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The Hoverfly Didea fasciata Macq. (Diptera: Syrphidae) in Essex
Didea fasciata is a large and rather striking hoverfly, with black and yellow bands on its
abdomen. As far as can be told, the only Essex records until now were those made by
Harwood at Colchester High Woods in May 1900 and again in 1908. These specimens
are in the collections at the Hope Department of Entomolgy at Oxford University
Museum and were cited by Ron Payne in his 1973/4 review of Essex hoverflies published
in Essex Naturalist 33, on page 88. It therefore came as quite a shock to find a solitary
female example in the ultra-violet light "electrocution trap" in the gate security lodge at
Beckton Sewage Treatment Works in the eastern London portion of South Essex during
July 1993.
Little is actually known about the ecology of this hoverfly. It is quite frequent but rather
local in distribution across southern England, always in well-wooded areas. There is a
very clear association with ancient deciduous woodlands and it may well be useful as an
indicator of such habitats. However, the only positive information on the larvae relates to
a single example collected at Kew Gardens where it was feeding on willow aphids; the
adult was bred through (Rotheray, personal communication). Beckton lies on the former
Thames flood-plain in east London and is probably the last place this hoverfly would be
expected to be resident; the nearest woodland of consequence is Oxleas Wood across the
river in West Kent but the insect is not recorded from that site at present. There are,
however, a number of willow trees in the area.
The trap at Beckton was emptied in early July by Charlie Gibson of Bioscan (UK) Ltd
and the fly was given to me for identification. It is not known when the trap was last
emptied before then. The moths in the trap included spring species such as Diaphora
mendica Cl. (Arctiidae) and Orthosia gothica L. (Noctuidae) but none of the autumnal
or winter species. Thus, one can be fairly certain that the specimen was taken during
1993 but a more precise date is impossible to discern.
Given the apparently unsuitable habitat of the Beckton area, the possibility that the fly
was taken here as a primary immigrant can not be ignored, though there was a distinct
lack of other immigrant hoverflies, and of immigrant Lepidoptera, in the first six months
of 1993. It is difficult to imagine the fly being a local resident, though perhaps finding it
may require specialist techniques. If willow aphids are the sole larval food then there is
no real reason why the insect should not be present in areas away from old woodland,
including the suburban zone of London. I have taken it occasionally in malaise traps in
other parts of the country so perhaps entomologists should try this technique in areas of
willow woodland/scrub in their local area and see if the fly is present. Either myself, or
Roger Payne at Southend Museum, would be delighted to identify any hoverflies which
Field Club members collect.
Colin W. Plant
East Ham Nature Reserve (Newham Museum Service),
The Visitor Centre, Norman Road, East Ham, London, E6 4HN