Diversity key to pollination
One challenge for the avocado industry is to improve orchard productivity and pollination is right up there.
Eight years of avocado pollination research work can now be used by the industry in practical ways, Dr David Pattemore told the New Zealand Avocado Industry International Conference.
He’s the team leader, apiculture and pollination at Plant & Food Research, which has 19 staff in both New Zealand and Australia. His diverse research projects cover the behaviour of pollinators for fruit, nut and vegetable seed crops.
He said New Zealand’s climate pushes the avocado’s boundaries beyond its natural range.
“We’re really on the edge of the avocado world. Fruit set rate is a low 0.3 percent. Three fruit from 1000 flowers isn’t unusual for mass-flowering trees from diverse tropical ecosystems. It was never the plant’s intention to set all flowers into fruit.”
Deliberately cross-pollinating by hand on selected branches can get set rates of up to five percent or 50 fruit from 1000 flowers.
“Such work shows the gap between normal pollination and the potential, even to go from 0.3 to 0.6 percent, represents a doubling in yield,” he said.
Avocados have bizarre flowering behaviour. Each flower opens once in a female phase, then closes and subsequently opens on a different day in a male phase. It’s complicated further with two different types of cultivars. Hass is a type A cultivar in which female flowering typically occurs during mornings, then close and open as male the following afternoon. In contrast, type B cultivars open as female in the afternoon, close and next morning open in the male phase.
Only male flowers produce pollen and this is why orchards are traditionally established with a mix of cultivars, so when Hass flowers are female, male type B flowers are producing pollen and providing enough to be moved onto them.
“Cultivars are entirely synchronistic – all trees go female, they close, and then go male but there can be a period of overlap in the middle,” he said.
Most overlap is around midday when temperatures are warm before female flowers have fully closed and males begin opening, but the key ingredient now is insect activity.
Researchers had to find out how much pollen different insects deposit when they land, their activity patterns and their cultivars of choice. Studies show that as the number of avocado pollen grains on the stigma deposited increases, so does the probability of fruit set.
“If you have one pollen grain there’s a lower probability of fruit set than if you have 50 pollen grains, there’s a direct relationship,” he said.
“Interactions and competition occur with pollen grains and it might also be due to the viability of pollen grains – that means, to have a good probability of fruit set, dozens of pollen grains need to be deposited on that stigma.”
Over several years scientists studied flower stigmas and counted how many pollen grains were deposited. Time after time Australian and NZ orchards showed similar patterns with the vast majority of flowers at the end of the female flowering period having zero pollen grains deposited.
“We need to find ways to increase pollen deposits on avocado flowers, because that will remove pollination as one of the limiting factors for production,” he said.
Most honey bees landing on Hass female flowers had less than 20 pollen grains.
“If you just have 20 pollen grains on a bee the probability of getting enough pollen grains on the stigma is vanishingly small and shows a problem; bees visiting flowers don’t have sufficient pollen grains.
“Our studies show bees carry so few pollen grains that there’s no harm in increasing other pollinator activity in orchards. More insects increase visits during male flower opening, and this increases the probability of more pollen grains being deposited on stigmas, which builds up enough pollen grains for good fruit set.”
Research shows the critical effect temperature has on flowering. One three-year study had cameras and temperature monitors take recordings of avocado flowers every five minutes during flowering. As temperatures declined at night female flowers opened later the next day. Minimum overnight temperatures of 11°C and 16°C had flowers open in the ideal time for honey bee activity from 9am to midday.
But as temperatures dropped to 9°C – 11°C flowers opened between midday and 4pm, past the peak of honey bee activity. Even colder temperatures from 4°C – 8°C had flowers not opening until 2pm, and sometimes as late as 6pm.
And studies found whatever phase the flower is in when the sun sets, it essentially freezes in that phase overnight.
“Not only do flowers open in the late afternoon when honey bees are heading for bed, but they stay open all night and don’t close until about around 9am the next day,” he said.
“This is important to know if we want to maximise the amount of pollen grains being moved and deposited. In cooler temperatures, honey bees are not the ideal insects because they’re not active in those conditions.” Honey bees have a distinct activity patterns which peak around midday to 2pm, but other insects are active in afternoon, evening and overnight.
NZ observations show significant numbers of male flowers open and producing pollen most nights, regardless of the temperature which represents a real opportunity to harness pollen transfer.
Diversity of insect pollinators means diversity in insect activity patterns. Australian research proves some orchards don’t have any honey bee activity and good pollination is secured almost entirely by flies and beetles. NZ orchards have low numbers of alternative insects but David’s team is looking at ways wild pollinators can be used and heavily boosted in numbers.
Moths could provide help because they’re present and busy at night in avocado orchards, however as yet it’s not known which species might transfer pollen.
Bumble bees are an exciting alternative to honey bees because their large and hairy bodies cross-pollinate more effectively.
Variability is the rule.
“We know that pollinators vary in activity, behaviour and efficiency,” he said.
“Key in thinking about the pollination cycle is there’s not much we can do about biological facts about the avocado plant’s flowering, however, we can do things about the overlap of cultivars when setting up orchards.
“We can use different pollinators and that’s what our work has come to – focusing on how we can pick the right pollinators, then boosting their numbers so we can ensure we can get sufficient pollen flowing onto the female avocado flower.”
“We need to find ways to increase pollen deposits on avocado flowers, because that will remove pollination as one of the limiting factors for production.”
Dr David Pattemore