The early spring flowers are well out now, and in common
with most other places our garden is showing mostly yellow ones. There are lots of daffodils, lesser
celandines, some Forsythia, and some dandelions. They all look wonderful until you look a bit
closer and see that almost every flower has got tiny blue-black beetles
inside. They are pollen beetles and they
are doing what comes naturally, eating pollen.
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| Pollen Beetles in Daffodil |
Pollen is amazing stuff.
First of all, it is very, very small.
The smallest pollen belongs to the forget-me-not and it is only six
microns in diameter – that is 6 thousandths of a millimetre. (The largest, maize, is about 100 microns
across and more easily seen.) You would
need a good microscope just to see the rough shape of the pollen grains of any
plant. If you wanted to see each grain
in detail, then it would be handy to have a scanning electron microscope in
your potting shed. Pollen grains are the
equivalent of sperm in the animal kingdom, and are what transports the male
genes to the seeds of the plant. As in
the animal kingdom, each plant species wants to fertilize, or be fertilized by
plants of its own species, and plants achieve this by making the pollen grains
unique. So each plant produces pollen
grains that have a certain size, shape, and surface texture. Only when a matching pollen grain lands on
the female stigma does the stigma accept the pollen and allow fertilization to
occur.
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| Pollen Beetles in Lesser Celandine |
Different plants also produce pollen of different
colours. Yellow flowers, like the ones
that the pollen beetles are so fond of, tend to have yellow pollen, and that is
the sort of default colour. But if you
examine the pollen baskets of bumblebees as I am in the habit of doing you find
many different colours – comfrey has pale straw-coloured pollen; red clover has
dark grey, almost black pollen. One of
the most startling colours is that of Viper’s Bugloss which has very dark blue
pollen. Just occasionally, I have seen
bumblebees with bright red pollen on their legs, but so far I have yet to see
which plant they have collected it from.
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| Brown-banded Carder Bee on Viper's Bugloss |
The equivalent to ‘man cannot live by bread alone’ in the
insect world would be ‘insect cannot live by nectar alone’ and one feature of
pollen is that it is rich in protein.
For most insects that eat pollen, it is their only source of
protein. There are three main groups
of insects that feed on pollen – the bees, of course, including the bumblebees
and the solitary bees I wrote about last month.
Then there are the beetles – not only pollen beetles, but there are a
range of other beetles like the thick-kneed flower beetle and some of the
longhorn beetles as well. Another group
– the second largest group after the beetles – are the flies (Diptera), though
only a few of them are pollen feeders, the most conspicuous of these being the
hover flies and the soldier flies. But
what about the moths and butterflies I hear you ask? Well, in the main, the adult insects don’t
feed on pollen, they only take nectar (and some of the moths don’t feed at all
as adults). Whilst their caterpillars
may take a bit of pollen, they are probably more interested in your brassica
leaves.
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| Peacock Butterfly on Buddleia |
The plant and pollinator relationship is a neat one, they
each get something from it, but the interesting thing about the relationship is
that the four groups of insects mentioned above all evolved before the plants
developed their showy flowers. They
developed flowers when the primary disadvantage of having their pollen eaten
was outweighed by the advantage of having their pollen moved from flower to
flower and plant to plant. This happened
about 140 million years ago when plants started to become more attractive to
insects by evolving their colourful and scented flowers. So without the pollinators, there wouldn't be
such flowers – what a dull thought.
