Bug Day (18): Killer Wasp

Ichneumonid wasp, genus Netelia

As I was poking around in the fennel bush looking for aphids, ants, and ladybugs, this reddish creature flew in and crashlanded in a dense tangle of green tendrils in front of my eyes. This bug had a lot of extremities: long antennae, long legs, big wings, very long belly. Getting all its appendages clear of the young fennel was quite a struggle. When it finally managed, it flew off, landing briefly on my forehead. I had no idea what it was, but there’s an app for that! The Seek app from iNaturalist thought it was an Ichneumonid wasp, possibly of the Ophion genus. I checked that against other web sources and it seemed OK. But getting a confirmed ID was another matter. There’s an app for that too! Actually, not an app, but a group of dedicated volunteers with deep bug knowledge who run the BugGuide.net website. I submitted the above photo at 9 a.m. with a request for ID, and had a positive ID the same afternoon. It’s not an Ophion. The genus is Netelia. And it’s a female. The BugGuide expert based the ID on “Presence of an occipital carina, dark-colored ocellar area, scutellum with a lateral carina, propodeum with lateral glymmas, forewings with closed areolet and an ovipositor that projects noticeably beyond the tip of the metasoma.” Translation: the bug has a circular ridge between its mouth and its eyes; the area around its simple eyes on the top of the head is dark; there is a small hard plate with a sideways ridge; the place where the abdomen attaches to the thorax has lateral glymmas (whatever they are); the forewings have a small dark area surrounded by veins; and the egg-laying spike projects beyond the rear end. Identifying bugs at this level requires a whole new language; for diagrams with labels, see this page.

Much remains unknown about these bugs. Wikipedia says:

The Ichneumonidae, also known as the ichneumon wasps or ichneumonids, is a parasitoid wasp family within the insect order Hymenoptera. This insect family is among the most species-rich branches of the tree of life. At the same time, it is one of the groups for which our knowledge most severely lags behind their actual diversity. The roughly 25,000 species described today[1] probably represent less than a quarter of their true richness, but reliable estimates are lacking, as is much of the most basic knowledge about their ecologydistribution and evolution.[2]


The Netelia genus contains about 320 known species. BugGuide advises that for a human to identify the species usually requires dissection of the male genitalia (of the bug, not the scientist). I’m guessing the reason is that the bug’s male genitalia are like a key that only fits a “lock” of the same species. I doubt that the bugs engage in random fit testing with different species before mating successfully. The bugs must have a way of determining a mate of their own species without a lot of embarrassing trial and error. We humans just don’t know what the bugs know. In any event, male genital ID is not a determination that can be made from a photo at this scale. Particularly since this individual is a female.

Whatever species it may be, It’s one of hundreds of Netelia reported to iNaturalist in California, including six in Alameda County. The most recent was reported in June on a trail in the hills above UC by evolutionary biologist Silu Wang. This one is the first reported in Cesar Chavez Park.

Netelia wasps, as noted above, have long stingers. Their main function is to deposit eggs. But the Netelia, unlike almost any other known ichneumonids, can and sometimes do sting humans, causing painful irritation. However, it didn’t sting me.

Ichneumonid wasps generally inject their eggs inside caterpillars of other species. The Netelia females plant an egg on top of a caterpillar. A strong tube attaches the egg to the host, and the larva that grows out of the egg has specialized bristles on its rear end that attach it to the host caterpillar. The larva then uses its mouth to feed on the host, gradually emptying it out and killing it. This is called koinobiont parasitism, as distinct from the idiobiont method, which paralyzes or kills the host immediately. This bug’s lethal method of reproduction has made it an interesting candidate for biological control of other insects, but so far nothing in the way of commercial applications seems to have developed.

More about them: Ichneumonidae BugGuide Brit ID Parasitism BugEric

7 thoughts on “Bug Day (18): Killer Wasp

  • Marty, This brief article, https://www.amentsoc.org/publications/bulletin/articles/observations-of-netelia-vinulae.html, may help somewhat to explain the mechanics of how the Netelia egg is attached, and how the larva behaves. The study was not sufficiently thorough to reveal all the details you ask about, nor does it suggest that all (or most, or some, or just this species of) Netelia have this same egg-attaching and larval behavior. But it’s a start; google “Netelia feeding by larvae” and/or similar search phrases to see if more details are published.

  • Thanks for the clarification. I’ve removed the “straw” reference. Not clear, actually, what happens to the stalk that initially attached and anchored the egg to the host. This punctured the host’s skin. Does the larva remove the stalk and feed through the resulting hole? Or does it chew a new hole or holes when it feeds?

  • Marty, The bristles referred to by Eaton are on the larva’s rear end and are used to “hold on” to the egg/stalk. They are not straws. The larval wasp eats using its own mouth parts.

    I don’t know what Bob may have wished to infer about the ID of the wasp, but I think that while the characteristics enumerated (by you; by the BugGuide person) lead one to Netelia, the photo does not show all of those characteristics as clearly as one might desire. The specimen in the photo certainly “looks like” many other photos of Netelia.

  • Good to know, thank you. Bob, are you saying that this wasp was misidentified? Wrong genus?
    Peter, yes, I should have said larva, not egg. And I guess when you say bristles plural you’re saying the larva sucks its host’s fluids through not just one but multiple straws.

  • Bug Guide (and some of it’s participants) while certainly helpful, are not always without error. In this case, the glymma that you mentioned are basically deep indentations not on the propodeum, but rather on each side of the first metasomal segment, in what APPEARS to be the first segment of the abdomen. In fact, the propodeum is the first true abdominal segment, but in most Hymenoptera, this segment is fused with the thorax, so the narrow part of the body immediately behind the propodeum is the abdomen (less the 1st segment) and called the “metasoma”). In some species, these indentations are SO deep that they almost touch each other, making for a virtual hole through the segment.

  • BTW, related to this topic of Ichneumonidae, one of the famous ichneumonologists was Gerd Heinrich. His son, Bernd, was at one time on the faculty of the UC Berkeley Entomology Dept. Bernie is an accomplished author of great books popularizing various biological topics. One book, which details some of his father’s biological collecting activities is:
    Heinrich, Bernd (2007). The Snoring Bird: My Family’s Journey Through a Century of Biology. New York: Ecco. ISBN 978-006074215-7.

    It’s a wonderful read.

  • Great photo.

    ” A strong tube attaches the egg to the host, and serves as a straw through which the wasp egg feeds on the caterpillar’s insides until eventually the host dies.”

    Actually, as Eric Eaton states, “The larval wasp that hatches from the egg remains attached to it via specialized bristles on its posterior end. The wasp larva feeds on the caterpillar as an external parasite.”

    I.e., it’s the larva –not the egg– of the wasp that eats the parasitized caterpillar. So now, be on the lookout for caterpillars with wasp larvae hanging on the outside of them, to feast and turn those butterfly larvae into wasps !

    To deep-dive into the external anatomy of this wasp, and to clarify* where to find some of the structures named above, check out this really nice graphic elaboration: http://www.amentinst.org/GIN/morphology.php
    * It will still be very difficult to discern some of those structures on the photographed wasp.

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