Mallota cimbiciformis: An Encounter with the Rot-hole Hoverfly

Having had some success with a pheromone lure for Emperor Moth, we decided to try our luck with some Clearwing lures this year and purchased ones for Hornet Moth, Lunar Hornet and Yellow-legged Clearwing.

On Sunday 26^th June 2022 I decided to try the Hornet Moth lure by the native Black Poplars (Populus nigra) in Earlham Cemetery in Norwich. The moth has previously been recorded in the Cemetery but on the other side of the ring road (we had tried luring by hybrid poplars the previous week but with no success). There are three large native Black Poplars left in the Cemetery out of an original planting of six that were from cuttings taken from a local tree.

I hung the lure on some low growth and as I did so I noticed something dark flying close to the trunk just above head height. I stepped back for a better view and saw a large hairy fly with dark abdomen and orangey-brown thorax fly somewhat lazily in to land on the trunk just below the scar of a long missing branch. It dropped down to land much in the manner of a large robberfly and for a brief moment I thought it was a Bumblebee Robberfly which would have been an amazing discovery (and highly unlikely in Norfolk!). As soon as I got a proper view I realised it was actually a hoverfly, one of the bumblebee mimics, but one I hadn’t seen before although something in my head said ‘Mallota’. Luckily it had chosen to land only just above head height so I was able to get some reasonable photos from different angles without making it fly off.

A look through the Hoverfly books confirmed my suspicions, it was indeed Mallota cimbiciformis, also known as the Rot-hole Hoverfly. Females lay their eggs in water-filled rot-holes in trees and males will guard their particular hole against other males.

I revisited the next day, taking Jeremy with me, and we were lucky to see the hoverfly once again guarding his rot hole. Again that rather lazy flight, lifting up to investigate any large fly that came near before dropping back down; shifting position every so often but mostly just sitting on watchful guard duty. It was surprisingly approachable and we were again able to get some decent photos without disturbing it.

On the following day the weather was rather dull and windy and the hoverfly failed to put in an appearance but conditions were better on Wednesday and I made another lunchtime visit on my own and spent a good half an hour watching him until I had to leave. The hoverfly was there when I arrived in the same spot. There were four or five Episyrphus balteatus hoverflies hovering about in the dappled shade of the tree but these were completely ignored. However when a larger Myathropa florea appeared the Mallota cimbiciformis flew up to investigate, then back to his perch.

Sometimes he would land right on the lip of the rot hole facing inwards then would shift around looking outwards. Other times he was settled a few inches below on the trunk of the tree on an area of darkened bark. Sometimes he took up his position slightly to the side of the rot hole and above the lip. A couple of times he flew up in a slow, slightly wavering flight in amongst the leaves and around the back of the tree but soon came back down again to the rot hole.

As well as I my lunch time visit, I also had a look later in the early evening as the weather was still good but there was no sign of him. The following day I tried looking in the morning but the weather wasn’t so good. The weather on Friday was better but there was no sign mid-morning or early afternoon and no further sightings were made on subsequent visits.

These hoverflies are known to visit flowers so I also checked out the Bramble blossom on each occasion and looked at other flowers in the Cemetery in the hope of seeing the male there or, better still, finding a female. The Lime trees on the boundary had several Eristalis spp along with the usual Honeybees but nothing as impressive as the Mallota cimbiciformis.

Added Extras

Along with the surprising encounter with this scarce hoverfly, the Black Poplar yielded some other interesting insects. In particular, a large number of petiole galls caused by the aphid Pemphigus spirothecae were very obvious on some of the lower branches.

When James Emerson went to look for the Mallota cimbiciformis (successfully I might add) he found a number of leaf mines caused by the micromoth Phyllonorycter comparella.

The final bonus was the rather lovely looking larva of the sawfly Cladius grandis which was spotted by Steve Judd.

Spending time around one particular tree proved very productive over just a few days. Although I drew a blank in regard to the clearwing lure, this lovely tree yielded up some wonderful wildlife secrets.

Vanna Bartlett.

Postscript

I returned to the Black Poplar at the end of September to look at the petiole galls again in the hope of seeing the actual aphids. Several galls had twisted open and were empty but I retrieved a few whole ones. Inside they were packed with aphids at different stages of development and some strange waxy-looking globules. These aphids spend a long time within their gall and are quite fascinating as a result. They actually produce ‘soldier’ aphids (differentiated by having thicker legs) that act as guards preventing would-be predators from entering the gall and also keep the inside of the gall clean by removing waste.

It is well known that aphids produce honeydew as a waste product; free-living aphids dispose of this by simply ejecting it from their bodies, often to attending ants which eat the sticky substance. Inside a gall, this sugary liquid could trap the aphids, potentially drowning them and would soon accumulate mould. The gall dwelling aphids have overcome these problems by producing a waxy substance that coats the inside of the gall. As honeydew is secreted, it comes into contact with the wax and forms ‘liquid marbles’. These are then rolled out of the gall by the soldier aphids.

On opening up the collected galls, I also found a couple of Heringia sp hoverfly larvae which are known to inhabit these galls and prey on the aphids within. So yet another secret yielded up from the Black Poplar. Unfortunately, I had no luck in trying to rear them through to adulthood. Something to try again another year perhaps.

References

How aphids lose their marbles: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1691028/pdf/12065036.pdf

Pemphigus spirothecae aphids https://influentialpoints.com/Gallery/Pemphigus_spyrothecae_Poplar_spiral_gall_aphid.htm