Some Parasitic Wasps in the Garden

23rd July 2013
The rear wall of our Warwickshire house faces due west and catches the afternoon and evening sunshine. This makes it a great spot for observing bees and wasps that use old walls for nesting, or that parasitise bee and wasps nests. In my experience, July and August are particularly good for spotting the parasitic varieties.

With the really warm sunshine we have been enjoying recently, there has been lots of activity. Most easily spotted is the rather bizarre and extraordinary-looking gasteruptiid wasp, Gasteruption jaculator.

These (typical of many parasitic wasps), carefully reconnoitre the wall, investigating every hole and crevice. They are looking for the nest site of other solitary wasps and bees. If they find one, they use their long ovipositor to deposit an egg into the brood chamber. When the egg hatches; the larva eats the host egg/larva and the pollen/nectar food store collected by the host.

Also in reasonable numbers this year, are Ruby-tailed Wasps. These are instantly recognisable (once spotted), because of their their distinctive iridescence and colouration. They are small though (up to 10mm).

Again, they exhibit very characteristic behaviour when searching for potential host nests. With their downward-curved antennae constantly twitching (as they seek the "scent" of their host), they carefully explore the wall surface. They are looking for nests still under construction. If they find one, they will enter and lay an egg in one of the host's brood cells. When this hatches, the wasp larva devours the host's egg or larva and then the food stocks. They then complete their development and emerge as adults next season.

There are several related and similar-looking species of Chrysis Ruby-tailed Wasps in the UK. Some are known to parasitise the nests of Osmia mason bees. There are some active Osmia leaiana nests in the wall, but I haven't actually seen one of the wasps entering these.

We have quite a few completed Red Mason Bee (Osmia bicornis) nests, both in a "bee hotel" and in the house wall. The Red Mason Bees prefer the east-facing stone wall of the house, but there have been a couple of nests in the west wall. Earlier in the season, all these nest holes attracted activity from the parasitic fly Cacoxenus indagator. The flies are now gone and interest is now from this parasitic chalcid wasp, possibly Monodontomerus obscurus (I understand that the similar Monodontomerus dentipes is also associated with Osmia bicornis nests).

This is very small (3-4mm). I don't recollect seeing them last year, but they could easily be overlooked. Once magnified, they show a "stout" build, a bronze/green iridescence and dark red eyes. The ovipositor is about the same length as the abdomen. After all the hard work that mason bees put into the construction and provisioning of their nests, it seems sad that these "cuckoo" and parasitic wasps can take such easy advantage of them. But they have their babies to raise too!


Photo comment By George P: Great photo of mono! A very real pest if they get a hold of red mason bee cocoons. Not had a problem this year with caco flies but monos, different matter!! Cheers GP
Photo comment By Dudley: I have placed about 80 tubes for Red Mason bees this year, most of which have been successfully filled. One day I noticed a wasp that I had never seen before and see now it was Gasteruption jaculator. I will remove the tubes later in the year and separate the bee cocoons out. Thanks

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