April, 1997
By Roger Hoopingarner
Michigan State University
The design and management of fruit orchards to maximize fruit set for economical production has changed during the last few decades.
Management for effective pollination has not always kept up to the changes within the orchard. For example, the switch from standard trees to dwarfing rootstock makes a great change in the number of pollinator bees needed, and possibly placement of colonies. Successful orchard management occurs when you can produce an optimal amount and quality of fruit year after year. Either over- or under-pollination of potential blossoms can cause a cyclic production of fruit from year to year.
In this article I will summarize what we
currently know about the pollination of various fruit crops, and how to
build into the orchard management equation the effective use of honey bees
for pollination. This will be broken down into four sections. The first
will detail the biological limits on foraging behavior of pollinators. The
second will focus on orchard parameters that affect pollination, such as
variety incompatibility. Then I will merge foraging (pollination) dynamics
and orchard management systems. Finally I will outline how to monitor your
orchard for pollinator activity and how you might enhance fruit set within
the limits of your system.
Pollinators
There are three types of major bee pollinators that may be found in an orchard - solitary bees, bumble bees and honey bees. There are several to hundreds of different kinds of solitary bees. Their name indicates much about the biology of these bees. Each individual female bee builds its own nest in hollow weed stems, or makes nest burrows within the ground. In a typical nest the bee would line the fairly straight tube with leaf disks that she cuts from various plants. The bee would collect pollen sufficient to make a small ball in the nest section and then lays an egg onto the pollen. She then caps off the section and repeats the process several more times. Such nests may produce only one generation per year or some species may have several cycles in a season. Since each cell must have pollen for the young larva to develop, the foraging solitary bee is always a pollen collector. Some orchards have an abundance of solitary bees either because of favorable site locations or agricultural practices beneficial to their preservation. Many of our practices have discriminated against these bees because of reduced nesting sites (fence rows), large orchard blocks and by the use of pesticides during the year. Some species of solitary bee can be managed within an orchard system, for example the Osmia spp. With these bees the orchardist can provide nest blocks made out of wood. These blocks can be kept in a cooler and removed prior to the blossoming period.
Bumble bees have an annual nest that is started each spring by a queen that has overwintered in a freeze-protected location such as the leaf litter in a woodlot. During the fruit bloom period it is usually only the queen that is foraging. She is collecting pollen and nectar to produce workers that may number to a few dozen by late summer. The colony in the fall produces new queens and males that mate, and the new queens hibernate until the next spring. Bumble bees forage at lower temperatures than either the solitary bees or honey bees, and are less affected by increased wind speed. The technology for management of bumble bee colonies has advanced to where managed colonies are being used for glass-house pollination, and to a limited extent for fruit crops.
The European honey bee (Apis mellifera
L.) is worldwide in distribution. The early colonists brought them to the
New World and they spread throughout the country via swarms. Pollination
of fruit trees was mainly provided by nearby colonies, either wild (feral)
or managed, until recent years. With increased size of fruit orchards, more
insecticide use, and fewer varieties within each orchard, there has been
an increased need for planned pollination. Beekeeper colonies used for pollination
are, more and more, moved on pallets holding two or four colonies. Such
mechanization has allowed beekeepers to effectively move more colonies in
a shorter time. It is one of the few labor-saving devices that beekeeping
has been able to enjoy.
Foraging (pollination) by bees
Bees use pollen as their source of protein and nectar for energy (movement). The solitary bees are always pollen collectors. However, even a nectar collecting bee is almost always a pollinator if she is carrying compatible pollen. When bees clean their bodies to remove the pollen to feed to their young they still leave thousands of pollen grains on the branched hairs that cover them. Thus, when they move from flower to flower they accidentally pollinate the fruit. Most bees visit only one species of flowering plant on a foraging trip. This flower constancy favors effective pollination except that if such constancy would narrow the beeís collection to a single variety within a self-incompatible crop, such as apple.
Most fruit crops have a good nectar sugar concentration and thus are competitive for bee visits. However, pears have a low sugar concentration and bees will sometimes leave pears for other crops that are in bloom at the same time.
Foraging trips from a colony are often limited by low temperatures. Honey bees and solitary bees do not forage much below 55° F. (13° C.). Solar radiation may enhance flight somewhat at these low temperatures. Bumble bees can elevate their body temperature by muscle activity and often forage below 55° F. Between 60° and 90° F the increase in available foragers is essentially linear. At the highest temperatures there is a reduction in foraging as more bees are used to circulate air in order to cool the hive.
Increasing wind speed has just the opposite effect on available forging flight. When the wind speed exceeds about 25 mph most honey bee flight is stopped. Wind speeds can also influence pollination directly. For example, when the wind makes landing on a moving fruit blossom too costly in energy the bee will forage on the dandelion blossoms on the orchard floor where the wind speed is reduced.
Foraging within an orchard will depend
upon the size of the tree and often the planting pattern. A honey bee will
often find sufficient forage on a standard-size apple tree to complete a
load of pollen (or nectar) on a single tree. This would prevent any cross
pollination from occurring on that flight (see pollen transfer mechanisms
below). If flower constancy extends to varieties then the problem of compatible
pollen transfer is increased. The between-tree spacing is usually much less
within a row than between rows, and bees will often continue down a row
rather than cross to another row. This foraging pattern is caused by the
beesí attempt to maximize their energy gain, as the greater distance
of flight between rows uses more flight energy and thus fewer calories to
be gained.
Pollen compatibility and pollinating
fruit crops
Many fruit crops are pollen self-compatible - tart cherry, peach, some plums, pear. Others are completely, or partially, self-incompatible. Examples of these are apple, almond and sweet cherry. Then there are some varieties of sweet cherry, or polyploid apple, that are also cross- incompatible.
The problem of self-incompatibility and the optimal fruit-set level becomes intertwined in determining the optimal concentration of pollinating bees. For example, in pollinating peaches we have a self-compatible variety along with a low fruit-set requirement. The number of honey bee colonies recommended is generally less than one colony/acre. When we move to a self-compatible crop such as tart cherries that has a high fruit-set requirement then we need more bees ñ one colony/acre. A self-incompatible crop along with a low fruit set (e.g., apple) generally requires about one colony/acre. When we move to high fruit-set levels (35%-40% of blossoms) along with self-incompatibility (almonds and sweet cherries), pollination requirements are between two and three colonies/acre.
There may be an important disclaimer to
these recommendations. Most research on fruit set was conducted prior to
the loss of the wild (feral) colonies of honey bees due to the newly introduced
parasitic mites. The combination of the tracheal mite and varroa have essentially
removed most feral colonies from the landscape. Estimates of the past abundance
of these feral colonies has varied from about 50% to 80% of the pollinating
population.
Orchard planting patterns
In pollinating self-incompatible crops the orchardist must consider the pollinizer variety, the timing of pollen dehiscence and the mix of pollen within the orchard. For example, in pollinating Delicious apples care must be given to selecting the pollinizer if the king blossom is to be set. The pollinizer variety should begin to bloom at least a few hours before the first flowers on the main variety. I have seen growers plant a mix of Golden Delicious and Delicious and ask why they are not getting king blossom set on the Delicious. While Golden Delicious is a good pollinizer variety it often blooms too late to provide pollen for the early (king) blossoms. It is also possible that temperature could cause asynchrony in blossoming period as well. McIntosh has a development base threshold of about 34° F whereas Delicious has a base temperature of about 40° F. Other varieties may have different thresholds, and certain critical temperatures could cause a certain amount of bloom asynchrony.
Care must also be given when selecting crab apple pollinizers. They can have color differences and be asynchronous with the main variety. Both conditions can interfere with pollination and cause poor fruit set.
Orchard planting patterns have been developed
to take into consideration both the beeís foraging patterns and also
to maximize the yield of the main variety. Self-incompatible varieties may
have pollinizer rows alternating with the main variety, or alternating with
two or more rows of the main variety. Growers also may plant a pollinizer
tree within the row to circumvent the beeís tendency to work down
the row. The difficulty with this pattern is that having the pollinizer
within the row may conflict with pest management practices or in harvesting.
One orchard plan has a pollinizer tree every third tree, every third row.
This pattern breaks down into eight main-variety trees surrounding a pollinizer
tree. In theory the pollinizer is only one tree away from a main-variety
tree. However, at least two factors may be working against this orchard
plan. The first is that there is still the problem of a greater distance
between rows, and also the ratio of pollinizer pollen to the main variety
may become too low under certain pollinating conditions. We often get a
cool, wet pollination period. Then fruit set may be reduced since we just
do not have enough pollinizer pollen transferred because of few available
pollination periods. Systems analysis of pollination of apples
A few years ago we undertook to try to analyze the complex variables of pollination and fruit set in Delicious apples. We wrote a computer program using systems analysis techniques. This type of program attempts to organize variables such that they can be given values by which they can be graded for importance. Certain variables can be held constant while others changed to assess the impact of the change. When we compared all of the variables within the model, two came out as having the most importance for affecting fruit set. The first was that a foraging bee had to be carrying compatible pollen. That is, a bee going from flower to flower on Delicious was not setting any fruit even if it moved to another tree. The other major variable was weather. This last one was not very surprising as weather drives the whole system. If bees were to be carrying compatible pollen the published literature said that they had to wander from one variety to another during a foraging trip. There were also indications from the literature that certainly not all bees would be wandering during a trip, especially while foraging on standard apple trees. If competition for pollen and nectar were great enough then wandering would be increased.
Our initial model would only produce about 1% initial fruit set. Clearly not enough. After checking the computer program for accuracy we then decided to increase the number of wandering bees. However, even if we made every bee a wandering bee at some time during the foraging trip we still had difficulty getting sufficient fruit set in our computer simulations. Then we happened to see some old publications that hinted that bees could be accidentally exchanging pollen with their nestmates when they returned and moved about the hive. Then upon leaving the hive they would be carrying compatible pollen that would cross pollinate at least the first few flowers that they visited even if they remained on the same tree. Thus, under certain conditions all bees would be carrying compatible pollen. When we created this possibility within the computer program we instantly ìproducedî a normal apple crop. We were fortunate in that we were able to validate this concept by comparing the pollen on bees within some solid blocks of one variety of apples. By using the scanning electron microscope we were able to see which variety of pollen was on the bee. Our analysis of these bees indicated that almost all bees, even those that had never left the hive, had a mixture of pollens on their bodies .
We also decided to see what happened in
real orchard situations. We found an orchard that had one row of pollinizer
trees with five rows of Delicious on either side. If wandering were the
only mechanism for pollen transfer then there should be a decline in fruit
set the further away from the pollinizer row. This declining pattern did
not appear. This in-hive pollen transfer also explains how isolated, single
apple trees get pollinated. The in-hive transfer also makes the honey bee
a much more efficient pollinator of the self-incompatible crops.
Planned pollination
The first step in getting a fruit crop
is having sufficient pollination. Without pollen transfer all other management
practices are only meaningful for a future yearís crop. Adequate
and sustained pollination helps stabilize fruit set from year to year. Currently
it makes sense to rent sufficient colonies for your pollination needs, as
almost all of the wild colonies have died because of the accidental introduction
of the two mites parasitic on honey bees. There has been another change
in orchards over the last few years. Standard-size trees have been replaced
with dwarfing rootstock. There now may be four or five times the number
of trees per acre as were previously planted. The blossom density has more
than doubled. Yet many orchardists have not increased the number of colonies
of bees. They are using the same number of colonies of bees as their father
or grandfather. This is true even though the orchard has increased block
size, decreased the diversity of cultivars useful in cross pollination,
removed the nesting sites for solitary and bumble bees, and planted dwarf-rootstock
trees.
Monitoring pollination
It is difficult to give good guidelines
for observing your trees to assess whether you are getting adequate visitation
by bees to ensure pollination. The weather, time of day, intensity of bloom
and the type of fruit crop all affect the number of bees that may be seen
in a tree. Pollen dehiscence is usually early in the day and if weather
is good bees will collect most of what is available in the morning. If you
are examining your trees for bees, or honey bee colonies for flight, remember
to consider the temperature and wind speed in your determination. We like
to see one bee/minute on a dwarf fruit tree under average conditions. Remember,
not every bee is carrying compatible pollen on crops such as apple. In apples
each flower that is to be pollinated needs about five viable, compatible
pollen grains on the stigma in order for that apple to grow and remain on
the tree.
Pollination agreement (contract)
Fruit growers and beekeepers should draw up a written contract. If it is written and signed the agreement protects both parties. The contract should specify the size of the colony in number of frames of bees and brood that is minimum. There is a growing trend to pay the beekeeper on the total number of frames rather than colonies. This arrangement benefits both parties. The beekeepers benefit because they could move fewer colonies for the same price, and growers because there are more foragers per relative population in larger colonies.
Hoopingarner is with MSUís Department
of Entomology and is the stateís apiarist
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