see a featureless, bright, white plane rather than a barrier, and in a more natural setting bees can fly towards and through bright areas. Some trials of black mesh which apparently lessen the problem probably just relocate it. Bees flying inside the canopy appear to have considerable difficulty relocating their hive, possibly because of the uniformity of the geography within.


By far the most serious problem for hives working under cover is bee mortality. In a fully enclosed canopy there is nothing available to sustain large colonies of honeybees. They must be provided with a water source and will have to be fed. A newly delivered pollination unit will have a great need for pollen by design, because we normally want a hive at this stage of growth to maximise the effect it has on pollination. However, it will only contain a few days of pollen reserves (bees do not store large quantities of pollen), and the pollen gathered from the vines in the orchard is poor quality, without the complete set of amino acids the bees require for adequate protein nutrition. Supplementary feeding is possible with manufactured protein supplements and carbohydrates (sugars) but it's important to know these are not replacements for a natural diet. Bee mortality from hives in an enclosure is extremely high. Large numbers of foraging bees may fail to return because of an inability to navigate adequately, and those that do return deliver 'junk food'. A hive starved by both quantity and quality deteriorates very quickly. The loss of bees affects the colony's ability to regulate temperature and care for brood, young bees that normally care for brood begin foraging prematurely, protein deficiency encourages brood cannibalisation, the queen will stop laying eggs that produce new bees, and the hive enters a spiral of decline that takes months to correct. Not only are the hives unsuitable for follow-on pollination work they are incapable of honey production too, effectively rendering them useless for that season. Rather than pollination being priced at a marginal cost, as it is now, a beekeeper would have to attribute the full annual cost of a pollination unit to a 'one-time' use.


Honeybees use a variety of strategies to navigate, and are able to communicate (in distinct dialects) distant locations to nest-mates. They have good memories, understand time, and can measure distance. They are able to use the sun as a compass and compensate for its apparent movement across the sky but, as they cannot actually see the sun itself, it’s probably more accurate to think they are using a combination of brightness, size, UV, and light polarization. When the sun is not visible (to a bee) they use the angle of polarised light in the sky, and there is plenty of evidence to suggest they are sensitive to magnetic fields. Bees also have a good memory for landmarks. These have to be substantial given their visual acuity; patterns have to fairly large or really close to be any use, on a geographic scale lines of trees, the shape of the horizon, shorelines, roads, boundaries, buildings and so on. Close up, bees use their eyesight and olfactory clues (scents), and have good pattern recognition, colour vision peaking at the UV, blue and green end of the spectrum, and fast, accurate odour perception.


It's quite possible that for much of the time the light transmitted through a canopy does not contain the navigational 'data' that bees need, although there doesn't seem to be any studies to either discount or support this assertion. There is evidence of this in the case of some type of greenhouse. [See Tjeerd Blacquière, Jeannette van der Aa-Furnée, Bram Cornelissen & Jeroen Donders. (2006). Behaviour of honey bees and bumble bees beneath three different greenhouse claddings. Proc.Neth.Entomol.Soc.Meet – Vol. 17]. It is also very likely that an enclosure will limit both the aspect and size of navigational 'landmarks' the bees might use, and the height and angle from which they can be viewed. They have no view of any horizon. In the absence of good spatial information they would need to adopt a more 'route-based' strategy and will be looking for 'close-up' features to use for image matching and as waypoints, and may depend on odour trails. These are fairly easy features to provide, various coloured disks nailed to the top of each post for example, or large graffiti on the canopy! No doubt, aids to navigation are better placed above the canopy than below, and some should be associated with the hive location only.


Another thing to consider is whether the current standard for a pollination unit is appropriate for enclosures. Previous trials with honeybees in greenhouses and tunnels suggest it’s probably well worth trying the use of hives that are composed of 'naive' foraging bees, that is, bees that have yet to take their orientation flights. Bees that have already learnt to orient and have to 're-learn' the new location will do so within a pre-existing paradigm, whereas naive bees will learn within the constraints provided by the enclosure. Such hives are fairly simple to provide, but it would be worth experimenting with both queen-right and queen-less versions to understand their foraging capacity, hive density, and so on. There may be advantages combining the use of these hives with dry artificial pollination. Such a system would be comparable to the use of bumblebees, often used in enclosed spaces, but provide greater numbers of foraging bees.


Any enclosed hive is going to need dietary supplements for both pollen and nectar, adding to the burden of maintenance and cost. Liquid protein feeds may be particularly suitable in this instance (eg: Megabee). It's not just the cost of feed and labour. Beekeepers have to be careful about feeding hives at this time of year because any food the bees choose not to consume and store instead invariably ends up in the crop produced at the end of the year. In particular, it is easy to contaminate a honey crop with sucrose sugars that render the product unfit for export. Hives that are supplemented may be set aside for pollination use only, limiting their productivity for the beekeeper and potentially increasing the cost to the orchard. The use of small hives that are less demanding of their foraging force may be appropriate, and lessen the problem of ‘overstocking’ the enclosure given the limited forage inside. They may also better control the risk for the beekeeper, and cost for the grower.


While growers may think it important to place hives as close to the vines as they can, if a sufficient number of units are used there may be no good reason for distributing bee colonies inside the canopy. In blocks where hives are placed by the edge of the covered area (there are no side panels) perfectly adequate pollination has been achieved for the vines inside. This may not always be the case, but beekeepers are much more likely to favour this kind of arrangement as they are more able to keep control of their asset, and the hives are likely to be self-sustaining. Placing hives at a (say) 10m by 6m opening in the canopy side is possible without creating a wind tunnel (the hive between the opening and a matching 10x6 screen, and at this time the sides are not necessary for excluding birds - buds and soft growth are over, their normal food is available, and no fruit is in the orchard. Alternatively (we don’t know the answer yet), placing hives throughout the area, and particularly in corners, may help to ‘mop up’ lost and drifting foragers, giving them a home even if it isn’t the right home.


Are there viable alternatives? Artificial pollination clearly is one, although beyond the scope of the discussion here. However there is no doubt there are high performing orchards where this is the method of choice. There is a current project studying the feasibility of bumble bees as an alternate pollinator (Zespri/Plant & Food). New Zealand is not particularly well-blessed with ‘manageable’ pollinators (that is after all why bees were introduced) but the Bombus bees are worth considering. It’s not clear that the advantages claimed for them apply to kiwifruit, or why in large blocks (rather than greenhouses) they would not suffer from the same navigational issues as their smaller cousins. They are however known for adopting a ‘trap-lining’ strategy when foraging, and this might be an advantage. Perhaps looking at the needs of bumble bees will go some way to re-evaluating the commercial and ecological value of other unmanaged and ‘minimally managed’ pollinators.


Not only kiwifruit growers face challenges. Besides the usual seasonal variation that impacts on honeybee survivability, beekeepers already battle the effects of two invaders, the varroa mite and species of Vespid wasp, and there are more to worry about on the horizon. In the autumn wasps, which arrived in the 1940s and 1970s, can completely destroy a honeybee colony in a few hours, and they seem to have a greater impact each year. Since the turn of the century varroa and its associated viruses necessitate a permanent combination of costly management and medicinal measures to keep colonies in good health or they will die. World demand for honey is high, altering the balance between the competing interests of production and pollination in favour of production, while the local demand for pollination is growing as the new gold variety expands. While the market for services will react to the cost and supply of pollination units it is far better to be proactive and anticipate change, rather than cope with its consequences. It's more important than ever that growers think about and plan their pollination requirement well ahead, and that they keep a communication channel open with their beekeeper, particularly when they are considering changes (like covered blocks) to their growing practices. That way beekeepers will be able to adjust the service, technique, and supply to suit the change in circumstances.

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