THE ECONOMICS OF CROP POLLINATION: A COMMENTARY, WITH SPECIAL REFERENCE
TO BUMBLE BEES, CRANBERRIES AND BLUEBERRIES
R.C. Plowright
Department of Zoology
University of Toronto
Toronto, Ontario
Canada M5S 1A1
A friend of mine recently demonstrated to me how one could calculate how
many bumble bees would be required to pollinate an acre of lowbush blue-
berries. It was a nicely reasoned piece of work: he had previously
obtained data on the working speed of bumble bee workers (number of
flowers visited per minute), the length of time that each blueberry
flower remained open and receptive, the number of flowers per sq. metre,
and so on. From all of this, using some standard logical tools (such as
assuming that if a flower bloomed for X days, then the average age of a
flower picked at random should be X/2 days) he was able to demonstrate
that some number (call it N) of foraging Bombus workers should be able
to visit and pollinate each and every blueberry flower within their
designated area.
As mentioned, I thought that my friend's approach, which seemed to be a
fairly typical example of the methods that have hitherto been used to
calculate application rates for insect-pollinated crops, was well-
reasoned. But the problem with it, I believe, is that it is highly
misleading and would under most circumstances greatly underestimate the
actual number of bumble bees required to do a good job on an acre of
lowbush blueberries. The reason for this is that the method makes some
exaggerated assumptions about the perfection of bumble bee foraging
behaviour, and therefore misconceives the nature of the relationship
between crop yield and pollinator density.
Now I would certainly be just about the last person in the world to
admit that the foraging efficiency of Bombus workers is anything other
than wonderful and just about the best there is in the business (or at
least in that part of the business where the insects are not able to
communicate with each other by dancing and stuff). To appreciate this
one only has to follow a Bombus impatiens worker as she proceeds method-
ically along a row of tomato plants in a greenhouse. At times it seems
as though her foraging performance is well-nigh perfect: she hardly ever
skips even a single flower (although she may decide not to actually land
on some of the blossoms that she inspects), and often she may even fly
back to visit one that she nearly missed.
Similarly, although the foraging path of foraging bumble bees across a
meadow of flowers (or an insect-pollinated crop) is not quite as
straight as it is along a row of tomatoes, the flight path is usually
linear enough to avoid much re-visiting (Galen & Plowright etc.). But
the problem is that although we may visualize many bees proceeding in
approximate straight lines across a field, there is no guarantee that
all these approximately straight lines will be arranged in a perfectly
regular pattern so that the whole field will be exactly covered with no
overlap. Far from it: in most cases the actual state of affairs will be
that many flowers will be visited more than once, and numerous other
flowers will never be visited at all.
It was for reasons like this that Lester Hartling and I, in the course
of our study of the dynamics of red clover pollination (Plowright &
Hartling, J. Appl. Ecol., 1981), felt it necessary to remind our
colleagues that the shape of the curve relating pollinator density to
percentage seed-set must generally look like this:
| .
P | .
E | .
R | .
C | .
E | .
N | .
T | .
| .
S | .
E | .
E | .
D | .
| .
S | .
E | .
T | .
|.
+----------------------------------------------------------------
POLLINATOR DENSITY
[The foregoing is intended to depict a smoothly saturating curve--i.e. a
"diminishing returns" curve--often described by a simple mathematical
equation known as the "Michaelis-Menton" formula. The saturation point
in such a diminishing returns curve may indeed not be fully 100%, for
after all, the seed set in most insect-pollinated crops is not only
limited by insufficient pollination--other factors come into play at
high pollinator densities].
Is all this terribly obvious? Well, I would hope so, but the signifi-
cance of the argument sometimes seems to be lost under the well-inten-
tioned mantle of: "But surely, we HAVE to be able to recommend SOME
justifiable application rate to the grower, don't we?"
MARGINAL-VALUE CONSIDERATIONS:
The approach we have taken in recent years has been to ask what is the
marginal-value (to the grower) of each extra hive that is placed on the
crop. We start with the limiting condition of a hypothetically "empty
field"--empty, that is, of all pollinators. (This is not intended to
depict a realistic state of nature: the realism will be added later on).
We can then ask: "How many dollars will a single hive placed on such an
empty field return to the pocket of the grower?". Because in the limit-
ing condition of the empty field we can assume that virtually each and
every flower visited by a foraging bee will be in a virginal condition,
it is relatively easy to make some plausible calculations. Here follow
some data which we collected last summer, as part of a study of cran-
berry pollination in Wisconsin, in collaboration with Elden Stang and
Dan Mahr:
No. of bumble bees leaving hive in a the course of one day 1,540
Average duration of each foraging trip (mins.) 10
No. of flowers visited per minute by a single bee 14
Duration of pollination period for the crop (days) 14
If we multiply all these numbers together we deduce that foragers from
the bumble bee colony would visit about 3,018,400 cranberry flowers
during the 14-day pollination period. (I will comment upon the plausi-
bility of each of these figures later in this talk). Making the assump-
tions that (1) each bee visit to a virginal flower results in 1 extra
cranberry (remember that we are at present considering only the "empty
field" situation), (2) that there are approximately 400 cranberries per
pound, and (3) each pound of cranberries returns $0.50 to the grower,
then we conclude that the bumble bee hive is "worth" $3,773 in terms of
added value of the crop.
It hardly needs to be said that the notion that a single bumble bee
colony could be worth over three thousand dollars is not about to be
widely believed by either growers or bee biologists. Let me quickly
discuss, and then modify, our figures so that we can move toward a more
plausible estimate. First, some comments about the data in my table:
(1) Nest-traffic data: Our count of 1540 sorties from a bumble bee
colony was obtained from an extremely large B. impatiens hive
which, at the time the data were taken contained over 300 worker
bees. A more realistic value would be about half this (i.e. bet-
ween 700 and 800 sorties, according to values obtained by Lisa
Guy and Steve Buchmann this summer).
(2) Foraging-trip-duration. A value of 10 minutes might seem rather
low (20-mins would seem closer to the mark for an experienced
Bombus forager), but for our purposes this figure must represent
an average for every sortie from the hive--in other words it must
include "play flights", orientation flights, etc., and not just
the flights made by real foragers. We will be able to get a more
precise estimate for this parameter after this summer's data are
analyzed, but the 10-minute estimate seems likely to be pretty
close to the mark.
(3) Rate of flower visitation. This is probably a good estimate, as
far as it goes: we found that the average of 14 flowers per
minute was a very constant value for bumble bee workers visiting
flowers on the Wisconsin cranberry marshes during the summer of
1991, at least during the middle hours of warm sunny days. How-
ever, we still need more data to fill in those parts of the
blooming period when environmental conditions are less than
perfect.
(4) It is probably an overestimate, even in the "empty field"
situation, to equate each bee visit with "one added cranberry".
Not all flowers visited would necessarily have receptive stigmas
at the time of visitation, and even in an empty field, the forag-
ing bees from a large B. impatiens colony would doubtless
sometimes cross each other's tracks--i.e. some revisiting of
flowers would take place.
Taking the foregoing cautionary remarks into account, we would probably
do well to halve our estimate of the "value" of a B. impatiens colony in
an empty cranberry bog, and then to halve it again: $943.25 sounds much
more believable!
But even if this new estimate might persuade a few people (and I know
that some of my audience would at this point be shaking their heads and
muttering "Well, COME ON!! Until recently this guy made his living from
the sale of bumble bee colonies! Does he really expect us to swallow
this sort of exaggeration?!"), it certainly bears little relationship to
what is happening out there in the real world. Because, of course, in
reality fields are usually far from "empty": other pollinators, includ-
ing other bumble bees both wild and introduced, are out there doing
their part to assist in the production of cranberries . . . [And even if
it were not so, the poor cranberry farmer would scarcely have remained
in business for long under the limiting "empty field" situation]. So
what use can be made of the calculations that I have presented?
POLLINATION DEFICITS:
To proceed futher, I should next introduce what we call the "pollination
deficit". This is simply the difference between (a) the maximum possi-
ble percentage of fruit that could be obtained if pollination were
perfect and (b) that actually realized under field conditions. Note
that, as mentioned above, the pollination deficit is NOT the difference
between the field conditions and 100% because even under the most
perfect pollination regimes it is hardly ever the case that each and
every flower on a plant will bear fruit. Pollination deficits are not
always easy to measure, again, largely because of this same difficulty:
what WOULD be the maximum possible fruit-set under conditions of satura-
tion pollination? Nevertheless, in order to calculate the economic
impact of a bumble bee hive, or a colony of honeybees, or some other
pollinator, it is absolutely essential to know what is the maximum
possible yield that could be obtained.
For some crops, the pollination deficit is known with fair precision.
Elden Stang has told me that for Searle's, a cranberry cultivar grown
widely in Wisconsin, a pollination deficit of about 8% is a fairly typi-
cal occurrence. From this, we might conclude, as a simple beginning,
that the field, so far from being empty, is "92% full" or, if you like,
that only 8 out of 100 flowers visited by a foraging bee are actually
available to be pollinated. From this it follows that the marginal
value of a bumble bee colony set out in the marsh would be only 0.08 of
the "empty" value, i.e. $75.46. Now, while I am not about to admit that
improvements in rearing technology might not eventually result in our
ability to produce bumble bee colonies at a cost of less than $75 each,
that day is certainly not yet with us. So I have to say that those
growers who are working with Searle's and getting pollination rates of
92% are doing extremely well and are extremely unlikely to be calling on
any of the commercial bumble bee raising companies in the near future.
Such is not always true, however, for cranberry crops of other cultivars
in other parts of North America. We hear, repeatedly, that growers in
Massachusetts have reported substantial pollination deficits. The same
has been reported for lowbush blueberry producers in areas such as the
Lac St-Jean region of Quebec. As is so often the situation with regard
to insect-pollinated crops, the worst pollination problems seem to occur
in those farms which have the largest monoculture stands. This makes
sense ecologically, because if you think of one of these Vaccinium farms
as being a huge "sink" which pulls in wild pollinators from the sur-
rounding areas, then it would seem reasonable that the pollinators are
going to be spread most thinly in circumstances where the crop-acreage
is relatively large. In these huge farms, it is likely that the
marginal value of an introduced bumble colony (a large colony of B.
impatiens, for example) may reach $500 or more. So in these cases we
are probably within the domain of commercially feasible bumble bee
pollination.
To extend the analysis which I have presented, so that some sort of
economic break-even point can be determined, requires other types of
knowledge of the foraging biology of the bees: we need to know the
EXTENT TO WHICH THE FORAGING INSECTS WILL OVERLAP WITH EACH OTHER--
because it is this that determines the shape of the diminishing returns
curve. In the simplest case, I suppose one could fall back on the
notion of the flowers in the crop as being like balls in an urn: two
"colours" of ball are present, one being a previously visited flower and
the other a flower that is still virgin. Then one could develop a model
from probability theory to track the changing dynamics of flower visita-
tion throughout the blooming period. But such an approach is probably
at the same time both too sophisticated and too crude: too sophisticated
because the necessary mathematics are probably in advance of the data,
and too crude because in most instances we do not think of bees as
moving randomly through the crop (or at least not entirely randomly).
So it seems likely that some spatial parameters should appear explicitly
in models used to estimate overlap frequencies--and this in turn leads
us to the conclusion that data on "foraging ranges" are likely to be
needed. There is plenty of scope for useful research here, and this is
indeed where a large part of our efforts are currently directed.
BLUEBERRY POLLINATION IN THE SOUTH-EASTERN UNITED STATES:
I must here return again to the topic of "pollination deficits", because
they are crucial, in my opinion, to some of the most interesting contem-
porary problems in the ecology of crop pollination. I have recently
been attempting to gather data on the pollination deficits to be found
among blueberry farms (highbush and rabbiteye) up and down the eastern
seaboard of the United States--i.e. from Florida through Georgia,
through North Carolina, and so finally to New Jersey. There seem to be
major differences between the major regions with respect to the growers'
perceptions (and also the perceptions of the scientists who advise the
growers) of how well their crops are being pollinated. At the risk of
oversimplification, one might summarize by saying that Florida and Geor-
gia admit certain problems with regard to pollination, North Carolina
steadfastly maintains that pollination is not a problem for the industry
there, and New Jersey leans toward the view that pollination is diffi-
cult in some years. But even WITHIN each region, it is rare to find
strong unanimity on the subject of pollination: some will tell you that
poor fruit-set is due to scarcity of pollinating insects, while others
are convinced that other factors are responsible.
To some extent, these differences in opinion are merely reflections of
differences in the floral morphology of the various cultivars: for exam-
ple, the rabbiteye blueberry (Vaccinium ashei) has a distinctly longer
and less accessible corolla than the commercial highbush varieties. It
is commonly assumed, therefore, that rabbiteye flowers are more diffi-
cult to pollinate (especially because of the greater need for cross-
pollination in many cultivars of this species) by short-tongued bee
species--i.e. honeybees--than are highbush flowers. Though plausible, I
haven't yet seen much strong evidence for this: but it certainly was
true in North Carolina in both 1991 and 1992 that in farms containing
mostly highbush with only a minority planting of rabbiteye, the long-
tongued bees (mostly Bombus spp.) were to be found predominantly on the
rabbiteye bushes.
THE ROLE OF HONEY BEES:
Overlaying all of this, I suspect, is a tapestry of misgivings regarding
the efficacy of honey bees as pollinators of Vaccinium. Again at the
risk of oversimplification, it seemed that until the onset of Varroa,
American beekeepers enjoyed a fairly comfortable relationship with the
cranberry and blueberry growers of North America. The rental charges
for the hives were fairly modest, and the growers, even when they
weren't perfectly convinced of the value of the honey bee colonies as
pollinators of their crops, were smart enough to accept the wisdom of
paying a certain amount of "vigorish" if that was what it took to
provide a little extra pollination-insurance.
However, there are signs that this cozy partnership may be falling
apart. In the first place, beekeepers have been hard pressed in the
past few years to be able to provide hives of adequate quality for blue-
berry pollination. (Cranberry pollination is easier, in the sense that
the crop blooms so much later in the season). But more ominous for the
beekeeper, some of us have begun to question what may be the ecological
consequences of indiscriminate "laying on of hives". We were most
impressed in the summer of 1991 to observe that while most of the forag-
ing bumble bees that we saw working in the Wisconsin marshes were carry-
ing pollen in greater or lesser amount, almost NONE of the foraging
honey bees (less than 1%) were carrying any pollen at all. We muttered
dutifully about the fact that honey bees, in contrast to their Bombus
cousins, cannot buzz-pollinate the cranberry flowers, but a point of
greater significance appeared to be the following: if, as seemed
likely, the honey bees were foraging only for nectar, what influence
might they be having (in terms of exploitation competition) on the well-
being of the bumble bees? Could it possibly be, we thought, that by
renting honey bee colonies in their fields the cranberry growers are
actually having a negative impact upon the more efficient (bumble bee)
pollinators? In fact, might it be the case that the cranberry growers
are actually spending money in order to LOSE money? What a heretical
notion that would be!
SUMMARY AND RECOMMENDATIONS:
The two main points that I have tried to make in this paper are:
(1) That any estimate of the economic value of bees to growers of
insect-pollinated crops be based upon realistic knowledge of
insect behaviour;
(2) That because of the diminishing-returns relationship between
pollinator density and crop yield, it makes the most sense to
start from marginal value considerations as a prelude to working
out specific recommendations regarding application rates.
I have not attempted to carry my arguments forward to the point where I
would feel confident in recommending to growers of cranberries or blue-
berries how many bumble bee hives they should use to pollinate their
crops. This is a topic which will be addressed by other contributors to
the Non-Apis Pollination Workshop. My only recommendation is that any
recommendations that ARE made should be carefully scrutinized to deter-
mine whether, during the course of their formulation, the issues raised
in this paper have been properly taken into account.
ACKNOWLEDGMENTS
My warmest thanks to all those who have enriched my life with interest-
ing discussions about Vaccinium crops: Phil Emmer, Guy Gottschalk, Lisa
Guy, Kenna MacKenzie, Mike Mainland, Phil Marucci, Elden Stang, Alto
Straughn, and last but certainly not least, my good friend Paul Lyrene.
And, as always, I would like to thank my bombolongical colleagues, Rick
Fisher, Nelson Pomeroy, Bill Stephen, and James Thomson, for their
patience and wise comments.
|
