Houdini fly alert for owners of bee hotels

If you have an insect hotel, you should check your mason bee nests for Houdini flies (Cacoxenus indagator) before the season starts. These non-native flies consume the pollen inside mason bee nests, causing the bee larvae to starve to death. They are very easy to get rid if you have nesting tubes that can be taken apart.

Killing them at the larval stage is easy. Just unwrap your nesting tubes during the winter and squish the larvae (below) before they mature into adults. If you’re not into squishing things, submerge them in alcohol or soapy water before discarding. Or freeze them (just don’t forget about them in case spouse would freak out).

Houdini fly larvae

I had a half-dozen or so cells infested with the flies, so killing them removed approximately 100 individuals (below) from my local population. That won’t eliminate them from my yard but at least my hotel is not part of the problem.

Houdini fly larvae

Here are photographs of the adult. They look like common fruit flies but are darker and considerably less skittish when you approach them — they are so chill you can often squish them with a finger (do that if you can). You’ll notice them lurking around a bee hotel waiting for female mason bees to leave their nesting tube unguarded, then they’ll slip inside and leave a load of eggs.

If you find Houdini flies at your hotel, please upload a photograph to iNaturalist so we can track their spread.

Below are mason bee cocoons, for comparison. You can clean off the frass and mud, wash them, then store outside in a way that protects them from birds and moisture. For details on all how to do that, see this page. If you store them in an unheated garage in a container, check them at least every day so that you can release them.

Mason bee pupae

3 thoughts on “Houdini fly alert for owners of bee hotels

  1. Lisa

    I went out to look at my mason bee hotel here in Virginia. There was one housefly (live one) in there and it flew out and landed on me. Didn’t see any of the Houdini flies but I have seen them in the past. Also only one deceased mason bee adult stuck in there which popped out when I tapped the hotel. There were only two dirt filled compartments and they had holes in the dirt plugs.

    Reply
    1. Colin Purrington Post author

      For the nests with dirt plugs, even though the most visible one has a hole you might have viable bees deeper into the tube. Do you have paper inserts so you can pull out and check?

      Reply
  2. Thyra McKelvie

    Hi Colin
    Thank you for teaching more people the importance of proper care of their mason bees. So many people never remove their blocks at the end of spring and leave them out all year round and don’t use the proper nesting material.
    Great pictures and great information. Thank you!
    Thyra

    Reply

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