What’s Bothering My Mason Bees?

Bob Askey, Oregon, asks:

I think gnat sized wasps are going after my Mason Bees. I have some bees still working. I started to take the house down but some bees are still working. I may be too late now to do anything about the wasps if they have already laid eggs. Anything I can do? I mainly have the bamboo reeds and some cardboard reeds.

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Rusty Burlew replies:

It’s it certainly the right time of year. The parasitic wasp genus Monodontomerus shows up just as the mason bee season is coming to a close. The wasps are very tiny, perhaps fruit fly size, and fly with a nervous, side-to-side pattern that makes them look guilty.

The females have extremely long and thin ovipositors that can go right through cardboard tubes and sometimes bamboo. They deposit their eggs in the developing mason bee, and then the wasp larva eat the bee from the inside.

I would take your mason bee housing down immediately to try to save as many bees as possible. Your remaining adults that are still active will find another place to put their eggs, such as reeds or stems out in the environment. These might actually do well because when the nests are scattered throughout the environment, they are much less likely to be targeted by the wasps. Mason bee condos make it very easy for the wasps to find lots of prey.

I take down my mason bees as soon as the activity starts to slow in spring. Then I cover the filled house with a fine but tightly-woven fabric that will let air in but not wasps. No-seem-um netting works, too. Store them in a cool, dry place until you are ready to put them out in spring. A shed or basement usually works.

Sometimes the wasps will hatch in mid-summer. If you happen to notice them inside the netting, you can kill them. As far as I know, they will not mate inside the netting, so they can’t lay fertile eggs in other tubes as long as they remain captive.

If you don’t want to take the housing in this early, the only other thing  you can do is catch the wasps in a butterfly net as they hunt for places to lay their eggs, then squish them. I’ve spent many hours doing this, but with only so-so results. Taking the bees inside is better.