According to a research article published in Nature,  the idea of pollinator independence was put to the test using a popular self-fertile almond variety: the Independence almond variety.

The research team included people from these institutes and companies:

  • Grupo de Ecología de la Polinización, INIBIOMA, CONICET-Universidad Nacional del Comahue, San Carlos de Bariloche, Rio Negro, Argentina
  • Centro de Investigación en Abejas Sociales (CIAS) (IIPROSAM-CONICET), Universidad Nacional de Mar del Plata (UNMdP), Funes 3350, CP 7600, Mar del Plata, Argentina
  • Beeflow Inc. - Smart Pollination Services, Los Angeles, CA, USA

There is a growing need of animal pollination in global agriculture, with about 10% of the world's agricultural output provided through pollination-dependent crops. However wild pollinators are on the decline (cite) and addressing pollination-dependence is becoming more of a problem each passing year, despite growers using managed honey bees.

To understand the effect of pollination dependence on the almond industry, one has to imagine the fact that over 1.8 million colonies were shipped into California in 2018, compared with the approximately 600,000 colonies native to California (USDA, 2018; California Department of Food and Agriculture, 2018). Almond pollination now makes up a third of U.S. beekeeping income, and many commercial beekeeping operations depend on the almond pollination market (Lee, Sumner, and Champetier, 2019).

During the last 50 years, managed honey-bee population has increased by 45% globally. However this increase is still not enough to address the global pollination crisis because pollination-dependence has increased by about 300% during the same period. Despite conscious efforts to increase domesticated honey-bee hives around the world, the growth is slow to address pollination needs without the help of wild pollinators.

Growers also need to consider the cost of maintaining effective pollination: the average rental rate for a single honey bee colony has increased from ~$70 in 1995 to an average of ~$180 in 2018. After accounting for inflation, costs of renting colonies are now ~60% higher than two decades ago.

Recognizing the complexity of this problem, governments set up breeding programs that focused on producing plant species that could yield crops of same quality with a reduced need for pollination. The Australian almond breeding program can serve as an example of these self-fertile almond breeding programs. In this program, 76 different almond cultivars were used as parents and over 29000 almond seedlings were produced for evaluation.

Such breeding programs have multiple goals when seedlings are evaluated. In a country lacking a diverse set of almond cultivars such as Australia (Nonpareil is accounting for over 50% of plantings), the main goal is producing a variety of almond seedlings that adapt to Australian soil, climate and ecosystem better.

Australian almond breeding program intends to address these significant shortcomings:

  • low quality (kernel & shell)
  • disease susceptibility
  • physiological disorders
  • virus infection
  • poor coincidence of flowering times

Old cultivars and rootstocks fail to perform well under Australian soil and climate conditions. While optimizing for the Australian environment, self-pollination is also among the selection criteria for new almond cultivars:

  • High yield
  • Self pollinating (self-fertility)
  • Nonpareil type and shape
  • Compatibility with Nonpareil
  • Blanchability
  • White kernel color and golden testa
  • Large kernel size i.e. minimum kernel weight 1.24 g
  • Double kernels at < 5%

Self-fertility ranks highly together with improvements in nut quality in such breeding programs (see other examples from UC Davis and Agricultural Research Service)

As a result of these breeding programs, growers are able to access many varieties labeled as "self-fertile" and sold as pollinator-independent. However the level of this pollinator-independence and how it affects yield quantity or quality remained largely unknown. Controlled experiments investigating the effect of pollinators on almond quality or quantity were needed to clarify the impact of bees on self-fertile almonds.

According to 2019 California Almond Acreage Report, a self-fertile almond variety from Zaiger Genetics - called Independence quickly gained popularity among growers, rising to be the second most popular almond variety grown in California. We previously wrote about Independence almond variety on as well. This was a breakthrough for almond industry considering the high capacity for autonomous self-pollination of the Independence variety and the fact that it has seen massive adoption: the area cultivated with Independence increased exponentially since 2008. However there wasn't much detail on how pollinator-independent and how productive this variety was before this study in Nature.

The hypothesis in the study is that if this variety were fully self-fertile and capable of autonomous self-pollination, a similar fruit set, kernel yield and nut quality should be observed in these trees as in those not visited by bees. Meaning, the tree would be able to fully pollinate itself so that there would be no room for improvement to be gained from wild or managed bees.

Self-fertile trees, better with bees

  • 10 trees out of 30 almond trees of the Independence variety were covered with fine mesh (from Green-Tek) that could keep bees away from the trees.
  • 10 other trees were kept open and fully accessible to bees.
  • 10 remaining trees served as control, covered with a mesh above and along a fringe on the sides. This limited mesh coverage (which still left the tree accessible to bees) was performed to understand possible effects of reduced radiation and frost.

Surrounding the experimental field, honey bee colonies were placed at a rate of 5 colonies per hectare, which is regarded as a standard rate for varieties that are not self-fertile.

Honey bee colonies were placed at a stocking rate of five colonies/ha surrounding the experimental study fields, which is a standard stocking rate for self-incompatible varieties

60% higher fruit set in Independence with bee pollination

The experiment accounted for the June drop by taking two measurements of developing fruits: One after bloom and one right before harvest. David Doll, who served as a Nut Crop Farm Advisor for the University of California, explains these three periods of nut drop:

  • After bloom, there will be defective flowers falling from the tree.
  • A month after bloom, pea-sized non-pollinated flowers fall together with some non-fertilized larger nuts.
  • 6-7 weeks post-bloom, resource competition hits some of the fertilized nuts and these resource-constrained nuts drop.

Recorded about 3 weeks after the end of flowering, initial fruit set was observed to be 90% higher in the trees visited by bees.

  • Bee-isolated trees: 0.42 ± 0.02 fruits/flower
  • Open-pollinated trees: 0.79 ± 0.01 fruits/flower

Measured right before harvest, final fruit set was 60% higher in bee-pollinated in the self-fertile almond trees of Independence variety.

  • Bee-isolated trees: 0.19 ± 0.01 fruits/flowers
  • Open-pollinated trees: 0.30 ± 0.02 fruits/flower

These results were statistically significant.

The increase that is still existing right before harvest means that the fruit set increase supplied by bee-pollination persist until fruit maturity despite many fruits aborting.

20% higher kernel yield in Independence with bee pollination

Kernel yield was 20% higher in bee-pollinated trees.

  • Bee-isolated trees: 4.49 ± 0.18 kg per tree
  • Open-pollinated trees: 5.53 ± 0.39 kg per tree

A high oleic/linoleic acid ratio is regarded as a good indication of increased shelf life and improved health benefits. Fat content composition is tracked as a determinant of longevity and stability of almond and almond oil as well. According to (Kester et al. 1993) and (Senesi et al. 1996), high oleic/linoleic acid ratio of fatty acids affects the degree of oxidation and points to a more stable oil that has better shelf life.

Similar nutritional quality was measured with oleic/linoleic acid ratio in kernels from both bee-isolated and open-pollinated trees.

  • Bee isolated trees: 2.63 ± 0.04
  • Open-pollinated trees: 2.52 ± 0.06

Key takeaway: Mind the bees

The research article clearly states that growers would achieve higher crop yields if they rely on either wild or managed bees to pollinate their self-fertile almond varieties.

Advertising these varieties as pollinator-independent might be pushing growers to lose out on a significant 20% extra return on their almond production.

Independence, possibly the best breed out of current self-fertile varieties, is truly, effectively self-fertile but not totally independent from pollinators in maximizing yields.

Recognizing that bee pollination introduces a colony renting cost on the grower's part, researchers calculated that even with colony renting costs, growers would miss out on 10% more profit if they relied on fewer colonies than the standard stocking rate.

These proposed benefits were reported considering young trees in the experiment. There could be additional benefits when a full production plantation is considered.

Key takeaway: Independence keeps a high nutritional quality

In a previous work from 2014, Brittain et al. concluded that self-pollination lowered nutritional quality of almonds. This work indeed shows that we cannot generalize this conclusion to all almond varieties.

Independence variety that was selected to be efficiently self-fertile from the start. As researchers note, Brittain et al. observed the effect of self-pollination on Nonpareil which is known to be a self-incompatible variety with an existing handicap in pollinating its own flowers.

Independence demonstrated a nutritional quality measure (oleic/linoleic acid ratio) of 2.5, which was well within the expected range (1.79–3.79) of Californian almond varieties.