Natter’s Notes

Jean R. Natter, OSU Master Gardener

Early in 2020, a new pest of mason bee, Cacoxenus indagator, was identified in Washington State for the first time. It’s often referred to as the Houdini Fly because of the unique way it escapes from the mason bee’s nesting cell. It’s also nicknamed the Red Devil due to its large red eyes, or just Devil Fly. It’s presence in Oregon is suspected to but not yet verified.

Fig 1 – houdini fly, BugGuide

The arrival of the Houdini fly is suspected to be an unfortunate example of moving bees without carefully inspecting them and their nests prior to the move. “In New York, the first two records were in 2011, although it may have arrived there earlier. It had presumably come there from Europe, probably someone moving an unclean nest block,” said Josh Vlach, from the Oregon Department of Agriculture during an interview by Andony Melathopolous during PolliNation Podcast #154 (2020).

What damage does the fly cause?

“The flies don’t actually attack the bees; they’re kleptoparasites,” continued Vlach.  “The fly is in the same group as Drosophila fruit flies that fly around a bowl of over-ripe fruit.” They closely resemble their fruit fly cousins – about the same size, with large red eyes, but otherwise a dull brown color. They move rather sluggishly, and are often seen near the entry to a nesting tunnel.

After the mother bee leaves the nesting tunnel, the Houdini fly enters the tube, lays eggs on the pollen ball, then quickly exits. After the nesting cell is closed by the mother mason bee, the fly larvae hatch and eat the pollen ball. As a result, the mason bee larva starves.

Fig 2 – Cacoxenus indagator

 How to recognize an infestation

Telltale signs of these kleptoparasitic flies are sticky clusters of small white maggots in a nest cell. The bee larva is dead or missing. [Note: Kleptoparasite may be spelled with a “c” as in cleptoparasite.] But beware! Another pest, a parasitoid, produces a similar cluster of small white larvae.

Be aware of a look-alike infestation by tiny wasps

Unfortunately, to the untrained eye, the white larvae of Monodontomerus wasps could be mistaken for Houdini fly maggots. These small black wasps – sometimes referred to as ‘Mono’ wasps – are much more active than adult Houdini flies. The adult wasps erratically flit about. They’re parasitoids which lay multiple eggs in a single mason bee larva. However, the end point is the same as with the Houdini flies: Dead mason bees.

Management suggestions for Houdini flies (WSDA Pest Alert)

–  Harvest mason bee cocoons – Open mason bee nesting materials before they emerge in the spring and destroy Houdini fly maggots.

– Control adult mason bee emergence – If you cannot open nesting materials, place your nesting materials in a fine mesh bag and close tightly. As the bees emerge, release the mason bees daily and kill any Houdini flies.

– Only use nesting materials that allow you to open, inspect, and harvest cocoons. Visual inspections can greatly reduce Houdini fly populations. (Ed. note: Kill the larvae on sight.]

– Before purchasing mason bees, ask the provider how they harvested and whether they inspected the cocoons for Houdini fly.

Only purchase pest-free mason bee cocoons.

A few final words

– WSDA suggests: “Please do not unnecessarily move bee blocks or boxes around.”

– If you’re having sizeable losses of healthy mason bee cocoons, seriously consider modifying your materials, methods, and procedures.

– A viable alternative to using clustered artificial housing for native bees is a healthy environment with modest-sized patches of suitable flowering plants that provide a year-round succession of bloom

– Perhaps the best habitats for native bees are patches of bare soil, along with naturally-occurring tubes, among them spent plant stems and old holes from boring beetles, all in a pesticide-free location.


– PolliNation transcript #154 – An interview with Josh Vlach, ODA. (

– “Parasitoids and Cleptos”- [Caution: “. . .artificial bee nests and hotels may be preferentially used by introduced bee species and native wasps, rather than native bees]

– Video: Houdini fly, a kleptoparasite of Osmia –

– Video: Life cycle of Montodontomerus wasp – 

– “How to Manage the Blue Orchard Bee”- An overview.

– “Orchard Mason Bee” (10-Minute University) — Orchard Mason Bees (

 – Pest Alert WSDA//  Houdini fly found in Washington; images and 2 videos:

Video: How does Cacoxenus escape?; Video:Devil fly on nesting tubes. “The accompanying fauna of Osmia cornuta and Osmiarufa” – Pests of Osmia in Europe, with images –

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2 thoughts on “A new Pest of Mason Bees: The “Houdini” Fly

  1. Hi Jean. Thanks for this report. It is now April of 2021. I live in inner northeast Portland and, while working from home, I can observe my mason bee nest. I had not seen Houdini flies before this year, and now I am killing 2 to 5 a day, and have been most warm days for the past two weeks. It is quite an alarming and dramatic change since just last year for me, and speaks to the risks of moving bees significant distances without being careful or aware of unintended consequences.

    Mace Vaughan
    Xerces Society for Invertebrate Conservation


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