Giant Ichneumon

Yesterday I spotted this stunning giant Ichneumon wasp (Megarhyssa macrurus) resting on a window high up in a parking garage. No matter how many times I see these, I’m still shocked at the length of their ovipositors. Last year I had an amazing experience watching dozens of giant ichneumons (Megarhyssa cf. nortoni) ovipositing into a burned patch of forest at Yosemite. Check out my post and footage and the wasps’ expandable disc membranes if you missed it! Wasps in the family Ichneumonidae are parasitoids of a wide range of invertebrates, including beetles, lepidopterans, and other wasps. This genus exclusively parasitizes a basal group of wasps called the horntails (family Siricidae), more specifically the pigeon horntail (Tremex columba). Like ichneumonids, horntails have modified their stinging apparatus into a serrated ovipositor and therefore lack the ability to sting. While horntails have stumpy short ovipositors, many ichneumonids have incredibly long ovipositors, both for drilling into solid bark though with entirely different intentions.


Horntails simply aim to break the surface to deposit their eggs, which will hatch into larvae and bore into the tree to consume its vascular tissues. The ichneumons parasitize the already existing horntail larvae by stabbing their ovipositors deep into the tree and deposit eggs onto the horntail larvae, where their larvae will hatch and consume their fellow wasp hosts. It might seem impossibly tough for ichneumons to get their ovipositors into the wood— sort of like wobbling a long rubber tube and applying pressure against a solid surface at the perfect angle. However, the wasps are able to achieve this daring feat with efficiency. The central chitinous filament of the ovipositor retracts and separates from two surrounding sheaths (like unveiling a sword) and coils back into a bizarre translucent disc-like membrane that expands upwards between two abdominal segments. The tension in the slender ovipositor increases substantially, and the wasp simultaneously excretes destructive enzymes as she slips her ovipositor past the bark’s surface. It seems that actual penetration occurs relatively rapidly, while the precise localization of horntail larvae probably requires more patience. It would be so cool to somehow look inside the tree as this process occurs, to see how exactly the ichneumon’s ovipositor moves and whether horntail larvae have developed adaptations to avoid having parasitic roommates.

Photographed after capture [5]


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