Odynerus simillimus (Morawitz) in N.E. Essex 2004
DAVID SCOTT
Ford Farm, Brightlingsea, Colchester, Essex C07 OSA
The discovery of nesting aggregations of the RDB1 Eumenid wasp Odynerus simillimus in N.E.
Essex has been reported in Essex Naturalist 18 & 20. Work funded by the Aculeate Conservation
Group in 2003 established that the prey identity, previously uncertain, was the larva of the Curculionid
weevil Hypera pollux. This feeds on Fool's Watcr-crcss Apium nodiflorum and Lesser Water-
parsnip Berula erecta. The female wasps search the plants for the larvae which they paralyse by
stinging, then carrying them back to their individual nests where they are packed into underground
cells as food for their own larvae.
A. nodiflorum is common in N.E. Essex (Tarpey & Heath 1990), and if the weevil is generally
present on the plant then O. simillimus may itself be more widespread than was thought at first.
Work in 2003, therefore, was both to monitor existing sites and search for new ones. The largest
known site, at Brightlingsea, appeared to have maintained wasp numbers satisfactorily compared
with 2002 and 2003. Counting the nest chimneys was difficult since the chimneys, constructed of
earth pellets around the nest holes, were at times washed away by heavy rain, and an additional
complication at this site is the presence of many nests of the commoner Eumenid Odynerus spinipes.
This species is quite similar to O. simillimus but nests rather earlier and constructs a much longer,
down curving nest chimney. However, it can be difficult to distinguish a completed short O. simillimus
chimney from a partially completed O. spinipes chimney without seeing the wasp itself at the nest.
The several sites at Alresford also maintained their numbers, and A. nodiflorum was also present in
quantity in nearby spring fed ditches. One new site was found about one kilometre cast of the
Alresford sites on the Brightlingsea side of Alresford Creek. This was another shared site, with O.
spinipes nesting on a low vertical bank and O. simillimus appearing later on a small level area,
lightly vegetated, at the foot of the bank. The nearest A. nodiflorum here is now on the opposite
side of the tidal creek. In 2003 there was a lot in a silted up pond two hundred metres away, but the
2003 drought killed most of this, although the few plants remaining had a lot of weevil damage.
The other area investigated successfully in 2004 was Howlands Marsh E.W.T. reserve at St. Osyth.
There was some A. nodiflorum in a wet ditch on the eastern boundary and the warden, Reg Arthur.
has seen both it and Berula erecta elsewhere. The A. nodiflorum was quite heavily attacked by
weevil larvae by 19th July, also with several of the distinctive white latticed Hypera pollux cocoons
(Plate 11). No O. simillimus wasps or nests were seen, but on a second visit on 21st July one female
wasp appealed and started searching the A. nodiflorum, confirming the wasps' presence in this
area.
References
TARPEY, T. & HEATH, J. (1990) Wild flowers of North East Essex. Colchester Natural History Society.
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Essex Naturalist (New Series) 21 (2004)