8. Pests & Diseases

Ensure Bee Health

8.3 Spring - Mason Bee Nesting

8.3 Spring - Mason Bee Nesting

The pests below are active while mason bee larvae are developing.

Parasitic Wasp: Monodontomerus (also known as mono)

  • What It Is: Also known as mono, monodontomerus is a gnat-sized (3/16" or 4mm) parasitic wasp.
  • What You See: Small, black, and with a long ovipositor (egg-layer), they hover and fly in a zig-zag pattern at nesting holes. Evidence of mono wasps are small holes in the sides of paper nesting holes where the adult mono wasps emerged. When harvesting cocoons, you may find a small hole in the side of an empty cocoon.
  • Effect on Bees: Female mono wasps lay its eggs within developing bee larvae by inserting their flexible ovipositor through cracks in mud, thin walls of paper nesting tubes, or other weak spots in nesting materials. The infected mason bee larva lives long enough to spin its cocoon and the parasitic wasps are now encased in what appears to be a healthy cocoon. The sneaky wasp’s development time is short and they emerge as adults from the mason bee cocoons by chewing a small hole in the side.
  • What You Can Do: Mason bee cocoons that did not emerge within a few weeks of release may be full of mono wasps and should be thrown away. Removing and protecting filled mason bee nesting materials removes the nesting holes while mono wasps are active. Check the BeeGuard Bag a week after removal from the bee house and you can easily kill parasitic wasps trapped inside of the bag.
    • Pro Tip: If you suspect mono within a cocoon you can candle the cocoons. With a strong flashlight, make a pinhole in a piece of cardboard, place over the flashlight, and hold the cocoon over the pinhole. Mono larvae look like a mass of small bodies and a mono-filled cocoon can also look much brighter or red in color under the light. An adult mason bee is large and dark in color within their cocoons.
  • Product Recommendations: In the late spring when female bees are done nesting, remove and protect filled nesting holes over the summer and store in a BeeGuard Bag.

Parasitic Fly: Cacoxenus indagator, Houdini Fly

  • What It Is: A well-known pest in Europe, these small flies are members of the fruit fly family who are active when mason bees emerge in the spring. Named after their ability to escape, the young flies inflate their heads to break out of mason bee mud walls.
  • What You See: Small gray flies with red eyes.
  • Effect on Bees: Within a mason bee nesting hole the maggots eat the pollen loaf starving the mason bee larva. The maggots overwinter in the nesting holes and emerge as adults in spring. Adults sit outside the nest and wait to lay eggs in freshly built mason bee nests.
  • What You Can Do: Harvest all mason bee nesting holes before spring and remove and crush any found maggots. If you cannot open your bamboo tubes or drilled blocks of wood, place the entire house and/or nesting holes in a BeeGuard Bag. Close the bag tightly and release the emerged bees daily in the spring. Crush any emerged fly to control this pest.
  • For more detailed information on this emerging pest, please visit here.

Parasitic Wasp: Sapyga spp. (Sapygid Wasps)

  • What It Is: Female sapygid wasps are fairly large wasps (about 1/3” long) and with their black and yellow bodies they look very similar to the well known social wasp, paper wasp, or hornet. There are a few different species of sapygid wasps and they may have white or yellow spots or a maroon band.
  • Effect on Bees: The female sapygid wasp finds a recently laid mason bee nest and inserts her egg layer into the mud cap, laying an egg or two near the bee egg. Sapygid eggs hatch within 1-2 days and consume any other sapygid larva and the bee egg. The wasp larva eats the pollen loaf, spins a cocoon, and overwinters along with the neighboring bees. They emerge about the same time as the mason bee cocoons.
  • What You Can Do: If you find this large parasitic wasp hovering near your bee nesting holes, spray it with a fine mist of water to stun and then kill it.


  • What it is: Earwigs are opportunists and are more scavenger than predator.
  • Effect on Bees: Leftover pollen is a wonderful earwig treat. They may destroy a freshly laid egg if the nesting female has just laid it and has left to gather mud. However, this seems remote.
  • What You Can Do: Loosely roll up a section of newspaper, bind it with a rubber band or string, and dampen it. Place this moisture "trap" inside the house, or physically attach it to the nesting house where you suspect the earwigs are gathering. Earwigs will be attracted to the moisture and crawl inside the coil. Dispose of the newspaper with the earwigs in your compost pile. Another solution is to build a moat by placing your mason bee house on bricks and set it in a tub of water (look at the ant section above for more details).

Topic Completion

Pests & Diseases

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