by Sandy Garland

Grass-carrying wasps (Isodontia mexicana) use hollow stems or tunnels in wood to build nests for their offspring

I love this species because it’s so easy to identify, especially if it’s building a nest. I was watching the insect hotel at the Fletcher Wildlife Garden one day and noticed that several “rooms” were stuffed with bundles of grass.

“Who would do that?” I exclaimed and bent down for a closer look thinking that vandals had plugged up potential nest holes. I was about to pull out the grass stems when a wasp flew over my shoulder and proceeded to poke another blade of grass into the hole.

I was astonished. I hurried home and Googled “wasp carrying grass.” More astonishment – and some laughter – when the search engine came up with “Grass-carrying Wasp (Isodontia mexicana).”

We are trying to find out as much as we can about all our local pollinators, and, as this one is now dear to my heart, I Googled its name recently and found a wonderful article by Heather Holm, containing just about everything I wanted to know about this creature: Meet the Grass-Carrying Wasp, a Gentle Pollinator of Summer Flowers.

Parchment-lined tunnel in wood has been used by a Grass-carrying Wasp

I learned that it IS a pollinator. Even though it gathers dead insects, not pollen, for its offspring, the adults visit flowers to feed on nectar and, in doing so, incidentally carry pollen from one plant to the next. And I learned that the cocoons I have in my refrigerator are likely a second generation of larvae and they will emerge in early summer – lots of time for me to get them back outside where they’ll find the resources they’ll need as adults.

Heather’s article mentions the kinds of wildflowers these wasps prefer, what sorts of nesting sites they use, and other interesting facts about the various stages of their life cycle. I enjoyed seeing the many photos she included, especially the ones showing the inside of a nest tunnel. At the FWG, we have not observed them using plant stems or the rolled paper tunnels we make for Mason Bees. But we do see them using holes in wood quite regularly. (Note: we line these holes with parchment, which can be replaced, making them easy to clean.)

Please let us know if you see these wasps in your backyard. By all accounts, they are NOT aggressive, so welcome them as another member of our local pollinator “team.”

One more thing: Grass-carrying wasps will nest in any small hole, so people often find bundles of grass wedged into spaces around window frames. This is NOT a reason for concern and the wasps know what they are doing, so please be content to share the space until the adults emerge in early summer.

Other articles on the Grass-carrying Wasp

29 Responses to “Grass-carrying Wasp (Isodontia mexicana)”

  1. Elaine Wilkinson McKean says:

    Thanks for posting this, and with a great picture, too. I have had these wasps building nests in my wind chimes for a couple of years. The grass litter on my deck and the chime tube stuffed with trash was a mystery to me until yesterday when I observed not one, but two “ladies” struggling to gain height with dried grass blades many times longer than they. My light bulb finally went on when the wasps disappeared down the tunnel of the chime pulling the grass in afterward. So cool. I’m very happy for them to continue to do their thing. I live in Paris, Tennessee.

  2. Kelly Derby says:

    I have them nesting in patio chairs (of tubular structure with small holes on under side). I’ve never seen them before. Glad to learn more about them. I live in Lakewood, CO.

  3. Anne says:

    I just found a nest in one of my upper windows. Took me a while to figure it out. It looks as if the prey in the nest is still
    Alive and possibly paralyzed?
    We live in the Beaches, Toronto

  4. JRoberts says:

    I live in Clinton Arkansas foothills of the Ozark mountains this is the first year I have them stuffing blades of grass in the siding of my house . Glad to read they are not aggressive. I will leave them be and let them do there thing

  5. Laura J. says:

    I just found out about these wasps today. I saw dried grass sticking out of the bottom of one of my metal wind chime tubes. I pulled it out wondering what did this. Six green tree crickets fell out. I then Googled and the grass carrying wasp came up. Nature is so amazing, I learn from it every day!

  6. Lulu Anderson says:

    I have these little workers in the track of my window screen and in the bannister of my porch. Love watching them. Often carrying huge Tufts if dead grass or weeds…several inches bigger than them and carrying them down into the tracks or using the little nest hanger I have. I love how the weeds stick out of the bamboo “homes”.

  7. Brian Squibb says:

    Indeed, I have photographed many of these wasps on our Trumpet Vine in our backyard. Not aggressive at all in my experience (yet).

  8. Dexter smith says:

    I have one building a nest in my shelter house

  9. Violet Bevins says:

    First time I have seen the holes with grass poking out. We have lots of carpenter bee holes on our porch and have never seen this. Still haven’t seen the bees doing this. Have seen this in three different holes around the property. We live in Virginia

  10. PATRICIA A GRELA says:

    I’m in western, PA. I found a small pile of dead grass in my front porch watering can a few days ago. Didn’t think too much about it, just emptied the grass and went about my watering. Today, again I found grass in my can this time blocking the pouring tube. Brought it inside to clean out and found one wasp and about 20 barely alive small grasshoppers. Now that I know what’s happening, I will try to give the wasp an alternate location to nest.

  11. Diane Winters says:

    Hey neighbor, I live in Mountain View. Just a few miles away. A week or so I noticed blades of cut grass all over my porch, wrote it off to the wind knowing that probably wasn’t how it got there. This evening I saw a little sticking out the bottom of a tube on my wind chimes, which led me here. This is my learn something new today.

  12. Robin Rebold says:

    I live in Western Pa. and have a metal wind chime on my back deck. I noticed brown blades of grass and weeds laying on the deck in front of my chair that I sit in almost nightly. Much to my surprise I learned about these busy little creatures and welcome them to my yard.

  13. Amy L Craig says:

    I never heard of them until a big storm came and broke one of my bamboo wind chimes and a whole bunch of grass fell out and I was confused so I thought maybe there was some weird little bird or something inside it so I put the grass back in and wrapped it up with a piece of string and hung it back up and then I physically saw the carrying grass into my Asian wind chime and I knew I also have probably 20 carpenter bees that missed on my porch in the beans I don’t know why people are so afraid of them they do a job that we cannot physically do so I just let them bee(I had too,lol) I live in inner city Columbus Ohio

  14. Norma Shaw says:

    Just washed some 2nd-floor windows and every one had the grass nests in the tracks. I’d seen them in years past. Finally Googled them and found this. Sadly, I had vacuumed them up, but what was I to do? The windows hadn’t been cleaned in some years, so I’m sure several generations have survived! Fascinating to learn about them. Thanks. I’m in Central Massachusetts.

  15. SB says:

    We saw one in Kentucky tonight. We were confused by seeing a wasp build a nest like a bird in one of our windows.

  16. Ann Lombardo says:

    Hi from central California. We have paper wasps often but recently I noticed a black wasp carrying a green katydid, and it flew to the side of the house. We have plywood panel siding on the house, and the grooves are vertical. Well, the wasp went into the groove just under the window trim and disappeared. I guess the plywood groove is a little like a tube, but there’s no grass sticking out of it. I’m glad to hear there’s just a few cocoons in there and there’s not going to be a big wasp nest in my wall. Thanks for the info.

  17. TC says:

    Thank you for sharing. I found bits of grass and weeds below my cable box, cleaned it up and returned to find more. Upon closer inspection noticed grass and small sticks protruding from the tube surround my cable line. I had been spraying for the stinging wasps and hornets but now after reading your article feel the need to just watch the beauty and mysteries of nature!

  18. Nina says:

    About a year ago, we noticed a wasp going in and out of a window on our deck. We couldn’t figure out what it was doing at first, but then we saw it carrying a large blade of grass, so we Googled it. Sadly, by then I had already disturbed the nest in the window tract. It had several larvae and tiny green tree crickets, all wrapped in grass. I’m happy to report that we are watching a mama wasp prepare the nest for this year’s brood. This time I will let them be.
    Nina, Southington CT

  19. Susan says:

    I have been noticing them for the last few years. They make their nests around the windows on the north side of our house. I live in Northwest Georgia.

  20. Challie Brooks says:

    I would appreciate some advice:
    How do I store the cocoons over the winter? Fridge? In a mesh bag in a finished Seattle, Washington crawl space? When and how do I release them? Local garden stores and friends are unfamiliar with the grass carrying wasps although I do recall seeing some of these on some of my flowers this summer.
    I’ve discovered at least 10-15 cocoons in my mason bee house. I opened a few of the wooden blocks up to harvest my mason bees and was confused by the grass blades sticking out of the small holes. With some research, I’ve come across this post. I’ve reassembled the bee house but it’s still sitting in my spare bedroom with cocoons inside.

    • Sandra Garland says:

      Sorry to take so long to find your comment, Challie (we get a lot of spam). I would try to duplicate natural conditions as much as possible. Here in Ottawa, we do take mason bee and leafcutter bee cocoons out of the nest box and put them into a refrigerator over winter (until there are blossoms on trees the following spring). But we’ve been ignoring grass-carrying wasp cocoons because we’re not sure what to do with them. If you have separated the cocoons, I think your crawl space might be most “natural” for storage. Because there’s so much interest in these wasps, I will try to find out more about their life cycle and what they need.

  21. Cindy from Michigan says:

    I was wondering what was plugging up the tubes in my mason bee house. There’s grass stuffed in several tubes and your article gave me the answer. I knew it wasn’t mason bees because they plug the holes with something that looks more like mud. Thank you for solving my mystery.

  22. Sarah says:

    I’m in California, in the SF Bay Area, and I have encountered this twice, about a decade apart, but in the same spot both times. Opened a French door, then dry grass and what looked like katydid (or similar) wings rained down with some dry grass still adhering to the top of the door frame. I’d brush it off but more would appear, at least for a few months. I’m glad to hear the wasps are little pals or at least not destructive pests. Thank you for posting!

  23. Kathleen says:

    Oops. When I shook my wind chime tubes which are right outside my door (the two that had dried grass coming out) suddenly something that looked like a wasp flew out and around my head—and my heart skipped a beat. My husband developed hives up and down his arm after a paper wasp sting. We were both outside on our porch, I literally happened to have a bottle of apple cider vinegar right next to me and poured it over the sting. A little while later, he came to me and showed me the sting. It had swelled and it was 3” by 2”, and there were a bunch of red spots going down his arm and around the sting. He refused to go see a doctor; wouldn’t take Benadryl (makes him fall asleep); so I ran to the corner store and bought the generic topical Benadryl (it says anti itch, but the same ingredients and strength) plus some triple antibiotic ointment and I applied them (he’d been using an alcohol swab to treat it). Seeing the wasp, knowing that the wasp spray would be hazardous to me and my carpenter bee “hotel”, I was going full out Ripley to this shiny black venomous killer while she was breeding death eggs. I waited until dark, took my mini aerosol hairspray and a lighter, and sprayed into the top of the tubes (hoping to delay escape with the stickiness of the hair spray) and up into the bottoms, then flicked my lighter at the bottoms. I went to the tops; the grass was burning…but by some miracle, the wasp was flying around me. DO NOT THINK THAT I AM A HORRIBLE PERSON. If you EVER see someone having trouble breathing, IMMEDIATELY YELL “DOES ANYONE HAVE AN EPIPEN,” and “SOMEONE CALL AN AMBULANCE” because an EPIPEN IS JUST TO KEEP SOMEONE ALIVE UNTIL THE AMBULANCE ARRIVES. NEVER think that IT ALONE WILL SAVE ANYONE’S LIFE . And hope that the worst case is they’ll wake up tomorrow in a hospital bed.
    BUT I am glad that she’s not going to kill my husband, and now I know how to identify them and that a flamethrower isn’t necessary. Also, don’t spend $50 on a “bee hotel”- take a 4’ piece of 2” x 2” wood and use a 3/8 inch bit to drill holes 1.5” into it spacing them out a bit, I sometimes use a 1/4” bit and a half inch just because some prefer different sizes; they can move right in, a bit of lemon balm essential oil attracts them and don’t forget to put out some water with rocks for them to land on to get a drink. I’m thinking about putting up another one for the grass wasps now!

  24. Tish Kernan says:

    Thanks for your information. What I thought was a hornet with a long stinger was going in a tube of my wind chimes. Poor guy isn’t having much luck. I have a pile of straw directly under the wind chimes. Too much wind? Olympia Washington.

  25. Chrisa says:

    Sadly, momma wasp makes her nest in the vertical track between my upstairs windows and when they mature the wasps can’t get out… and I can’t let them out because the screen is hard to get into place so I never remove it. For 2 years now they would mature and not know how to get out and they ended up dying between the windows. It was terrible to see their bodies lying there. Im allergic so how on earth do i move them?? Last year I filled the gap but this year, just tonight, I see her in the space at the side of the window… there’s not much grass but it’s there.
    I don’t want to kill any of them because I know now she’s the gentle type, but if I leave her there then next year all her babies will die inside my windows again… NOT a pleasant feeling.
    Milton, Ontario

    • Sandra Garland says:

      How is the mother wasp getting in? Wondering why the offspring can’t get out the same way. Also, I took the screen out of my bedroom window so that my cat could get in and out – maybe you don’t need the screen? Or replace it with one of those small ones that fits into the open window? Or make a hole in the screen?

  26. Dave says:

    I just photographed this Grass-carrying Wasp, first time saw one. It was feeding on flowers my wife has in pots. I’m from Cerro Gordo county, Swaledale, Iowa.

  27. Suzanne Davis says:

    I had a friend over who noticed a large wasp fly into a set of chimes I had on the porch. Upon further examination we noticed dried grass coming out of the bottom of several of the chimes. We looked it up and came up with the grass carrying wasp. Since it liked small spaces like the chimes, I checked a mason bee house someone had given me. Sure enough more dried grass coming out of several of the holes. Great to know it is harmless. Very cool! I have a large yard with many flowering plants and shrubs. I hope it/they thrive and stay! I’m located in the central Willamette Valley in Oregon.

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