METOECUS PARADOXUS (L)
On the 2nd August 2002 I received an email from Sarah Price of Oldham (greater Manchester) who said that the local council had recently found Metoecus paradoxus in her roof. She wanted to know if it was a rare species and should she be excited about the discovery! Quite why she had contacted me I do not know but guess it was through some internet loop via our web site www.wbrc.org.uk. At the time I knew next to nothing about this beetle but soon discovered it was a parasite of underground wasps' nests. Wishing to learn more I place a query at the coleopterist e-group and received several very useful responses which led to useful literature (see references).
Metoecus paradoxus was known as a beetle for 130 years before its life-cycle was understood. Elucidation caused vigorous argument and controversy amongst Victorian naturalists in the 1860s and 1870's. The development of the larvae in wasps' nests was described by Chapman in 1870 and he finally obtained the missing information in 1890on how the larvae entered the nests (reference list in Edwards 1980). To date I have seen no recent reports of study of the species - most texts repeat earlier observations - which is what I am doing in this article! The life history is reminiscent of that of oil beetles.
The beetle itself is a rather odd-looking almost fly-like insect about 10 mms long with narrow pointed elytra which do not completely cover the wings or abdomen. The larva is first an internal parasite of a wasp larva, soon emerging and wrapping round the larva as an external parasite sucking body fluids from its victim before finally eating the lot! After pupation the beetles emerge and leave the wasps' nest. The part of the cycle which took so long to discover is that the female lays her eggs on rotting wood in the autumn which is likely to be visited by wasps in the following spring when they seeks wood to chew and convert to nest-building paper. The first larva is a triungulinid about 0.5-0.75 mms long with long legs provided with suckers. The larva hatches in early summer and waits on the wood for a worker wasp to visit. The larva climbs on to the wasp and is carried back to the nest where it actively searches for a cell with a half-grown larvae which it parasitizes. The beetle larva grows rapidly, pupates, and the beetle emerges about 2 days after a wasp should have done.
The early reports and texts suggest that Metoecus parasitizes underground nests primarily of Common Wasp Vespula vulgaris, and sometimes Vespula germanica. The modern reports I have seen by Ives 1991, 1992 and Gillett 1992 reported the beetle emerging from wasps' nests in roof spaces or other parts of buildings, and of course the email which started me on the trail referred to a nest in a roof space. Interestingly, it turned out that the Oldham occurrence had been referred by Environmental Health to a coleopterist and comment came back to me via another internet loop - the coleopterist e-group! This group also produced a report from Warwickshire with records from 11 sites, and five of those from domestic premises, and mention of near-moribund adults on a first floor windowsill. Apparently my old friend Roger Juckes had reported 150 emerging from a crevice next to a central heating pipe after two wasps' nests had been destroyed.
The climax of this story came first as a phone message to the Worcs Wildlife Trust Office on 3rd September from Jean Osborne, Selly Oak, Birmingham. "She had got Metoecus paradoxus in her greenhouse - was I interested"! Well, Yes, I was! After contacting her she sent me a few beetles and also wasps to identify - Vespula vulgaris - and the following summary (dated 24th September 2002:
1. Our house is a dormer bungalow as we had a sitting room put in the roof space when we bought it 26 years ago.
2. We tend to get a wasp nest in the roof space almost every year, but live in mutual harmony most of the time.
3. This year the nest is above the kitchen and because of the adjoining lean-to greenhouse some of the wasps exit through the greenhouse while others enter above the greenhouse.
4. If we had beetles previous years we would not have noticed them, but because they turned up on the greenhouse windowsill they did come to my notice, though initially I didn't realise where they were coming from and thought they were something to do with our ring culture tomatoes.
5. I sent a beetle to Roy Ledbury, President of Birmingham Natural History Society, when it dawned on me that they were beetles and he said that he thought it was Metoecus paradoxus but it couldn't be as they were associated with underground wasp nests! I had not told him about the wasps.
6. Beetle numbers. They are now not appearing every morning, but for about 6 weeks we were tipping ten to a dozen out through the greenhouse door each morning, some got tied up with plant pots on the windowsill and died but they were quite numerous.
7. We have various plant pots standing on old tree stumps on our patio and some of this wood is getting soft and rotten while our compost heaps up the garden have wood sides that we have to replace as they rot every few years, so there is a source of dead wood available, but how do they know to use it?
Following this I calculate that at lease 500 Metoecus paradoxus must have emerged from a wasps' nest in her roof!
We have very few local records. One was found in Worcester two years ago, and whilst on an invertebrate foray near Birtsmorton 24th July 2002 John Meiklejohn collected one in his net when sweeping tall vegetation well away from buildings.
The RECORDER 3.3 database describes the species as widely distributed but very local. They must easily be overlooked unless you happen to have a parasitised wasps' nest in your roof! Please keep a look-out for these curious beetles and let us know if you find any.
Very many thanks to Sarah Price and Jean Osborne for starting and finishing this trail! Also to participants in the Coleopterist egroup for useful advice, especially to Matthew Smith who told me of the very useful publication in the Rentokill Library and kindly sent me a photocopy of the section on Metoecus paradoxus.
BILY S A. 1990. A colour guide to beetles pages 172-173. Treasure Press
EDWARDS R. 1980 Social wasps, their biology and control. Pages 151-155. Rentokill Library Series
GILLETT MTP. 1992 Metoecus paradoxus (L) (Rhipiphoridae) More parts to the puzzle. Coleopterist 1:4-5
IVES E. 1991 Metoecus paradoxus (L) (Rhipiphoridae) A puzzle. Coleopterist's Newsletter nos. 44 & 45:9-10
IVES E. 1992 Metoecus paradoxus (L) (Rhipiphoridae) A further note. Coleopterist. 1:17.
LINSSEN EF. Beetles of the British Isles. Pages 92-93. Frederick Warne
STEP E. 1946. Bees, wasps, ants and allied insects. Pages 40-41. Frederick Warne.
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