Mud Dauber Nest, With Spiders

2010 August 28

When we pulled our barbecue out in the middle of July, we found this mud nest partially constructed inside of it. Since it was going to be killed anyway by the heat when we used the grill, I carefully removed it so that we could get a better look, and see what was inside.

We didn’t actually see the wasp that was building it, but we can guess what type it probably was based on the fact that the nest was being built from scratch. We’ve previously confirmed in these pages that there are two kinds of mud dauber wasps living in this area: the Blue Mud Dauber, Chalybon californicum, and the Black and Yellow Mud Dauber, Sceliphron caementarium. Of the two of them, the Black and Yellow Mud Dauber is far and away more likely, because they are the ones that build their nests from scratch. Eric Eaton assures me that the Blue Mud Daubers will not do this, and that they always either re-use old mud nests that the Black-and-Yellows built, or use pre-existing cavities that they find.

Also, when we look at what is inside, we find that the mother wasp was concentrating heavily on crab spiders. Again, this is a vote against it being a Blue Mud Dauber nest, because they are supposed to be more partial to cobweb spiders.

The mother wasp had made enough progress to put in an egg, here it is adhering to one of the spiders.

So, what we have here is a rather sophisticated case of food preservation. If the mother wasp just killed the spiders and stuffed them into the mud nest, they would rot almost immediately, so by the time the egg hatched there would be nothing left to eat. Instead, what she has done is stung them in their main nerve ganglion and paralyzed them. Since they were still alive, their immune systems would keep them from decomposing. And, since they weren’t actually moving around or anything, and they were about to be sealed into an enclosed space that would prevent evaporation, it would take them a long time to actually starve or dehydrate. So, they would keep for the couple of months that it would take for the egg to hatch out and for the wasp larva to devour their helpless bodies. After which, the larva would make a pupa in the mud cell and hibernate through the winter. Come spring, it would finish maturing to a wasp, bore its way through the mud, and head out to start over.

If we hadn’t disturbed the nest, the mother wasp would have finished filling this tube, sealed it, and then built another next to it, continuing until something happened to stop her (old age, or maybe getting eaten by one of the very spiders she was hunting). By the time she was done, there might be a dozen or so mud tubes in the cluster.

Of course, she doesn’t necessarily have everything her own way. There are other parasitic wasps that will try to sneak into the nest while she is stocking it, laying their own eggs that will hatch first, eat her egg, and then settle down to devour all those preserved spiders that she so thoughtfully supplied.

And then the question comes up whether these are beneficial wasps or not. On the one hand, she is killing spiders, which are normally considered valuable pest control. That’s bad[1]. But, in this case, she’s concentrating on crab spiders, which mainly hide out in flowers and prey on pollinators that are needed by crops, so maybe that’s good. Then again, the mud nests tend to be built on walls, where they are messy and unsightly, which is bad again. And yet, they rarely sting, which is good. All in all, I expect that on the whole “Beneficial to humans/Not beneficial to humans” scale, she’s probably pretty much a wash.

Incidentally, just because a nest is made out of mud and sticking to the house doesn’t mean it is a mud dauber nest. This one was built in the crack at the bottom of one of our upstairs windows, and was obviously made by a different insect.

The cells are less than a centimeter long, which is much too small to be a mud dauber wasp – I think it was probably some sort of bee. The cocoons were almost all empty, so whatever it was had already matured and left. Except for that one on the end, which was still full of these:

Yes, they are yet another kind of parasitoids, that completely ate the hapless probably-bee larva that was in the mud cell originally[2]. Once you start keeping an eye out for them, the parasitoids are everywhere. Being slowly eaten alive by grubs is an astoundingly common way for arthropods to die.

I’m glad I’m not a bug. Or a spider.

[1] Shopkeeper: Take this object, but beware! It carries a terrible curse!
Homer: Oohh, that’s bad.
S: But it comes with a free Frogurt[3]!
H: That’s good!
S: The Frogurt is also cursed.
H: That’s bad!
S: But you get your choice of toppings!
H: That’s good!
S: The toppings contain potassium benzoate.
H: ???
S: That’s bad.
H: Can I go now?
[From "The Simpsons: Treehouse of Horror III"]

[2] So, why are the parasitoids still there after all the unparasitized whatever-they-were had flown off already? Well, probably because until the bees or wasps they were parasitizing get down to building and stocking new nests, the parasitoids don’t have anything better to do. So they might as well hang out in the relatively safe mud nest for a couple of weeks before they emerge, at which point their hosts should be ripe for the picking. This brings up something that I saw pointed out years ago, that while we commonly think of the adult insect as being the most important life stage, many insects actually spend only a vanishingly short period of their lives as adults. Like mayflies,which might spend a couple of years as nymphs compared to a couple of days as adults, or only about 0.2% of their lifespan.

[3] Frogurt = frozen yogurt

5 Responses leave one →
  1. August 28, 2010

    A very educational and entertaining post – thanks! (And nice job working in the Simpsons reference!)

    Also, how big was the mud nest in the barbecue? I’m trying to get a sense of scale.

  2. August 29, 2010

    I knew I was forgetting something – it was pretty big, about an inch and a half long.

  3. September 1, 2010

    There’s an additional benefit to the wasps – they give us something clever to discover and analyze. I’d say that’s a huge plus. Advantage: Wasp!

    Great post. Over on my blog, my post on Spider Intelligence gets about 6 hits a day. Since you inspired me to do that one, I now understand what you mean when you say that the wasp stings the spider in the central nerve ganglion. Way cool.

  4. September 1, 2010

    KT: I see that, if I do a Google search on “Spider Intelligence”, your post on the subject is the very first hit!
    We have an interesting book that is very nice for looking at spider anatomy: Uncover a Tarantula: Take a 3-Dimensional Look Inside a Tarantula. It has transparent, embossed 3-D pages that let you take progressive slices through a model of a tarantula, looking at all the important internal structures. The nervous system is just a big blob in the middle of the cephalothorax, sending individual nerves all through the body.

  5. Trena permalink
    July 30, 2012

    I am so glad to have found your site. I have found four of these mud packs with sacks inside and spiders too. couldn’t figure out what it was. It seems like the spiders around my house have tripled this year. not sure if it’s cause its so dry and hot. I am trying to figure out what kind they are. REally want to make sure my young children aren’t in danger of a very bad spider bite.

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