mason bees wasps

What happens when shrewd solitary wasps kill mason bees

Did you ever see a cloud of tiny insects nervously casing your mason bee house? If they reminded you or gnats or fruit flies, they may have been Monodontomerus parasitic wasps.

Inside: Monodontomerus parasitic wasps kill mason bees by laying eggs inside the developing cocoons. The eggs hatch into larval wasps that eat the bee’s insides before pupating.

What is a Monodontomerus parasitic wasp?

In short, these tiny wasps are insidious predators of tube-nesting bees such as Dianthidium, Megachile, and Hoplitis. Although in nature they don’t cause much of a problem, people who raise Osmia mason bees may find themselves suddenly overrun by these voracious predators.

In North America, we have about 18 species of Monodontomerus (sometimes called “monos”) which are Chalcid wasps in the family Torymidae. They are metallic green to black, although it’s hard to see their color without looking through a hand lens. Many of them have bright red eyes, which are a little easier to spot.

When I say these wasps are tiny, I mean the males are about 2-3 mm long and the females are a tad longer, around 3-4 mm. The females have a long sheath on the end of their bodies that looks like a stinger. But in fact, the sheath is a scabbard of sorts, a protective envelope for her delicate ovipositor.


An ovipositor is a long tube for depositing eggs. They come in various shapes and sizes, but they allow eggs to travel securely to the location the mother has chosen. Sometimes ovipositors are also used to bore holes, as in these solitary wasps. In many social bees and wasps, the ovipositor evolved into a stinger.

How Monodontomerus solitary wasps kill mason bees

These parasitic wasps hunt for the offspring of cavity-nesting bees. Once a female wasp locates her prey by scent, she inserts her ovipositor through the cocoon and into the body of a pupa or prepupa and lays her eggs.

To do this, she must first pierce anything that comes between her and the cocoon. She does this with ease, penetrating the sides of nesting tubes, paper straws, cracked plastic containers, or mud plugs. The ovipositor is so slender and long, it can make its way through a surprising array of materials.

Once the female wasp senses that her ovipositor has penetrated the pupa, she deposits a clutch of eggs—about 10—into the pupa. Then she moves to the next cocoon. One wasp may lay eggs in several different pupae before she dies.

The wasp eggs become larvae inside the bess

The wasp eggs develop into larvae that begin eating the bee pupa alive. Eventually, the wasp larvae pupate inside the now-dead bee pupa. After the wasps emerge as adults, which takes about a month, they pop a hole through the side of the host bee and find their way out of the nesting cavity. Ah! Fresh air. These new adult wasps will mate and repeat the entire sequence.

The number of generations per year can vary, but the species that decimate Osmia bees appear to have two generations per year. The last generation of wasps will overwinter as larvae inside the bee cocoons. These wasps will emerge as adults the following year near the end of mason bee season, just in time to feast on the new crop of larval bees.

Monocultures increase the numbers

Like other invertebrate predators of bees, Monodontomerus populations increase over time. They can quickly destroy large populations of host bees living close together.

A monoculture—a single species living in a compact area—is always irresistible to its natural predators. When we raise cavity-nesting bees as a monoculture, we make these wasps a problem. The wasps themselves are just doing what they are designed to do, but we’ve made it easy for them.

Bee boxes make it easy for the wasps

In nature, when cavity-nesting bees are spread randomly across the landscape, some Monodontomerus are lucky enough to find a host bee and some are not. Those who don’t find a suitable host die without reproducing. Those who do find a bee to parasitize, yield more wasps. But it’s largely a matter of luck, both for the bee and the wasp. That is the way it is supposed to work.

But when a parasitic wasp is lucky enough to find an artificially large population of her favorite prey that’s ridiculously easy to plunder, she can lay as many eggs as she wants. It takes only a few seasons to go from one lucky wasp to a great cloud of death and complete decimation of the mason bee community.

Limiting parasitic wasp reproduction

People who start off with a few mason bee tubes in their backyard seldom see these wasps. But the temptation to add more tubes and more nests leads to a large concentrated population. Once a parasitic wasp finds this banquet, the bees are doomed. The wasps reproduce much more quickly than the bees. One spring you suddenly realize that no bees hatched, and no early bees are tending your fruit trees.

How to recognize a Monodontomerus

The first photo below shows a live female Monodontomerus (the accented syllable is “tom”) that hatched in captivity inside my garden shed. She hatched from a mason bee cocoon, and as soon as she hatched I spent hours trying to photograph her.

The photo is deceiving because, without a powerful lens, these wasps look a lot like fruit flies. They appear black, nondescript, and kind of boring. They are so small you can’t see the colorful exoskeleton or the red eyes that are characteristic. Instead, they look like timid, shivering black flies. They sway from side to side, as wasps often do, but they are so small the movement looks more like a quiver. In no way do they look dangerous.

Another characteristic that I learned by messing with them is that they hop, rather than fly, short distances. I would focus carefully on one that was sitting stone still on my potting bench and then suddenly she was six inches away as if by magic. They move like grasshoppers.

I don’t know if this is technically a hop, but I learned to distinguish them by this unexpected motion. The males (second photo) were harder to photograph, as males often are, and they seemed to hop further. They look similar to the females but are slightly smaller and without the sheath on the back end.

6 ways to control monos at home

Controlling small wasps in a community of bees is next to impossible, after all a bee is just a vegetarian wasp. However, a few things can help.

  • Separate your mason bee houses as far as you can. The farther apart they are, the less chance of all of them becoming infected.

  • The female wasp searches for weak spots where she can insert her ovipositor, so the thicker and heavier the nesting material, the better. Although many mason beekeepers don’t like to use drilled blocks or tough bamboo tubes because of moisture problems, they do offer some resistance to parasitic wasps.

    • Take your mason bee tubes inside just as nesting starts to slow down. Don’t wait until nesting is totally finished because, if you do, the earliest mason bee cocoons will be ripe and ready for wasp predation before the last mason bees are done laying eggs.

      This may seem like a waste of mason bees but you’re just sacrificing some for the good of the majority. If you wait too late, you may lose all of them.

    • After you bring the tubes inside, cover them in fine mesh bags. These bags will let in oxygen and release water vapor, but they will keep the wasps from escaping and mating.

      If you check them every day or so, you can squeeze any wasps that are stuck inside the mesh.

    • At the normal time for sorting cocoons, say October through December, check each cocoon for soundness. A bee-filled cocoon will feel solid whereas a wasp filled cocoon will be light and soft. If you have a strong light source, you may be able to “candle” the cocoon and see shadowy creatures inside—either a bee or multiple wasp larvae.

    • Avoid “renting” mason bees or shipping them through the mail. Shipping and trading distribute bee diseases and parasites far and wide. Limit the number of mason bees that come and go from your property, keeping it as parasite-free as possible.

      Raising mason bees should be fun

      Indeed, mason bees are fun. But to keep them that way, you need to do a little extra work and a little advance planning. If you put in the time, you will be rewarded by healthy and plentiful mason bees for years to come.

      Honey Bee Suite

      This female Monodontomerus wasp inserts her ovipositor into the cocoons of mason bees to lay her eggs. On her tail end you can see the sheath that protects the ovipositor.
      This female parasitic wasp, Monodontomerus, lays her eggs inside the pupa of a cavity-nesting bee. The wasp larvae eat the bee pupa before pupating themselves. © Rusty Burlew.
      This male Monodontomerus has no sheath and no ovipositor.
      This is a male Monodontomerus. They are smaller than the females, and this one was nearly dead but still beautiful. Note that he has no sheath on his tail end because he has no ovipositor. © Rusty Burlew.


      • The pictures are beautiful. The writing is fascinating. My reaction is EEWW. Pretty sure this is the plot of a SciFi Horror movie.

        (Also, totally off topic, my surviving colony had pollen going in today. In February.)

        • Roberta,

          Thank you. These are some of my favorite pictures, I think because I took hours of blurry shots before I got anything halfway decent. Tiny doesn’t begin to describe them.

          Cool about your bees. It’s raining and cold here and my bees aren’t having any of it.

      • I really enjoy ALL your photography. These photos are lovely! I hope you’ll do a future blog post about your techniques.

      • Great blog post as always Rusty! Very informative. I shared with a friend, we raise, “husband” I guess you would say, mason bees. He recently had a health issue that he must rest for a week (a monumental task for him) so I told him of this blog and he is enjoying the posts. I wanted to share one memorable post you had from several years ago and I can’t seem to find it, I searched. You spoke of capturing bees in a handheld vac for observation….I’ll never forget it! Could you direct me to that post so that I may share it with my “reluctant to follow the doctor’s orders” friend? Thanks so much!

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