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by JOE TRAYNOR
Scientific Ag Co.
P.O. Box 2144
Bakersfield, CA 93303
Knowing
how much pollen a plant species produced would be useful in ranking
plants as a food source for both honey bees and wild bees. Beekeepers
have a good general idea, as to whether a particular species
is a good pollen source (or nectar source), but there is little
quantitative information on the subject.
Past studies have estimated pollen
produced per flower, but none have calculated pollen produced
per acre. Obviously, the way to calculate pollen production per
acre is to multiply the number of flowers per acre by the weight
of pollen per flower. These two parameters are estimated below
for almonds.
Flowers per acre
The flowers/acre of
almonds will vary from year to year (a lighter flower set often
follows a heavy almond crop and vice versa). Estimating the number
of flowers per tree, then multiplying by trees/acre may be the
best way. Trees/acre vary according to planting distance; the
most common almond plantings are:
25' X 25' = 70 trees/acre
24' X 24' = 76 trees/acre
22' X 24' = 83 trees/acre
Dan Mayer estimated that an acre of apples produces 1 million
flowers.(5) Almonds bloom more profusely than apples and 2 million
flowers/acre for almonds is not an unreasonable guess. Two million
flowers/acre would give the following flowers/tree for the above
3 planting distances: 28,500, 26,300 and 24,000.
Another method is to calculate flowers/acre from nuts/acre. About
3000 lbs. of nut meats per acre is considererd an excellent yield
and to achieve such a high yield, a relatively high percent of
flowers must be set. Kester & Griggs(4) obtained a 50% almond
set (50% of the flowers set an almond) as did Hill, et al(3),
although the latter study did not account for May (aka "June")
drop.
Assuming there are 350 nuts per pound (varieties vary in this
regard; Nonpareil produces significantly larger nuts than Fritz)
a 3000 lb/acre yield translates to 1,050,000 nuts/acre which
in turn translates to 2,100,000 flowers per acre if a 50% set
is assumed.
For this discussion, we will assume that an acre of almonds produces
2 million flowers (realizing that this figure can vary greatly
from year to year).
Weight of pollen per flower
DeGrandi-Hoffman, et
al(2) used sonication to remove pollen from almond flowers; Hill,
et al(3) removed anthers from 100 flowers, dehisced the
anthers at room temperature, then sieved and weighed the pollen.
Both of these research teams came up with a range of around 0.7
to 1.2 mg of pollen per almond flower and both obtained year-to-year
variations in pollen production. Oberle & Goertzen(6) also
showed significant year-to-year variations in pollen production
for a number of deciduous fruit species.
Traynor(7) used the method used by Oberle & Goertzen(6)
to count pollen grains per flower: Put pollen (dehisced at
room temperature from a flower's anthers) in 2 ml of 10% calgon
solution, shake and count the pollen grains in a small portion
of the solution in the counting chamber of a hemacytometer (a
simple glass slide used to count red blood cells; Spencer Bright
Line, Clay Adams Cat. #@-2440/B).
Traynor obtained a range of 42,000 to 67,000 pollen grains per
flower for eight commercial almond varieties and a figure of
44,000 pollen grains per flower for the Nonpareil variety, which
comprises about 40% of California's almond acreage. Traynor did
not count the grains in a given weight of almond pollen. However,
Bosch(1) determined that an almond pollen provision of Osmia
cornuta (a mason bee), that weighed 16.5 mg., contained roughly
300,000 pollen grains. Pollen from one almond flower (44,000
grains for the Nonpareil variety, as calculated by Traynor) would
weigh 2.4 mg, considerably higher than the 0.7 to 1.2 mg calculated
by the DeGrandi-Hoffman and Hill teams. The "glue"
that mason bees use to mold their provisions could have added
enough weight to account for the difference.
Pollen per acre
Assuming 2 million
almond flowers per acre, the figure of 1 mg of pollen per flower(2,3)
translates to 4.4 lbs. of almond pollen per acre, while 2.4 mg/flower(1,7)
translates to 10.7 lbs/acre. This is a wide range, but it is
in line with the 5 to 8 lbs. of pollen per acre that commercial
beekeepers report that they trap from almonds.
Summary
For well over 200 years,
biological scientists have counted and classified things
in order to achieve a better understanding
of Nature. Thomas Jefferson and Charles Darwin were two of the
more distinguished classifiers. Classifying plants by pollen
production would be of practical benefit as well as of academic
interest. From a practical standpoint, it would be helpful to
know which wild plants provided the most sustenance for bees
(both honey bees and wild bees) with the idea of making efforts
to conserve such plants. Knowing the nutritional value of different
pollens would also be helpful. Dr. Christine Peng (UC., Davis)
and others have done some work on this, but overall information
is limited.
Pollen production of plants that produce wind-borne pollen (e.g.,
Ragweed) is being determined from a public health standpoint(8),
but pollen production data of plants producing insect-collected
pollen are very limited. Perhaps the reason is that the subject
falls squarely in the "no-man's land" between the disciplines
of plant science and apiculture, with either side leery of intruding
on another's territory. The pollen production calculations given
above for almonds are simple and it is hoped that others will
be encouraged to perform similar calculations on other plant
species.
References
1. Bosch, J. 1994. The
nesting behaviour of the mason bee, Osmia Cornuta (Latr),
with special reference to its pollinating potential. Apidologie.
25: 4-93.
2. Degrandi-Hoffman, Gloria, Gerald Loper, Robbin Thorp and
Dan Eisikowitch 1991. The influence of nectar and pollen
availability and blossom density on the attractiveness of almond
cultivars to honeybees. Acta Horticultureae 288, 6th Pollination
Symposium.
3. Hill, S.J., D.W. Stephenson and B.K. Taylor 1985. Almond
pollination studies: pollen production and viability and cross-pollination
tests. Australian J. Exp. Agric. 25, 697-204.
4. Kester, Dale and W.H. Griggs 1959. Fruit setting in
the almond: The effect of cross-pollinating various percentages
of flowers and The pattern of flower and fruit drop. Proc.
Amer. Soc. Hort. Sci. 74:206-213 and 214-219.
5. Mayer, Dan 1995. How to figure number of bee colonies
needed. Good Fruit Grower, April 1.
6. Oberle, G.D. and K.L. Goertzen 1952. A method for evaluating
pollen production of fruit species. Proc. Amer. Soc. Hort.
Sci. 59:263-265.
7. Traynor, J. 1981. Use of a fast and accurate method
for evaluating pollen production of alfalfa and almond flowers.
Amer. Bee J. 121 1:23-25.
8. Ziska, Lewis and F. Caulfield 2000. Rising CO2 and
pollen production of the common ragweed, a known allergy-inducing
species: Implications for public health. Australian J. of
Plant Physiology. 27:893-893.
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