BRIAN'S COUNTRY DIARY AUGUST 2008
On the face of it, if you hanker for the quiet life, being a caterpillar is not such a bad idea. After all, you only have to chomp away on the greenstuff you happen to be standing on, to grow big enough to turn into a pupa or chrysalis. Then if you are patient you will eventually end up as a moth or a butterfly, to take to the skies for a short but pleasant adulthood.
The reality of course is very far from this. Some clue as to the hazards to be faced come from the fact that many species lay hundreds of eggs. Say your average adult female produces three hundred eggs. For the population to stay stable two hundred and ninety-eight will have to die before reaching adulthood. Any more surviving and there would rapidly be a population explosion of that species.
One particularly nasty way of bowing out is to be parasitized. This was brought home to me the other day as I sat in my garden on a pleasantly warm summers’ day. Close by was a sallow bush which I had nurtured from a seed wind-drifted into the garden many years before. About four feet high, it has this year been virtually stripped of its leaves by an attack of birch sawfly caterpillars. The birch sawfly belongs to the same insect group as the bees, wasps and ants. Like them it has four gauzy wings, but otherwise looks a bit like an oversized house fly, mainly black with orange and yellow markings on its body.
The hundreds of caterpillars, green with yellow and black spots, and shiny black heads, were hanging from the edges of the nibbled leaves, with their bodies held in a strange way, the hind end away from the leaf, which made them look like so many tiny question marks. They blended so well with their surroundings that until my eyes became accustomed, they were quite difficult to spot.
Among them was a slender, mainly black, narrow-waisted wasp-like insect. As it walked on long legs, across the leaf, it was tapping the leaf continuously with its antennae. It was obviously looking for something. That something turned out to be the sawfly larvae. For she was a parasitic wasp, on the lookout for somewhere to lay her precious eggs. As she approached the helpless sawfly caterpillars they reared up even more, in a futile attempt to scare her off. Then in an action so swift I could scarcely follow it, she pierced the skin of her victim with her sting-like ovipositor (egg-laying tool), and inserted her egg into the soft tissues.
To cut a long story short, the wasp grub would soon hatch within the sawfly, eat it alive from within, carefully saving the vital bits to the last because you do not want your host to die before you are ready, and the final outcome would be the emergence of a wasp, not a sawfly. What a way to go!
What with the danger of being eaten by a passing bird, small mammal, or even a questing predatory beetle, succumbing to a fungus disease, being swept off by a puff of wind to starve on the ground below, to mention just a few of hundreds of possible dangers, perhaps the caterpillar has a pretty tough existence after all.
Oh, and in case you are wondering what happened to the leafless sallow bush, it is already sprouting a new crop of pale green leaves, as I knew it would, so that next spring it will delight me with its abundant silky yellow ‘pussy-willow’ catkins, as if nothing had ever happened.
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