The Chalcid wasp that is a small silent assassin of leafcutter bees!

I have seen this over the years here and in my last garden. I overlooked it thinking it was a small fly! How wrong I was! I never realised how lethal this wasp is. Pteromalus wasp is very small. I hope you don’t make the same mistake! Hence this precautionary video!

Pteromalus wasp leaving leafcutter cocoon Nurturing Nature

It is actually a parasitoid wasp, possibly Pteromalus venustus, out to kill your leafcutter bees! Like the Monodontomerus wasp with its long blatantly obvious ovipositor,  used to kill your Red mason bee cocoons, this wasp has no long ovipositor but can do exactly the same to your leaf cutter cocoons. i.e. decimate them in a very short period. Depending upon the temperature, the first generation emerge soon after being laid. It is this generation that will oviposit inside as many leafcutter cocoons as they can as fast as they can. They don’t even need to mate, virgin females can lay unfertilsed eggs that hatch into males. The second generation may overwinter inside the cocoon, again depending upon temperature. Female wasps pierce the silken cocoon with their ovipositors, then anesthetises the bee cocoon with it, presumably so it does not injure herself or her offspring during and after ovipositing her eggs. There were 15 larvae inside just one cocoon…. I filmed one inside a Osmia Leaiana cell as well. It never got the chance to go any further!

I saw the male mount the female and fluttered his wings a few times before using his wings as if he was ‘stationary’ flying, for quite some time, probably some courtship ritual. At first she constantly groomed herself. There is a lot more to the mating ritual which I have not shown… it may get boring to some of you!

All filmed using my solitary bee nest box.

With thanks to Marian of north Devon for the photographs of her full leafcutter nest box used in the film

{ 2 comments… read them below or add one }

Marian May 9, 2015 at 4:33 pm

So informative George. I shall know exactly what to look for, the tell tale tiny hole. I’m going to inspect every one of my leaf cutters now. They are all individuall so it shouldn’t be too difficult. Think I must have about 150 so I shall be busy this evening!!
Nice and warm here today, my red masons are waking up now, lots of activity!!
Regards as always, and Thankyou for the info. Marian

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nurturingnature May 10, 2015 at 9:50 pm

My pleasure Marian, although once you have the holes it means they have been and left! Cheers, George

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