The caterpillar and the wasp

Outdoor Living

PRETTY DEADLY Yellow Ophions are parasitic wasps that lay their eggs inside sawfly caterpillars, with gruesome results.


Country Sights and Sounds
Michael Kingdon

I find myself drawn repeatedly to a small Scots pine growing at the lake shore, among the needles of which an extraordinary drama is being enacted.
The first player was the pine sawfly, a rather stubby insect fitted with a saw-like appendage at its rear end, which it used to make cuts into the bark of pine branches. This is where it placed its eggs.
On hatching out the sawfly larvae, which resemble green caterpillars, spend every waking hour feeding. They sit alongside each other in groups of three (there are some fours, I notice), with their heads all pointing the same way at the tip of a pine needle and nibbling their way back down the leaf until there is nothing left, at which they move on to the next. When they are disturbed each group throw their heads back in unison, creating the illusion of a small snake opening its mouth ready to strike.
The larvae are growing. Even as they do, some among them are being thinned out, or harvested, I suppose, probably by birds with little ones to feed. Being eaten by a bird probably isn’t nearly the first of their priorities, yet I think it infinitely better then the fate which awaits some of their friends.
By way of a supporting actress, a small, slim insect hovers nearby, making darting flights among the pine branches as if making an inspection of the sawfly family. She has been attracted to this tree by a chemical signal that is released on caterpillar attack.
I think she is a yellow ophion – a species of ichneumonid wasp – if not, she is a close relative. Her body is red in colour. Her transparent wings carry strong, dark veining. She has at her front end a pair of long, quivering antennae and two shiny black, angular eyes. Her waist is slim and leads to an elongated abdomen, at the tip of which is her ovipositor, a specially designed egg-laying mechanism. Although under an inch in length she is the stuff of nightmares.
She certainly likes the look of the sawflies. After flying among them for several minutes she seems to have made a selection and lands nearby. With her wings folded along her back she moves slowly toward her target. Before she gets within reach Lady Ophion changes her position. Standing on tiptoe she curves her abdomen down and forward, so that it now protrudes from between her front legs, just in front of her head. She holds it like a weapon. I see it quivering and glistening with menace.
To maintain this stance she uses her wings to support herself, holding them behind her, slightly spread and pressed against the branchlet along which she creeps. It has already taken her a minute to make an inch of progress, but now she is within millimeters of her first victim, which continues to graze unsuspectingly.  
The moment arrives. She makes the tiniest darting movement and strikes the caterpillar just behind the head. The poor beast rears by way of objection but too late! It now has, for the rest of its life, a terrible alien companion. Yes, it is only an egg, but it is a death warrant for the host.
When this egg hatches the tiny grub that emerges will begin to consume the caterpillar from the inside. It somehow knows to avoid the vital organs – after all, if it were to eat the creatures heart it would be out of house and home. So there it lives without a care, drinking the soup of its surrogate mother.
I saw my yellow ophion lay two eggs in this manner, in two out of three neighbouring caterpillars. She left the third alone. I suppose if she were to attack the entire colony there would be a danger of having no sawflies for her own offspring to make use of.
The afflicted caterpillars quickly settle down, feeding and growing alongside their friends as if nothing has happened. I imagine that being eaten alive from the inside would be an unpleasant experience, but there seems to be no distress involved.
It is all rather ghoulish and very bizarre. I’m just glad these paratizoid wasps are only small and that, for now at least, larger game is off the menu. A change in their choice of prey might make rambling a distinctly hazardous occupation.
 
Michael Kingdon formerly wrote these columns under the pseudonym John Shelley. A naturalist and keen fisherman, he lives close to the shores of Lough Carra.

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