My name is Dr Margriet Dogterom and am the founder and owner of Beediverse. I write this blog for all who love bees and who want to learn more about these wonderful creatures.

Parasitic wasp control

I really enjoyed your seminar in Courtney today and went home to open my nest only to find that about half of the cells (40+) had a 1/16″ hole drilled through the paper and cocoon. In reading your book tonight I have not found any mention of this type of predation on Mason bees would you please tell me what would do this and how to prevent it, thanks.  Alan A.

Sounds to me that these holes are the right size for parasitic wasps. These wasps parasitize the cocoon contents, and emerge as adults 3 weeks later during the summer. There are 3 ways of keeping these pests down:

1. Candling of cocoons after washing will identify parasitized cocoons and these can then be destroyed.

2. If there are large numbers during spring bee -flight, set out a sticky trap

3. Protect further parasitism after flight by placing bee nests into a wasp free net bag.

Dr Margriet

Adult parasitic wasps inside a mason bee cocoon

Fall: Adult parasitic wasps inside a mason bee cocoon.  Left alone, these adult parasitic wasps exit the cocoon and find others to parasitize.

Tiny parasitic wasp of mason bees

Spring- summer:  Tiny parasitic wasp preparing to parasitize a mason bee cocoon.


Fall:  The upper cocoon on right hand side has a hole- possibly an exit hole created by a parasitic wasp.

2013-10-06 margriet- wasp netting Mason Bee Workshop (6)

Summer-fall:  After spring bee flight, protect developing mason bees inside their nests, by placing nests into a wasp-free bag.



Fall: Cocoon contents: #3 and #4- dead bee pupae #5 parasitic wasp pupae.




“Back when I was cleaning up my cocoons I found three with a tiny hole in them. The hole was about the diameter of a straight pin shaft. Inside each I found 10 to 15 tiny winged insects metallic green in color. All of them were dead. They are about .03 to .04 inches long. Attached (I hope) is a microscope photo of one of them. Also attached is a photo of a cocoon with a cluster of the little guys. Do you have any idea what they are or why they were dead?”Hal

These are  tiny parasitic wasps that grow inside a bee cocoon.  Looks like there are more than 30 out of one cocoon.  Good thing you found it.  Candling cocoons can also identify them.  These critters can wipe out a mason bee colony.  Wasp proof nests are a great way of protecting your bees from parasitism.  If was probably too cold when they reached maturity.  The last batch of the season did not make it.  Dr Margriet 


IMG_8400 IMG_8403

From summer to early winter place mason bee homes inside a net bag.  This will prevent your mason bees from being parasitized while they are inside their cocoons over the winter.

Tiny parasitic wasp of mason bees


Can you verify that this picture is a Parasitic Wasp (or not)?

Valeri Wade
Wild Bird Chalet
705 Kentucky Street
Bellingham, WA 98225


Hi  Valerie, Thanks for sharing this photo with us and our friends.  Yes this tiny little wasp is a parasitic wasp of mason bees (and other insects).  You can see from the size of the cocoon in the background how tiny this critter really is!  At the moment that this picture was taken, this wasp was listening for bee movement inside the cocoon  ( it hears with its antennae).  Only live bees are parasitized!!  Great picture Valerie.

From Norm Z.
Can you ID these insects for me? If so what are it’s nesting habits? Thanks
Here they are-lovely photos- any body know what species these wasps are?  Probably parasitic types, since they are hanging around the mason bee nests.-Margriet

A comment on this blog asked for more pictures on candling cocoons. 

Just today I candled 4000 cocoons.  It seems like an awful lot, but when they are in  petri dishes it is easy to do candle them- about 30 mins or so.  I did see some duds that are of some interest.  I call anything that is not a fully developed bee a ‘dud’.  The percent ‘duds’ in this batch was 2.5%.  Anything under 5% is excellent.  But even with 107 duds there are some interesting ones.  Few had fully developed parasitic wasps- ready to emerge in spring.  Others were bee larvae that had not completed development into an adult. In the next day or so I will take some photos and put them on this blog.

I was teaching a group of people about candling the other day.  It is a straight forward procedure but the conditions have to be right.  The room that you do the candling in has to be completely dark- a bathroom without a window for example.  Any extra light besides the flashlight is too much light and you cannot candle the cocoons.

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March 2017
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