Tag Archives: insect hotel

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 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.

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

Macrosiagon cruenta parasitizing potter wasps at insect hotel

Last year I had a little surprise at my bee and wasp hotel. Although I didn’t know it at the time, some of the wasp larvae I found nesting inside the tubes were harboring parasitoids. The story starts in early March, 2021, when I took my hotel apart for its annual cleaning.

Here’s a photograph of one of the wasp larvae I recovered from the hotel. Each larva was in its own cell, and I simply unwrapped the paper straw and then plopped them all into a container.

Potter wasp larva in nesting tube

I got over two dozen of them, and kept them in my unheated garage.

Potter wasp larvae in container

By mid-May, the wasps were starting to look like wasps. But there was variation in how far along they were, which is probably because the eggs were deposited at different times.

Potter wasp pupae

The surprise

But four of the larvae weren’t progressing onto the next stage, and when I looked closer they each had a smaller larvae attached to the ventral region near the head. Here’s one:

Macrosiagon cruenta parasitizing potter wasp

Here’s a closeup:

Macrosiagon cruenta larva attached to Euodynerus sp. larva

The larvae were clearly sucking fluids out of the wasps, but also retarding their development in some way, which is a nice trick. The wasps below were approximately the same age. The ones with parasitoids attached never progressed to the pupal stage, instead just becoming shriveled bags. I was traveling when most of this happened so I don’t have additional photographs of this process.

Macrosiagon cruenta larva attached to Euodynerus sp. larva (right). Unparasitized wasp on left for comparison.

Luckily, one of the parasitoids survived to adulthood (below) and I was able to identify it as Macrosiagon cruenta, a wedge-shaped beetle.

Macrosiagon cruenta

Here’s a side view. The wings didn’t develop properly, probably because the containers I had them in didn’t have the proper humidity or were kept at the wrong temperature. Or, perhaps, their development suffered from all the fussing I did during the earlier photo shoots. If you’re curious what they should look like, here are images on iNaturalist.

Macrosiagon cruenta

Before this, I didn’t realize there were parasitic beetles that might arrive at insect hotels. But now I’m extremely interested in finding the dispersal phase of this insect, a tiny (less than 1-mm long) mobile larva called a triungulin that lurks on flowers visited by wasps, then jumps on, secures itself, and hitches a ride back to the nest the wasp is making. Here’s a photograph (not mine) of Macrosiagon limbata triungulins lurking on a mint inflorescence:

Macrosiagon limbata triungulins
Image by MJ Hatfield (CC BY-ND-NC 1.0)

Once inside the nest, the triungulin burrows into a developing wasp larva to feed internally for months, only later popping out to complete its development while attached externally. This is why I didn’t initially know the wasp larvae had parasites. The adults live only a few days, with females ovipositing onto plants (here’s one doing that) that are visited by wasps.

I’m not positive who the host was. In fact, it could be the case that the four larvae I found parasitized were different species. My confusion is because all of the unparasitized wasp larvae from the 2020 season turned out to be Euodynerus foraminatus. But one of the parasitized larva survived (because the I accidentally disturbed the parasite), and it was a Euodynerus hidalgo boreoorientalis. So I’m fairly confident that hosts were in the genus Euodynerus. I’m going to sort my wasps more carefully next time so that I can keep track of individual nest tubes.

In case of interest, below is a photograph of my bee and wasp hotel. And my guide to building your own.

Insect hotel