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Life inside rotting walnut husks

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There’s an eastern black walnut (Juglans nigra) on my neighbor’s property that rains down fruit every fall, and I finally got curious about what species might be inside. So far I’ve found four flies, five beetles, two mites, and a parasitic wasp. I’m a little disappointed with the number of wasps but am hopeful that over time I can find more. There are almost always more wasps.

Walnut husk maggots

Almost all of them were filled with fly larvae that are most likely Rhagoletis suavis (Tephritidae). If you collect walnuts for their nutmeat, these are unwanted residents because their activity can make the husks difficult to remove, stain the shell, and even cause the kernel to discolor, become bitter, and shrivel. Commercial walnut groves Juglans regia will even treat spray insecticide to minimize their presence. Below is a typical scene showing a healthy population of the larvae:

And here’s a close-up. It’s interesting that the larvae seem to accumulate dark pigment in each segment. I’m assuming it’s juglone, also known as Natural Brown 7. The presence is probably unavoidable given what it is eating but I wonder whether it confers any protection against predators. It’s certainly allelopathic, antifungal, and can kill fish.

Here’s an adult that I found a few years ago.

There are several species of tephritids that utilize walnuts. Rhagoletis suavis is restricted to the eastern half of North America (iNaturalist distribution map), with the western-most observation being in Colorado. The western species, Rhagoletis completa (BugGuide pics; iNaturalist map), seems to be expanding its range, arriving in Jonesboro, Arkansas in 2007. Another western species, Rhagoletis juglandis (BugGuide pics; iNaturalist map), occurs in Arizona, Utah, New Mexico, Mexico, and apparently British Columbia. If you live in an area with more than one of these species, you’d need to rear larvae to adulthood to get an identification (see fig. 8 here for wing pattern differences). Alternatively, you can try to use the key to larvae in Steyskal 1973.

Fungus gnat

This fly emerged from a pile of rotting walnuts that I’d covered with fine netting to catch whatever hatched when I wasn’t looking. I think it is Leia bivittata (Mycetophilidae). Per Weiss 1919 the larvae move around in a bag of mucous. It’s the only one that appeared. There is nothing in the literature that suggests they eat the fungi in rotting walnuts, so I don’t know how common they are in the fruit.

Vinegar flies

I found two species of vinegar flies (Drosophilidae) hanging out on rotting husks, Drosophila repleta and Drosophila tripunctata, the latter in large numbers. Per Buchner and Symmes 2014, Drosophila are attracted to the rotting flesh caused by Rhagoletis infestation. What’s notable to me is that larvae can tolerate the juglone and related compounds.

Beetles

This scooped scarab (Onthophagus hecate) was wedged inside a crevice in a walnut husk. They eat fungi but will also show up to eat dead animals.

I also found four different kinds of staphylinids. I’ve only been able to identify one, Belonuchus rufipennis, but will update this post if and when an expert on iNaturalist sees them (so probably at least a decade). I would love to know what they are eating. There are certainly plenty of fly larvae present but I don’t have any confirmation that those are their targets.

Anotylus sp.
Anotylus
Omalium sp.
Omalium
Aleocharinae
Aleocharinae
Belonuchus rufipennis
Belonuchus rufipennis

Mites

The rotting walnuts were crawling with mites, often numbering in the dozens. These are extremely small and fast moving so I’m not sure how many different kinds are present. I think there are at least two species, both in the order Mesostigmata. The first image might be in the suborder Gamasina, and possibly in the genus Lasioseius. I have no idea what they are eating but some mesostigs are omnivorous.

Parasitic wasps

Although I didn’t find them inside the walnuts, this wasp in the genus Coptera (Diapriidae) ended up in a yellow pan trap set up near a tray of Rhagoletis suava puparia that I situated atop a bin of several dozen walnuts. Here’s a really low-res video of it walking around on the tray after I rescued it from the pan trap. There was a second individual walking on the tray at the same time, so I don’t think the pan-trap individual was just a rando Coptera attracted to yellow. These wasps apparently dig down into the soil to find pupae. Some articles say they’ll drag the pupae to the surface prior to ovipositing into them.

Members of this genus are rarely observed and are difficult to tell apart, but based on proximity to walnuts it is likely Coptera pomonellae, the only species that arrived at pan traps set up under walnut trees in Iowa (two locations) and Indiana in Forbes et al. 2012.

Because I have trays of bait pupae, I’m also monitoring for but haven’t yet seen:

  1. Diachasmimorpha mellea (Braconidae), a wasp that is reported to parasitize Rhagoletis suavis (Krombein and Burks 1967, Wharton and Marsh 1978).
  2. Aphaereta pallipes (Braconidae), a pupal parasite of Rhagoletis suavis that was documented in Amherst, MA (Babb 1902). Per Joseph Fortier, it’s been reared from 15 dipteran families. The two observations on iNaturalist emerged from Chirosia gleniensis and Fannia canicularis.
  3. Phygadeuon sp. (Ichneumonidae), a species that attacks pupae of (the closely-related) Rhagoletis juglandis (Andrew Forbes, pers. comm.). I’m not sure whether Phygadeuon attacks Rhagoletis suavis, but there are observations in the eastern USA.
  4. Aganaspis alujai (Figitidae), a parthenogenic wasp that attacks Rhagoletis completa in Mexico (Ovruski et al. 2007, Takata et al. 2025). I have no idea whether there’s a member of the genus in Pennsylvania, so this is another unlikely arrival.
  5. Tetrastichus giffardianus (Eulophidae) “may parasitize Rhagoletis suavis completa CRESSON” (Narayanan and Chawla 1962, citing Marlatt 1933, which I cannot locate). Marlatt was writing in a USDA publication so this relates to the United States. But host species is now Rhagoletis completa, so this is probably a western wasp (confirmed: just California).

Absent from the list are hyperparasitoids, which is kind of annoying because I really like them. I will certainly be on the lookout for them, of course. I’ve also read that a fairy wasp attacks the Rhagoletis eggs, and that would also be fun to find.

I’ll post an update in August 2026 with details on what emerges from the several hundred Rhagoletis suava pupae I have. Hopefully there will be at least some Coptera in the mix, and with some luck I’ll get photographs of both sexes. I’m also very curious what types of wasps might seek out Rhagoletis completa that are on eastern black walnuts planted in the western USA, but that hinges on my sister reading this post and setting up some trays in a similar way (simple instructions are in next section). The first record of Rhagoletis completa in Oregon was in 1963 (Berlocher 1984).

Below is a photograph of the trays of pupae. The chair is so I can drink my coffee while scanning for wasps.

Collecting fly puparia

If anyone is curious how I’m getting puparia, I placed infested walnuts in trays filled with sand and vermiculate, and the larvae tunnel down into the medium to pupate. The plexiglass is to keep the rain from flooding the containers, and the whole operation is conducted inside a screened enclosure that keeps the squirrels from pilfering the walnuts. This is making the squirrels angry.

The larvae eventually burrow into the medium to form puparia, but sometimes they simply pupated underneath a walnut (e.g., as in photo below). At some point I’ll sift the media to collect puparia so that I can more closely monitor them. I’m curious whether they can be candled under a microscope to spy on the residents.

Here’s a sampling of the puparia along with some third-instar larvae. I suspect I have 500 or so puparia in the trays. Some pupae apparently stay dormant for two years (Stirret 1936).

Next year I’ll likely repeat all of the above but start monitoring for Coptera much earlier.

Some insects I found inside dried Turkish figs from Trader Joe’s

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During an online wasp identification course I participated in several weeks ago, there was a fun discussion about whether commercially-produced figs contained fig wasps (Agaonidae, in the Chalcidoidea). Opinions varied, plus some asserted, as do many internet pages, that even if wasps were once inside the figs that ficain (an enzyme) would completely dissolve their bodies. So I purchased a bag of figs and had a look. I found a wasp in the very first fig, in approximately two seconds. And I found much more than that. Photographic details below.

Photograph of a fig being examined under a dissecting microscope

The wasp was small, like most wasps, so here’s a photograph with an arrow pointing out the position. It was wedged in between some of the involucral bracts just inside the ostiole (opening) of the syconium (the “fruit” that is actually an inverted infructescence). I’m not sure whether the wasp was going in or going out.

Photograph of an opened fig with an arrow showing the location of a dead wasp

Here’s a close-up of the wasp, or at least what’s left of it. The shape and positions of the legs seem to be a good match for Blastophaga psenes, the sole pollinator (I think) of Ficus carica. In particular, the middle femorae are much smaller than the front and hind femorae. And the head is wedge-shaped, another attribute of the family.

The species exists in Turkey and in many other places where figs are grown. E.g., it’s in California because it was deliberately introduced there in 1899. This means, of course, that figs grown in California may have wasps in them, too. I’m just the messenger.

Photograph of a dead fig wasp inside a fig

Below is the dorsal side showing (I think) the quadrate scutellum, another family characteristic. The antennae appear to be folded medially into a streamlined position that I assume is useful for navigating the bract maze. I have more pics on iNaturalist.

Photograph showing the dorsal side of a fig wasp

In a different fig, I found two more wasps of a second species. I don’t have a high confidence in my identification but I think they might be Habrobracon hebetor (Braconidae). This species is a regular inside stored figs because it parasitizes caterpillars that eat dried fruit. This wasp, I’m guessing, oviposited into caterpillar-infested figs after they arrived in California (where Trader Joe’s presumably has a warehouse). I don’t know whether the species occurs in Turkey. I have more pics on iNaturalist.

Photograph of tiny dead wasp next to orange frass

Below is a caterpillar in the same fig as the wasp above. Caterpillars were present in several other figs, too, and they were often accompanied by fungal growth. I don’t have an identification guess but if you recognize it please visit my iNaturalist observation.

Photograph of a dead, shriveled caterpillar

I’m not sure whether it’s a different species, but the cocoons below seem to be lepidopterans, too. Here is the iNaturalist link.

Photograph of two caterpillars inside silk cocoons

Here’s the final type of lepidopteran I found, a pupa. Again, I’m not sure what family it might be in. Here’s the iNaturalist listing. The photograph also shows a good quantity of frass, something I found in many of the figs even when I couldn’t locate a caterpillar. The dark color on the left is from fungal growth.

Photograph of a moth pupa next to fig achenes and black frass

The final insect that turned up is a beetle, which I think might be Carpophilus hemipterus, a species native to Asia but now pretty much everywhere. It’s definitely in California. Here’s my iNaturalist observation if you’d like to weigh in on the identification.

Photograph of black and yellow beetle inside a fig

I’d like to point out that insects are extremely common in stored food so the above doesn’t come as a huge surprise to me. And the figs were clearly labelled “organic”, a term that to me greatly boosts the likelihood that there are organisms inside. Unless you have an allergy to chitin or some other insect component (it happens), inadvertent consumption of arthropods isn’t going to harm you.

What did surprise me was how common fungal growth was in this bag of figs. Below is a photograph of what the fungus usually looked like. Not sure what it is (Aspergillus niger?). Some of them were mildly lit up by 356nm UV light, by the way. I think that’s a bad sign.

Photograph of fungal growth next to achenes inside a fig

But a little research turned up what might be the obvious explanation: wasps that manage to crawl into figs via ostiole can be covered with fungal spores in the same way that they are often covered with pollen (that’s the whole point of fig wasps). Indeed, some fig farmers regularly spray anti-fungals onto figs before the females emerge to prevent them from picking up spores and transmitting them to the next fig. Without such treatment (i.e., at organic farms), it’s not rare to find smuts (Aspergillus spp.), endosepsis (Fusarium spp.), and Alternaria rot. Aside from potentially changing the taste of dried figs, I guess there’s a chance you could get a dose of aflatoxins. There’s research on the issue, even articles on the risk from dried Turkish figs.

When fresh figs start to arrive from California I might sacrifice some for a similar investigation. I’d love to find a fig wasp in better condition so I can photograph it. They are bizarre. But per several sources, many (most?) of the figs in California are produced by plants that don’t need fertilization, so finding one might be a challenge. Still, because fig wasps are naturalized in California, I wouldn’t be surprised if they’d still show up inside those varieties. As a demonstration that wasps are alive and well in California, you can order wasp-laden figs online at FigBid.com (e.g., this listing). Note that you don’t order such figs for eating. They are for placing near your crop of figs so that they can get pollinated. Some of the most delicious varieties of figs need such pollination. And some of the varieties that don’t need figs still develop much tastier fruit when they are pollinated by wasps.

Next on my to-do list is to find the coffee berry borer (Hypothenemus hampei).

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