Ichneumon wasp identification

Black Giant Ichneumonid Wasp (Megarhyssa atrata)

One of the wonderful things about this time of year is that when I go out somewhere hoping to see one thing, I often end up find myself captivated by something completely different. On June 16th I went to the South March Highlands hoping to find some new butterflies and dragonflies to add to the iNaturalist project I created a while back. This is such a large conservation area, containing a number of different habitats, that it seemed peculiar to me that I have not seen a corresponding variety in these two orders of insects…instead, I’ve seen only the most common species. I had a long walk (over 5 km in total) and saw some good birds, and even added a new dragonfly to the project (more on that later!), but it wasn’t until I was almost done that I came across something that absolutely fascinated me: an old stump covered in wasps.

When most people think of wasps, they think of the aggressive, stinging, social nest-builders that hunt for rotting food in garbage cans on warm summer days, terrorize anyone eating food in a park, and come swarming out of their mud or paper nest ready to sting if anyone dares to approach it. The truth is there are many types of wasps, and most are solitary – they do not nest in colonies, but rather use tiny tunnels built into the soil or in rotting wood. A few, such as the potter or mason wasps, build nests from material they have collected themselves. Many do not nest at all. Only social wasps sting in self-defense or become aggressive if you disturb their nest or try to slap them away. Solitary wasps are not aggressive, and do not sting in self-defense; those species that have stingers use them to subdue and paralyze other insects.

Ichneumon wasp sp. (male)

The wasps I saw crawling over the stump were solitary wasps, and I recognized them as Ichneumon Wasps – a family of parasitoid wasps that lay their eggs on the larvae of other insect species that have burrowed into the wood of decaying logs or stumps. Parasitoid organisms differ from parasitic organisms in that their feeding results in the death of the host. In this case, the host species (the Pigeon Horntail wasp, Tremex columba, another non-stinging solitary wasp) provides nutrients to the Ichneumon wasp larvae once they hatch inside the rotting tree. While some people may find the life cycle of parasitoid wasps quite gruesome compared to the fairy-tale life cycle of honey bees which industriously make honey to feed their young, parasitoid wasps are beneficial in that they keep the population of many pests under control.

Female Icheumon wasps can be recognized by the frighteningly long, thread-like ovipositor at the tip of the abdomen – fortunately this is not a stinger, but rather an egg-laying structure that is capable of boring down through the wood to reach the Pigeon Horntail larvae. Males are much smaller, lack an ovipositor, and often appear quite different from the females of the same species. It was the males I noticed first, though I wasn’t sure of the species – and while two appeared quite similar, one appeared much smaller. I took several photos before realizing three large, black females were hidden on the base of the stump by the vegetation growing next to it – I knew these females belonged to the genus Megarhyssa, and this provided a starting point for my identification research.

An iNaturalist user was kind enough to point me to an online key for differentiating the species. Fortunately, only four Megarhyssa wasp species are found in the northeast (common names taken from iNaturalist):

• Megarhyssa atrata (Black Giant Ichneumonid Wasp)
• Megarhyssa macrurus (Long-tailed Giant Ichneumonid Wasp)
• Megarhyssa greenei (Greene’s Giant Ichneumonid Wasp)
• Megarhyssa nortoni (Norton’s Giant Ichneumonid Wasp)

Males

Males are somewhat easier to identify than the females. First, determine whether there is a brown patch near the end of the wing (cell 2R1 for those who are knowledgeable about wing venation – I am not!). Those with a brown patch in the wing are safely identifiable as the Long-tailed Giant Ichneumonid Wasp (M. macrurus).

Long-tailed Giant Ichneumonid Wasp (male) (M. macrurus)

The remaining three males lack brown wing patches. One individual I saw looked almost identical to M. macrurus except it lacked the brown wing patch. Other males were smaller, with much redder abdomens.

Here the key requires us to get a good look at the side of the mesosoma (thorax). If the area below the wing insertion is chiefly reddish-brown with a single yellow spot, the wasp is a Norton’s Giant Ichneumonid Wasp (M. nortoni). If the area of the thorax below the wing insertion is mottled blackish-brown and yellow, the wasp is a Black Giant Ichneumonid Wasp (M. atrata). If the area below the wing insertion is mottled reddish-brown and yellow, the wasp is a Greene’s Giant Ichneumonid Wasp (M. greenei).

Black Giant Ichneumonid Wasp (Megarhyssa atrata)

Not mentioned in the key, but mentioned on BugGuide.net, the colour of the hind femur can also be used to separate M. greenei from M. atrata. The femur – the segment of the leg that is closest to the body – is entirely light brown in M. greenei, and is partly black in M. atrata. This is useful when photographing the wasps from above and the side of the thorax is not visible.

Ichneumon wasps (males) – M. atrata (left) and M. greenei (right)

Not mentioned in either the key or in BugGuide are the colour of the abdomen and the size difference – M. atrata males appear dark burgundy-black and and are larger than M. greenei males, which are a brighter red. The size difference was so pronounced that I didn’t take as many photos of the smaller M. greenei males because I wasn’t sure I could capture a great level of detail in the shady conditions.

Females

Similar to the males, one of the Megarhyssa females is easy to identify, while the others are slightly more difficult.

The easiest to identify is the Black Giant Ichneumonid Wasp (Megarhyssa atrata), whose metasoma (abdomen) is black with no yellow markings. Her ovipositor is about 3.7 times the length of her forewings.

Black Giant Ichneumonid Wasp (Megarhyssa atrata)

The female Black Giant Ichneumonid Wasp is the largest of the group, and she is unmistakable…and breathtaking. Her body length is 38 mm, her antennae are 24 mm long, and her ovipositor is 130 mm long…no wonder people who see her for the first time think she is something to fear!

Black Giant Ichneumonid Wasp (Megarhyssa atrata)

The other three females in the genus all have abdomens whose colours range from brown to reddish-brown with conspicuous bright yellow markings; M. nortoni is the easiest of these to identify as the yellow markings consist of distinct, round, yellow spots along the sides.

The remaining two females, however, have a series of yellow stripes running down the length of the abdomen that, when viewed from the side, appear to be angled. According to the key, there are two ways to differentiate M. marurus and M. greenei. The first involves a close look at the face, which is something I did not get when I encountered one female on a different log in the same area: M. macrurus has two black vertical stripes between the eyes just beneath the antennae, while M. greenei has none. Second, the length of the ovipositor is about 1.8 times the length of the forewings in M. greenei and about 3 times the length of the forewings in M. macrurus.

Greene’s Giant Ichneumonid Wasp (M. greenei)

I first noticed the second female species when she landed briefly on the stump before flying across the path to spend some time investigating the log there. At one point she landed on a green sunlit leaf, allowing me to get some bright, clear shots which I hoped would be enough to identify her. Unfortunately, my photo did not show either the face or the length of the ovipositor. Fortunately, BugGuide was able to provide some guidance on this: the basal vein of M. macrurus appears to have dark smudging along its length, while there is no dark smudging along the basal vein in M. greenei.

This led me to a series of searches trying to figure out where the basal vein was located in the wing. I couldn’t find a good resource for the wing venation of the genus Megarhyssa that mentioned the basal vein, but a diagram from a Japanese paper about a different family of bees was the most useful in determining what to look for as I paged through a number of images on BugGuide. The basal vein appears to be the cross-vein closest to the body:

Wing Venation showing location of basal vein

There is no smudging in this area, which makes this female a Greene’s Giant Ichneumonid Wasp (M. greenei). This meant that I had three different male species and two different female species all investigating the same two logs. The three species present – M. atrata, M. macrurus and M. greenei – all use the Pigeon Horntail wasp as the host for their larvae, while M. nortoni uses three different horntail wasp species altogether, explaining its absence.

Reproduction

I was able to see both females ovipositing, though neither species would do so in a nice, sunny, open area. Because it takes such a long time to drill down deep enough into the wood to reach the horntail wasp larvae – over an hour for M. atrata – the females are vulnerable while ovipositing, which is likely why they prefer to do so hidden behind vegetation or in the shadows.

Greene’s Giant Ichneumonid Wasp (M. greenei)

The life cycle of these wasps begins right at emergence – both males and females leave their burrows as adults immediately capable of reproduction. In fact, males will only mate with freshly emerged females, finding them by listening for the sounds of the females chewing their way through the wood. This results in large mixed-species swarms at a single stump where the original Pigeon Horntail wasps laid their eggs, which explains all the wasps I saw at South March Highlands. Mating occurs as soon as the female emerges, who then flies off to locate another piece of decaying wood containing the host larvae. The males remain at the same stump, waiting for other females to emerge.

Black Giant Ichneumonid Wasp (M. atrata)

While it is not clear how the female Ichneumon wasps locate the specific host larvae, it is believed that they are able to do so using sound or smell. The female uses her antennae to determine the position of a single larva deep inside, then moves into position to deposit her own egg on top of the horntail larva. She stretches her hind legs to make herself as tall as possible, then moves the ovipositor into position. Although the ovipositor looks like a single structure, it contains three filaments: a central tube and two side filaments which protect the ovipositor, provide stability, and contain a cutting edge at the tip. The two side filaments can often be seen while the wasp is laying her eggs, as they will arc out to the sides.

During the process of egg-laying, another structure comes into play: a membranous disc that forms when the wasp rotates the segments near the end of the abdomen. The inflation of this membrane helps push the ovipositor into the wood and gives it the force needed to drill down into the wood. Tissue on the inside surface of the membrane produces an enzyme-containing fluid that helps break down the wood, allowing the wasp to more easily insert her ovipositor to the required depths. It is a lengthy process, and I was able to see parts of it with four individual female wasps right in front of me.

Black Giant Ichneumonid Wasp (M. atrata) with membranous disc present

The Pigeon Horntail wasp larva provides all the nutrients an individual Ichneumon wasp will ever need; adults do not eat, and live less than a month after emerging (M. atrata live an average of 27 days). The Ichneumon wasp larvae spend the winter inside the burrow, emerging in the spring the following year to begin the cycle again.

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Author’s Note: These images were uploaded to iNaturalist, and one of them found its way into a new (September 2021) identification key for Megarhyssa wasps found in the U.S. and Canada! The guide, written by Zachary Dankowicz and Juan Cepeda Espinosa, covers not just the four species covered in this post, but subspecies found in southern and desert regions of the U.S. as well. It is quite user friendly, so check it out if you find yourself interested in photographing these colourful wasps next summer!

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Citations:

• Pook V., Sharkey M., Wahl D. (2016) Key to the species of Megarhyssa (Hymenoptera, Ichneumonidae, Rhyssinae) in America, north of Mexico. Deutsche Entomologische Zeitschrift 63(1): 137-148. https://doi.org/10.3897/dez.63.7619
• Hirashima Y. and 義宏 平嶋 (1962) Systematic and biological studies of the family Andrenidae of Japan (Hymenoptera, Apoidea) Part 2. Systematics, 7. https://www.semanticscholar.org/paper/Systematic-and-biological-studies-of-the-family-of-Hirashima-%E5%B9%B3%E5%B6%8B/3d4dbd82042aa02f615d373ce4dcd085b5691fff
• BugGuide. https://bugguide.net/node/view/4158. Ames, IA: Iowa State University [accessed September 29, 2020]
• BugGuide. https://bugguide.net/node/view/609755/bgimage. Ames, IA: Iowa State University [accessed September 29, 2020]
• BugGuide. https://bugguide.net/node/view/1327750/bgimage. Ames, IA: Iowa State University [accessed September 29, 2020]
• BugGuide. https://bugguide.net/node/view/119173. Ames, IA: Iowa State University [accessed September 29, 2020]
• Klein, S. (2012) “Megarhyssa atrata” (On-line), Animal Diversity Web. Accessed September 29, 2020 at https://animaldiversity.org/accounts/Megarhyssa_atrata/
• Sezen, U. (2016) Giant Ichneumon Wasp (Megarhyssa macrurus) Ovipositing (On-line), Nature Documentaries.org. Accessed September 30, 2020 at http://naturedocumentaries.org/3843/giant-ichneumon-megarhyssa-macrurus-ovipositing/

Special thanks to Paul Lagasi for photo-editing assistance on image #10.

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