According to this consultant, over-pollination can be as bad as too little pollination.

By Joe Traynor

CAUTION: Excessive use of this product will reduce your almond crop.

If almond growers saw this label on the hives of honeybees rented for pollination, they would think someone was making a joke. After all, everyone knows that honeybees are vital in setting an almond crop. Nonetheless, there is some validity to the idea of placing such a warning label on bee hives used for almond pollination.

There is the possibility that when a bee visits a tree often, it takes away energy the tree needs for nut development.

Almonds are unique among fruit and nut crops that require insect pollination in that a maximum set of blossoms is desired. In terms of percentage flower set, what an apple or plum grower would consider a bumper crop would be a crop failure to an almond grower. Crop thinning is a major expense for many fruit growers, but for almond growers, the more blossoms that set, the better.

A good pollination season in almonds is usually followed by a heavier-than-normal drop of pollinated fruit in succeeding months. Few trees could handle 100% flower set. Post-pollination fruit drop is nature's way of thinning the crop to manageable levels. Often, however, post-pollination fruit drop is heavier than anticipated. Such drop is felt to be caused by a lack of food to the developing nuts; in particular, a lack of carbohydrates. Developing nuts depend on carbohydrates from two sources:
  • Food stored in the tree prior to leaf emergence; and
  • Food manufactured by developed, functioning leaves.

Leaf development occurs after pollination. Until leaves are fully developed, the newly set nut crop must depend on stored reserves for it's nourishment. If cool, cloudy weather reduces food produced after the leaves are developed, then stored carbohydrate reserves assume greater importance than normal.

More to Fruit Set Than Just Pollination

Cross pollination in almonds occurs when pollen from one variety of almonds is deposited on the stigma of another variety. There is no question that honeybees are excellent pollinating insects for almonds. Pollination, however, is only part of the fruit setting process. The pollen tube must fertilize the egg and the newly set almond must survive the March, April, and May drop periods. It is usually late May before one can assess the final fruit set for almonds with a reasonable degree of accuracy.

As with other insect pollinated plant species, almond trees produce attractive flowers with significant amounts of bee food - pollen and nectar. Pollen is a rich source of nutrients and protein and is the primary food for pollinating insects. Nectar is the fuel that propels the insects from flower to flower - the gasoline that carries the insect to the grocery (pollen) store. Before the development of a honeybee industry, and before the advent of pesticides and intensive land cultivation, most crop pollination was done by wild bee species that were unrelated to the honeybee.

Unlike honeybees, most wild bee species are solitary insects; they collect very little nectar from flowers - only enough to fuel their flights to flowers and to serve as "glue" for their pollen loads. In contrast, honeybees are nectar collecting machines and have been bred to store nectar far in excess of their immediate needs. The stored nectar (honey) serves both as insulation for their domiciles and as a food reserve to carry the bees through times of flower shortage.

It seems logical that nature intended nectar reserves in flowers to be used mainly as a fuel for pollinating insects. A little dab of nectar is all a bee requires for this purpose. Compared to wild bees, the honeybee is a nectar hog.

Flowers Are Weakened By Bees Taking Nectar

Flowers keep their nectaries filled. When the nectar is depleted by honeybees, the flower will secrete additional nectar.

Nectar secretion is dependent on temperature and sunlight. Most plant species do not secrete significant nectar until temperatures approach 70ºF. Maximum nectar secretion on almonds probably occurs when temperatures reach the 80s.

By continually bleeding nectar from the flower, honeybees turn almond trees into nectar producing machines in warm, sunny days. This causes trees to "bleed" carbohydrates from thousands of tiny cuts (flowers). If the day was cool and cloudy, nectar secretion would be low and this bleeding would he much less.

University of California studies hive shown that almond flowers secrete most of their nectar after pollination has occurred. With reasonable weather, honeybees accomplish the pollination job the day the flower opens, or at least by the next day. The flowers, however, will continue to secrete nectar for several more days, right up to petal fall and even after petal fall. Honeybees can he seen working "flowers" for nectar after the petals have fallen off.

This post-pollination period is when the young developing almond embryos need all the food reserves they can get. If warm weather occurs during this period, trees can lose considerable reserves to honeybees. Instead of feeding almond embryos, this food source is feeding bee larvae or is being stored by honeybees.

Thus, weather during almond bloom can be "too good." A warm spell right after pollination could cause excessive nectar flow and nectar collection by honeybees. This could partially explain why disappointing almond crops sometimes occur after ideal weather throughout the blooming period.

There is some evidence that lends support to the preceding thesis. Recent UC tests compared nectar and pollen production of various almond varieties. The results were the opposite of what was expected. The varieties that produced the least amount of nectar (and pollen) set the highest percentage of blossoms; and varieties that produced the most nectar set the least, in spite of the fact that bee activity was significantly greater on the high nectar producing varieties.

A better test would be to cage an almond tree to prevent honeybee visitation. The caged tree should have a limb graft of a different variety to act as a pollinizer. A wild bee species that doesn't collect nectar should be put in the cage. Fruit set on the caged tree could then be compared with fruit set on trees outside the cage that were pollinated by honeybees.

Just such a test was run in 1977 by Phillip Torchio, USDA. The purpose of the test was to evaluate Osmia ligneria (a wild bee species) as a pollinator for almonds. Trees caged with the wild bees out-yielded open pollinated trees by over two to one. Torchio concluded that Osmia ligneria was a far superior pollinator of almonds than the honeybee.

Torchios results and conclusions were met with one-handed applause by those in the honeybee industry and by many in the almond industry. Even conceding that Osmia ligneria is a much more efficient pollinator than the honeybee, Torchio's detractors felt that the massive numbers of honeybees used for almond pollination more than made up for it's lower efficiency. Anyone who has spent time in an almond orchard during bloom would probably agree that honeybees are excellent pollinators of almonds and get the pollination job done.

A more plausible explanation for Torchios results is that his data reflect better carbohydrate reserves in the caged trees.

Excessive nectar collection by honeybees during a period of high nectar secretion could deplete carbohydrate reserves of almond trees to the point where final nut retention was affected. If post-bloom weather conditions were warm and sunny, the newly formed leaves could make up this drain on reserves; if not, nut drop could be excessive.

A grower could reduce potential nectar-carbohydrate losses by having the honeybee hives removed from his orchard immediately after pollination has been accomplished. Since nectar secretion continues well after pollination has been completed, flowers could still remain on the tree, but these flowers would be pollinated. UC tests have shown that the final flowers that open on a tree have an extremely low chance of setting. In some years, trying to get bees to set that last straggle flower that can be seen on a tree might not be worth the trouble.

The author is the manager of Scientific Ag Company, 1734 D Street, Suite 2, PO Box 2144, Bakersfield, CA 93303.