Giant Ichneumon Oviposition

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It’s difficult to get a good glimpse of ichneumonids, even though they are among my favorite groups of wasps. In Yosemite we were so lucky to stumble upon a section of charred forest where about 30-50 ichneumonids were flying around and ovipositing into the tree trunks. I was thrilled to see this behavior, especially in such a charismatic species (Megarhyssa cf. nortoni). These giant ichneumons are large and colorful, boasting one of the longest ovipositors in the arthropod world. The ovipositor in this species was around 6cm long, but in other Megarhyssa species the ovipositor can be more than three times the length of the body. The middle filament is actually the ovipositor, surrounded by two sheaths that retract into loops when the wasp lays eggs. Giant ichneumons seek out horntail larvae (family Siricidae) that are developing within trees. Usually horntails select trees with decaying wood or recently fallen trees, where their larvae may take years to feed and grow prior to pupation. Ichneumons are keen at detecting a fungus that horntail larvae use to digest wood fibers. After landing on suitable trees they walk around and tap their antennae to the substrate, detecting minute vibrations made by the horntail larvae.

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Once detected, the ichneumon will position its abdomen away from the substrate, arch its body and legs outward, rotate the distal segments of the abdomen, and push the ovipositor stylus outward. This moves the two valvulae (sheaths) aside and extends membranes to form a distinct translucent disc. Mechanoreceptive sensory organs are also present in the stylus and likely aid in further locating a suitable site of oviposition. The ichneumon will then pierce her dagger-tipped ovipositor through the wood to lay an egg in the tunnel where the horntail resides. Pores on the stylus secrete substances that aid in breaking down the substrate and/or act as a lubricant, but it is still a mystery to me how exactly their flimsy long ovipositors manage to penetrate solid wood. After the egg hatches the young ichneumon larva becomes an ectoparasitoid, attaching itself to the horntail larva and gradually consuming its tissues.

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Curiously, I often find deceased horntails still attached to trees by their short ovipositors, dying in the process. Ichneumons do not appear to have the same problem, possibly due to their more flexible and thin ovipositors. Ichneumons consistently eluded me when I was young, and I always considered them to be like fairies — flying through the air in a relatively straight path with wand-like ovipositors trailing behind. Large pompilids that I saw flying around in Costa Rica also joined ichneumons in the “fairy” category.

All wasps in this post filmed and photographed in situ / after pursuit [1-3]

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We saw at least five species of ichneumonids, three of them quite small, but here are photos of another about 3/5 the size of Megarhyssa cf. nortoni.
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Footage of ichneumonids ovipositing:

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Horntail (Siricidae), a possible host of Megarhyssa ectoparasitoids
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