The Wasps and the Fly

You literally don't have to do anything but step out the door to observe fascinating phenomena in the world of insects. I did just that on August 9th, and found myself watching peculiar behavior between a nest of European Paper Wasps, Polistes dominula, and an unidentified fly.

The paper wasp nest has been on the back of our fence gate for months, now. The occupants are amazingly tolerant, even as we come and go. They may get a bit disoriented if we leave the gate open for a lengthy period, but otherwise they are innocuous and we let them be. Imagine my surprise when the thing that has agitated them most was what appeared to be a house fly.

Upon closer examination, the fly had slightly different markings than a house fly, was spinier on the abdomen, and had perhaps longer legs. It walked a lot, nervously, and it was difficult to even get an image of it. Finally, it stopped walking and started, well, stalking towards the wasp nest. One wasp took notice and adopted a very alert posture. Suddenly, the fly flew off its perch and looped around the wasp nest at dizzying speed. It was so quick I was not sure what I was seeing.

The aftermath of the fly's reconnaissance mission was even more dramatic. The one wasp that had seen the fly in the first place suddenly began running frantically and erratically all over the comb, for probably at least ninety seconds. The fly was long gone, but I was suspecting that maybe it had laid an egg and that was the object of the wasp's energetic searching behavior.

Still puzzled by the fly's identity, I took to the internet for the most likely suspects: a tachinid fly, family Tachinidae; or a parasitic sarcophagid (Sarcophagidae). I found a tachinid that looked promising, but viewing images of the species it became readily apparent this was not the right one. Sarcophagids came up empty as well. Furthermore, I have witnessed paper wasps eating sarcophagids, not falling victim to them. Next I looked to my library and found a potential match in a European field guide, of all things. Eustalomyia festiva, a member of the diverse but obscure family Anthomyiidae, looked good. According to the text, it "Breeds in [the] bodies of flies stored by solitary wasps." Ok, but paper wasps are social....

I posted the images shown here on the help group "Hymenopterists Forum" on Facebook, and got this reply from Rui Andrade:

"It looks like Eustalomyia (Anthomyiidae). The larvae are kleptoparasites of wasps."

This makes sense to a degree, but social wasps do not store their prey for later consumption by their larval offspring. They feed masticated prey directly to the larvae. There is therefore no opportunity for a fly larva to develop in a social wasp nest under those circumstances, as a kleptoparasite feeding on prey intended for the host's offspring. The only alternative I can fathom is that the fly is parasitic on the wasp larvae themselves. Cursory review of the literature does not inform my opinion, as they all stubbornly state that solitary wasps are the victims of Eustalomyia. The wasps are not going to let me peer into each cell to see if there is something other than a wasp egg or larva inside, either.

Perhaps this one individual fly was just confused as to the proper host. Maybe it was initially attracted to the bee block ("bee condo") hanging up on another part of the fence, where we have indeed had solitary wasps nesting. This year the bee condo has been devoid of activity for the most part, so maybe scaring the paper wasps was an amusement borne of frustration for this fly. We may never know.

Sources: Chinery, Michael. 2012. Insects of Britain and Western Europe. A Domino Guide. London: Bloomsbury Publishing. 320 pp.
McGavin, George C. 1992. The Pocket Guide to Insects of the Northern Hemisphere. London: Parkgate Books, Ltd. 208 pp.

"New" Insight Into Behavior of Some Cuckoo Wasps (Chrysididae)

There is always something new to learn. That is the promise of entomology, and it is validated almost daily. Just last week (January 11) I came across this amazing image of Omalus puncticollis by Emily Hobson on my Facebook newsfeed. Since I know that many species of cuckoo wasps feed on the "honeydew" secreted as a sugary, liquid waste product by aphids and related true bugs, I suspected that the wasp in this photo was stimulating the little bug into producing some honeydew.

Female cuckoo wasps do not have stingers, so I knew that this was not a case of the wasp stinging a victim to then either consume herself or take back to a nest.....or so I thought. Cuckoo wasps are parasites of other kinds of wasps, and it turns out that this is what the wasp was up to, but indirectly.

One of the comments on the image revealed the extraordinary truth:

"This is really interesting. The species is probably Omalus violaceus, and the pic shows a behaviour, what is not formally described until now. The female Chrysididae wasp lays its egg into the Aphid. When a Crabonidae wasp (e.g. Pemphredon) takes the aphid as prey and carries it into its nest, then the chrysidid larva will appear earlier and develop in the nest instead of the larva of the crabronid wasp. Until now, many wasp researchers still think that the Chrysididae infect the host nest directly, which is not true. This behaviour is observed or suspected for Omalus and related genera, and for Holopyga." - Christian Schmid-Egger

So, the chrysidid wasp larva is a "cleptoparasite" that eats the food provided by the host wasp for its offspring. It literally steals from the mouths of the host larva, starving it to death.

These images were taken in Europe, but we have related species in North America, including a few that have immigrated from overseas as a result of accidental introductions on global commerce. I wrote about one such species, Pseudomalus auratus, in a previous blog post.

The comment thread from the Facebook "Entomology" group post reveals this might be the first photographic documentation of this behavior in this particular species, as noted by Alexander Berg, who also gives references:

"Actually it is described, for Omalus by Winterhagen P (2015), Holopyga by Veenendaal R (2012), and we also discuss it in Paukkunen et al 2015 (doi: zookeys.548.6164) for Pseudomalus (something Veenendaal has also postulated)....I don't think anyone doubts it occurs in all Pseudomalus, Omalus and Holopyga...."

Science and social media is becoming a marriage made in heaven, not only for informing the general public, but for communication between scientists, too. I am left now to wonder how much else I think I know, that I really don't know. In any event, you'll know when I know.

Special thanks to Emily Hobson for sharing her images, and for Christian Schmid-Egger for permission to use his quote here.

Wasp Wednesday: Stizoides renicinctus

NOTE: I failed to make it clear how many wasps, and even some flies, look nearly identical to this species if you are not an entomologist. There is the Blue-winged Wasp, Scolia dubia, spider wasps (Anoplius atrox and others), and even the enormous fly Mydas clavatus that are equal to or larger than Stizoides renicinctus. Stizoides renicinctus is also not nearly as common as these other wasps. So, those of you insisting you are seeing this species have likely mistaken it for something else. Thank you, and my apologies.

One of the cool “surprise bugs” from the ”wasp tree” was a kleptoparasitic wasp (I’ll explain that term below), Stizoides renicinctus. This insect is so obscure that it has no common name in English, but that doesn’t take anything away from its unique life history.

S. renicinctus is one of only two species in the genus occurring in North America. Globally, there are thirty species in the genus, most found in Africa, Mediterranean Europe, the Middle East, and India. The only other species in the U.S. is S. foxi, restricted to Arizona (and south into Mexico).

Field marks that help to identify S. renicinctus include an all black body with a red or orange band on the second dorsal abdominal segment (tergite), dark wings with translucent wingtips, and an overall elongate appearance. Both genders are nearly identical in appearance and size, at about 16-18 millimeters.

One usually encounters the males taking nectar from flowers. I was surprised at the fair number of them on the saltcedar tree during the latter half of June here in my Colorado Springs neighborhood. I have also seen this species in southern Arizona, coming to the blossoms of Seepwillow, Baccharis salicifolia in dry arroyos or along rivers.

Females are mostly busy seeking hosts for their offspring. The term “kleptoparasite” refers to an animal that essentially raids the refrigerator of its host organism. That means the parasite feeds on the material stored as food by its host. S. renicinctus is known to exploit the food caches of other solitary wasps in the genera Prionyx and Palmodes. Female Prionyx hunt grasshoppers (Acrididae), while Palmodes hunts katydids (Tettigoniidae). The wasps dig burrows and stock at least one prey item per cell that a single larva will consume to grow and mature into an adult wasp of the next generation. The female Stizoides renicinctus locates the closed burrow of a host, digs it open, destroys the host egg, and replaces it with one of her own. The larva that hatches will eat the food the host mother wasp took such care in providing.

It has been noted that S. renicinctus is not always solitary. Individuals will gather in “sleeping” clusters on vegetation at night. They apparently do this in the face of impending inclement weather, too. I found a loose aggregation forming in the saltcedar bush on the afternoon of June 16 as storm clouds began to gather. These wasps will sometimes be found amid sleeping clusters of other solitary wasps as well.

These wasps are quite efficient parasites, as documented by I. LaRivers in a 1945 paper in American Midland Naturalist. He found the species parasitizing Palmodes laeviventris in Elko County, Nevada. The Palmodes were doing local agriculture a service by slaying Mormon crickets (a type of wingless katydid), but the Stizoides wasps were also reaping the benefits. LaRivers estimated a parasitism rate of about 7.5 percent, which he extrapolated to roughly 10,000 destroyed Palmodes nests in about 350 square yards.

Ok, so Stizoides is an outlaw of the Old West (it actually ranges from Michigan and Wisconsin south to North Carolina and west to Alberta, British Columbia, California, Arizona, and Mexico). I would like to think that seeing a good number of specimens means that there is also a thriving population of host wasps. That in turn means abundant grasshopper prey, and thus a healthy habitat.

Be on the lookout for Stizoides where you live, and see if you can find one in the act of finding a host. You may discover a relationship previously unknown to science. You just never know.

Sources: Bohart, R.M. and A.S. Menke. 1976. Sphecid Wasps of the World: A Generic Revision. Berkeley: University of California Press. 695 pp.
Evans, Howard E. and Kevin M. O’Neill. 2007. The Sand Wasps: Natural History and Behavior. Cambridge, Massachusetts: Harvard University Press. 360 pp.

Spider Sunday: Dewdrop Spiders

Last week I wrote about one of North America’s largest spiders, the Golden Orb Weaver (Argiope aurantia). While photographing the specimen from Ohio last month (see below), I discovered something amazing: a dewdrop spider was living on the web of the orb weaver.

It turns out that this itty-bitty spider (below) lives as a “kleptoparasite” of the orb weaver.

Kleptoparasites are animals that steal the food of their hosts. Indeed, that is what a dewdrop spider does. Whether or not its activities impact the host spider is debatable. A study of a species that lives in the webs of Nephila orb weavers revealed that host spiders do not gain weight as much as spiders that do not host dewdrop spiders, and that they relocate their webs more often than non-host specimens (Grostal and Walter, 1997).

Dewdrop spiders are in the cobweb weaver family Theridiidae, and genus Argyrodes.
There are three species in North America, all confined to the United States, and mostly the southern U.S. The species imaged here might be Argyrodes elevatus. Interestingly, in this genus the males are usually larger than the females. The modified pedipalps of “my” specimen reveal it to be a male.

Some Argyrodes have been recorded as actually preying on the host spider. I found a paper online that documented this for a species that uses labyrinth spiders (Metepeira sp.) as a host (Wise, 1982). That makes sense. Argyrodes are tiny, only 2-4 mm in body length, and I can’t see them killing something as large as a female Argiope or Nephila. Maybe they can kill intruding males, though. Given that male Argiope and Nephila are several orders of magnitude smaller than females, they could indeed be vulnerable to an ambitious Argyrodes. Both kinds of spiders tend to frequent the perimeter of an orb web, too, where they would easily come into conflict.

Argyrodes may be considered as being “commensal” when it only takes prey in the host web that is too small for the host to bother with (like the tiny winged ant in the images here). Commensalism is defined as a relationship whereby one organism benefits and the other is not affected positively or negatively.

However, another negative impact that Argyrodes can have on its host is damage to the web. Some dewdrop spiders are known to actually eat the silk itself, and others create gaping holes in a snare when they actively remove prey to an area outside of the web where they can dine without fear of detection by the host.

Next time you come across a large orb web, take a minute to scan for dewdrop spiders. Our understanding of the relationships between Argyrodes and their hosts is still in its relative infancy, and observations you make and record could shed light there.

Sources: Grostal, Paul and David Evans Walter. 1997. “Kleptoparasites or commensals? Effects of Argyrodes antipodianus (Araneae: Theridiidae) on Nephila plumipes (Araneae: Tetragnathidae).” Oecologia, 111: 570-574.
Wise, D. H. 1982. “Predation by a commensal spider, Argyrodes trigonum, upon its host: an experimental study.” Jrnl. Of Arachnology, 10: 111-116.