Old wasps’ nests in compost heaps - an investigation full of surprises

Wasps are eusocial insects that form large nests made of wood pulp, a bit like paper. (Bees make them out of wax.) Their nests are elaborate constructions made of several tiers containing the brood cells separated by supports.

Wasps are known to make nests in all sorts of places: trees, inside human dwellings, and compost heaps to name a few. Indeed, last year there were several wasps’ nests in the Drury Rd allotment site and here is a report about what we found in a couple of them in compost heaps.

On a cold January day, when the life of a wasps’ nest had safely ended with the death of the colony’s queen and workers, together with Jo and Diana we decided to check the nests in a couple of plots, 47 and 45a. What we found is reported here after a very long time, because we were in for quite a few surprises and a long wait.

By a stroke of luck, we were able to dig out the nest in Plot 47 without much damage.

First surprise

Even though it was somewhat decomposed, the nest was crawling with life. To our surprise, we found quite a few big fat larvae dipping into the brood cells!  

Could these be the larvae of the Hornet Mimic Hoverfly, I wondered? I knew that their larvae develop in wasp’s nests by mostly feeding on the debris. But more of that later.

We weren’t so lucky with the nest in plot 45a. The compost heap was very dry and we found only fragments.

Note how the cells are beautifully made. Wasps rear their offspring in hexagonal cells like the bee’s honeycomb, but they have evolved independently. Somehow the bees’ hexagonal cells seem to have stolen all the limelight, but wasps are just as clever.

Second surprise

Fortunately, we were able to find some workers’ corpses in both nests, which enable me to identify the wasps from their faces.

They were the Common Wasp (Vespula vulagaris) in the first plot and the German Wasp (Vespula germanica) in the second. Interesting, because those plots were rather close to each other. These wasps are the most common wasps in the UK! Mind you, in the UK there are approximately 9,000 species of wasp. This includes smaller wasps’ species and microscopic parasitic wasps that can't be seen with the naked eye.

As for the hoverfly larvae in the first nest, I brought home quite a chunk of it so that I could rear them. This was important because I soon found out via the Facebook UK Hoverflies Larval Group that there are other hoverflies that can develop in wasps’ nests. Complicated twist.

Third surprise

Back at home, when I first inspected the nest remains (they had the most horrendous odour), I was in for another surprise. Besides the plump hoverfly larvae, there were lots of much smaller larvae - all very lively and rather grubby. Below is a photo of a few larvae after I gave them all a wash.

So I went back to the above-mentioned Facebook group and was told that the smaller larvae were carrion flies probably known as Fannia; they feed on all manner of decaying organic matter including carrion (about 288 species). So, I set out to rear them as well.

Rearing results

I kept the nest remains in a terrarium in rather cold conditions and by the beginning of March things started to change.

The fly larvae weren’t so frisky and towards the end of March, I noticed small flies crawling in the terrarium. They were great masters at escaping.

• Flies

I showed this photo to a Diptera expert, Phil Collins, and I was told that this fly was definitely a Spiny-winged Heleomyzidae, a different family from the Fanniidae. And so were the ones in the other photos, probably Tephrochlamys species. They develop in decomposing organic matter and carrion and probably came at a later stage. Not much of a surprise.

• Hoverflies

The hoverfly larvae pupated also in March and I found 20 puparia. Hoverflies pupate inside the larva’s skin, which hardens up in the process.

After quite a while they they started emerging gradually from 21 April till 4 May always at dawn: 4 males and 15 females. All of them were the Hornet Mimic hoverfly Volucella zonaria, which was my first guess. No surprise here.

The adults came out of their hard puparium head first by breaking it always in the same place. Shown in the photo below.

This happened far too quickly for me to actually see it - frustrating. Their bodies were very distended and almost transparent and their wings were still folded. Sometime soon after that they inflated the wings and, at the same time, retracted the abdomen. They were extremely beautiful and, at around 2 cm long, are our largest hoverfly.

I released them in our garden. They are great pollinators.

Conclusion

This investigation revealed that one of those old wasp’s nests hosted a couple of species: first, the Hornet Mimic Hoverfly, our biggest hoverfly and a good pollinator; second, lots of carrion flies, good food for the birds. Both species are part of the food chain, thus enhance our biodiversity.

Wasps are a very beneficial carnivorous insect, that predates on a variety of insects like aphids and in the process are also good pollinators. It is only towards the end of the summer that the workers revert to a sugary diet.

Even though they can be aggressive, please, leave their nests well alone; not that difficult if they are on compost heaps.

Acknowledgements

I’m grateful to the support from Facebook UK Hoverflies Larval Group. Also to Phil Collins for his help with the flies identification.