We recommend using bumblebees in combination with honeybees for the pollination of avocado, the addition of bumblebees greatly improves pollination reliability.
Cool temperatures during flowering can change the timing of flower opening, bumblebees provide greater reliability by pollinating during cool weather and by being active for more of the day.
- A minimum of 5 standard hives per hectare, spaced evenly throughout the crop when used in combination with managed honeybee hives. Bumblebee hives should be placed as far away from the honeybee hives as possible, while remaining with in the crop. The bumblebee hives can be used to offset some honeybee hives to minimise the additional cost.
The Biology of Avocado
While avocados (Persea americana) are self-fertile, the need for cross-pollination is climate dependant. In South America, where avocados originate, and in the tropical weather of Florida, cross pollination does not appear to be an important yield limiting factor in some varieties, with trees showing high levels of self-pollination. In other parts of the world, including New Zealand, self-pollination is much more limited, and cross-pollination by bees is important.
In New Zealand, flowering lasts from 5-8 weeks, and can occur from September to December, with peak flowering typically occurring in October and November. A mature avocado tree may bear in excess of 1 million flowers in one season. Flowers are small, usually less than 1.5 cm in diameter. Avocado flowers are perfect, bearing both female and male organs (at different times though, see below), and produce nectar.
Avocados exhibit an unusual flowering behaviour that promotes cross-pollination. Each avocado flower opens twice, over two consecutive days. Initially flowers are functionally female (stage 1 flowers) as the stigma (♀) is receptive, but the anthers (♂) do not shed any pollen. On the next day the flower is functionally male (stage 2 flowers), as they release pollen, but the stigma has by now dried up, turned brown, and is no longer receptive. In South America and Florida, the climactic conditions often result in the stigma remaining receptive during stage 2 flowering. Each stage lasts 3-4 hours, and cultivars are classified according to the time of day that they flower. Type A cultivars flower during the morning of the first day, then the afternoon of the second. Type B cultivars flower during the afternoon of their first day, then the morning of their second day.
Bumblebees are ten times more efficient in performing close pollination (within cultivar) and at least 20 times more efficient at cross-pollination than honeybees in avocados.
Studies have shown that honeybee activity is not correlated with avocado flowering, however, bumblebee activity is, resulting in greater initial fruit set and higher overall yields.
Bumblebees are much less susceptible to adverse weather conditions such as cold cloudy weather, wind and rain. Weather conditions can delay the opening and closing of flowers. Cool nights or cloudy mornings can cause both stage 1 and stage 2 flowers to occur on trees of the same variety in early afternoon, allowing for within cultivar pollen transfer. Cool weather is more detrimental to type B cultivars, as stage 1 flowers (functionally female) may occur so late in the afternoon/evening that pollinating honeybees are no longer active. Bumblebees forage until later in the day than honeybees, and can go a long way towards mitigating this. Bumblebees are also better suited to earlier flowering varieties than honeybees.
Potential Pollination Issues
Avocado pollen and nectar are not particularly attractive to bees (more so for honeybees than bumblebees), so ensure that wild flower levels are kept to a minimum (i.e. keep grass cut). Other flowering trees such as citrus and stonefruits may be more attractive to bees, leading to reduced quantities of bees actively foraging within the crop. In some cultivars, high levels of pollen may be non-viable. Pollinizer trees should be planted closely at a ratio of 1:9 (i.e. one pollinizer tree to 9 main crop trees).
Flower opening is temperature dependant. During prolonged warm weather a significant percentage of flowers will open, and it is essential to have sufficient pollinators present. Stage 1 flowers (functionally female) may not open if temperatures are sub 15˚C, and frosts can damage early season flowers. Trees may flower earlier in the season if the previous crop was too light, which can have adverse effects on pollination. Bumblebee hives are better suited to these early flowering trees than honeybees, which may spend significant amounts of time within the hive, and may not forage at all on some days.
As with all flowering plants, the health and nutrition of the plant can affect pollen production and viability. Poor quality/quantities of pollen will have negative impacts on the bumblebee hive, and may lead to bumblebees searching for alternative pollen sources away from the crop. It is important to ensure that the plants are vigorous and healthy during flowering, and that their nutritional requirements are adequately met. Boron, zinc and nitrogen are particularly important for pollen health in avocados, with leaf boron levels ideally above 28 parts per million (ppm), and zinc ideally above 50 ppm.
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