9
THE ROBBER FLY Asilus crabroniformis
This large and impressive nationally notable robber fly was presumed to be extinct in the
county until it was found by Roger Payne last year at Broom Hill, Chadwell.
1 saw at least 5 individuals at this site on the 6th August this year resting on horse dung. It
was also reported from Rainham Marshes during one of the surveys conducted there in
recent years but this record was apparently treated with some skepticism.
However now Colin Plant has received a report of some numbers of the robber fly
hunting all over the foreshore at Crayford Marshes on the 28th August this year, just
across the river from Rainham Marshes. Colin suggests that the fly may migrate from one
locality to another in the Thames Estuary region.
According to the English Nature publication "A review of the scarce and threatened flies
of Great Britain" the fly is recorded widely in southern England with about 40 known
post 1960 sites mainly within Hampshire and the south-west but greatly declined away
from these areas. Typically the fly is found on dry heathland, chalk downland and rough
unimproved pastureland. Adults are found around cow and horse dung but they are said
to lay their eggs in cow dung. At Broom Hill only horses are present and there is no cattle
grazing in the vicinity so it is probable that eggs are also laid in horse dung. It seems
reasonable to suppose that the fly may have to move at least small distances each year as
suitable grazing conditions change. The adults are large and presumably strong fliers but
are alert and difficult to approach. 1 have only seen them fly short distances before they
alight again, usually on another patch of dung.
The occurrence of the fly on the Thames grazing marshes again emphasises the national
importance of these areas. A look at a pre-war Ordnance Survey map shows the
staggering amount of Thames marsh that has been lost to industrial and housing
development. With the additional conversion of grasslands to arable, there are now only
sizeable areas of Thames grazing marsh left in Essex at Rainham, Fobbing, Canvey and
Kersey (Benfleet). These areas nearly all are or have been or will be under serious threat
from developments such as the Rainham Marsh MCA/Havering saga, there have been
proposals for a huge housing development at Canvey and there are various road
"improvement" schemes that continue to appear. The situation is little better in Kent
where the North Kent Marshes are under increasing pressure from development proposals
and for the siting of silt lagoons. Proposals to develop the "forgotten" East Thames
Corridor are presumably only waiting for an upturn in the economy to become a reality.
Peter Harvey