|
I have a colony of aphids living
on a tall angelica plant outside the back door. It is
conveniently at head height and I frequently take a
break to see what is happening. For all the distress
they give gardeners and farmers, aphids are
fascinating animals. I could fill pages (or megabytes)
with their curious ways.
This colony is attended by black
ants that run all the way up the angelica stem,
‘milk’ honeydew from the aphids and run all the
way back to their nest under a paving stone. The
‘milking’ of aphids by ants is one of the great
stories of natural history. It is clearly a worthwhile
from the ants’ point of view, but what is the
advantage of the relationship to the aphids?
The obvious reason is that ants
drive away ladybirds and other predators, but there is
a second benefit. Ants coax the aphids into extruding
a droplet of honeydew on demand, but if there are no
ants present, droplets are flicked away with a hindleg
and contaminate the leaves. Plant sap is very rich in
sugar but poor in protein and other food substances.
So an aphid has to eat a lot of sap to get a proper
diet. The surplus sugar is passed out as honeydew.
If honeydew is allowed to
accumulate on the leaves, it encourages moulds that
can infect the aphids. Predatory hoverflies prefer to
lay their eggs on leaves sticky with honeydew, perhaps
in the knowledge that there will be plenty to eat
nearby.
|
