Wednesday, February 9, 2011

Wasp Wednesday: More on Isodontia

Last week I showed a short video introducing you to a puzzling phenomenon in suburban areas of eastern North America. A quick review: Some homeowners in the U.S. and Canada, upon sliding open their windows, have been startled to discover the tracks filled with dry grass, dead insects, and live “worms.” Even the odd wasp has flown out. What is going on here?

Certain solitary wasps called “grass-carriers,” genus Isodontia, seem to have found window tracks make ideal nesting sites. Each female wasp makes her own nest, usually a series of cells along the length of the tunnel (though one species forms a communal brood chamber much like a bird nest).

After selecting a site, she goes in search of tree crickets or small katydids which she paralyzes with her sting and carts back to the nest. Once a cell is filled with victims, she lays an egg, and makes a partition of dry grass. Then she begins the provisioning process again, until the tunnel is filled with cells. She finishes the nest by plugging the entrance with dry grass, such that it looks as if someone has shoved a tiny broom into the hole, handle-first. Her job complete, she leaves permanently.

The larva that hatches from each egg grows by consuming the fresh prey insects. These are the “worms” that people find in the window tracks. After finishing its meals, the larva then enters the pupal stage, and an adult wasp emerges later (the following summer in northern climates).

Grass-carriers are not pests, and not aggressive towards people or pets. You will not get stung unless you physically grab a female. Males have no stinger at all.

My initial theory was that the wasps much prefer using natural cavities in dead wood, but few such natural resources exist in suburban settings. My revised theory is that the wasps may find window tracks to be a superior nesting site. Perhaps the space is “roomier” than conventional natural cavities. Maybe the parasitic flies and wasps that plague Isodontia have not yet caught on to the new nesting strategy, therefore giving Isodontia an advantage that yields more success in terms of offspring reared to maturity.

Please tolerate these wasps if you can, they are fascinating to watch. Otherwise, simply flushing the wasp and cleaning the track should discourage her from trying again.

Meanwhile, look for the adult wasps on wildflowers, especially sweetclover, sumac, and grape. They will also frequent aphid colonies, lapping up the “honeydew” secreted by the sap-sucking pests. Notice that these wasps usually have the wings splayed at rest, in contrast to other sphecid wasps that habitually fold their wings flat over their abdomen while sipping nectar.

The species shown in the images accompanying this article is Isodontia mexicana, very common over most of the eastern U.S. There are also five other species occurring in North America.

11 comments:

  1. Hi Eric! I just wandered across your blog and thought I'd stop in and say hello. This is a great little piece on grass-carrying wasps. Have yet to see any members of this genus for myself, but hope to soon! I enjoyed watching the video you put together too. I thought that "seabrooke" had a good idea for you doing more little segments like this on YouTube. You should definitely find a way to intersperse your fantastic photos inbetween the commentary -- I think they will really bring the insects to life for people. The dead bugs in little boxes made me a bit sad... no surprise there, eh? :-)

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  2. i see these in the summer, it looks like an ant/wasp combo, and i try to keep my distance haha

    randomramblingggg.blogspot.com

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  3. They come back again and again I clean them out and I they come back to the tops of my storm windows which I can not get to fit any tighter than they are. Is they any way to get them to not come back...I have them all at most windows in my house and it is a pain to have to clean them out 2 or 3 times a year.

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    1. With all due respect, I am not in the business of pest control. My goal is to generate respect and appreciation of insects. You can try installing "bee boxes" or "bee blocks," which have holes drilled in them for solitary bees and wasps to nest in, elsewhere on your property. Bundles of bamboo sometimes work, too. They might be more attractive to the wasps than your windows. There are many online resources on how to do this, most of them to be found under a search for "native pollinators." Xerces Society might be a good place to start.

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  4. Thanks so much for referring me to this post, that for sure must be the wasp I found. After hearing how non- aggressive they are I feel bad. I only wish the regular wasps would be the same. So angry, lol. That and they are killing all my orb weavers ! Oh well :(. Thanks again!!

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  5. Thanks for your great article! I've one building nests in the tubes of a wind chime and find it fascinating to watch, especially now that I know she won't bite me. Question: There is a small fly (like a house fly) that hangs around with her. Is it waiting to eat the larvae or what? most curious

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    1. Yes, that would probably be a "satellite fly" in the family Sarcophagidae, many of which are parasites of solitary wasps.

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  6. I'm glad I found your article. I've seen these for a few years and never knew what kind they were. I never saw a colony of them in one location so I never felt threatened or tried to exterminate them but I had no idea why my windchime tubes had grassy material sticking out if the ends. I thought that maybe some kind of bird was behiND it. Could these be the wasps that also chew my windchimes strings? Some of my other tubes have fallen and some of the string is missin and we happened to find a wasp on them one day, chewing away. Wasn't sure if they were the same kind or not. Anyway, my son and I are watching two of these wasps right now, making trips back and forth carrying straw-like pieces to 2 separate chimes and we are so fascinated by these wonderful creatures.

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    1. Thank you for sharing your experience. It seems reasonable that the wasps would want to convert the strings to nesting material. LOL! I admire your tolerance given the destruction of your chimes. Please consider taking images or video of their activities.

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