7
Cuckoo Bumblebees (Psithyrus)
Closely related to the true Bumblebees (and
included in the same genus by some recent
authors) are the 'cuckoo-bees', usually
assigned to the separate genus 'Psithyrus'.
These are very similar in appearance to true
Bumblebees, but lack the 'pollen baskets' on
the hind tibia. They also generally have a
thinner coat of hair on the body, and tend to be
more 'sluggish' in their behaviour. The
cuckoo-bees do not have a worker caste.
Instead, the fertilised females which emerge
from hibernation seek out and enter the
already established nest of a true Bumblebee
of an appropriate species. They subsequently
lay their eggs in the nest, and their resulting
larvae are nurtured by the host species'
workers. These give rise to mature male and
female cuckoo-bees. After mating, the males
continue to forage from various flowering
plants until they die off before winter, while
the fertilised females feed up for hibernation
as do true Bumblebee queens.
There is still a need for more research on the
relationship between cuckoo- and
Bumblebees. It is not known how intruder
cuckoos defend themselves against the
resistance of the host workers. Also, it seems
that in some species the intruding female
cuckoo-bee tolerates the continued presence of
the host queen, whereas in others she is
generally killed. In some cases (for example
Psithyrus rupestris and Bombus lapidarius).
but not all, there arc similarities in colouration
between cuckoo-bees and their hosts. It is not
known whether this is significant in aiding
them to avoid attacks from the host workers.
Finally, it is not known how exclusively
particular cuckoo species parasitise particular
host-species. Typically, the cuckoo-bees have
smaller populations than their hosts. However,
this is somewhat misleading, as the great
majority of individuals in the populations of
true Bumblebees are non-reproductive
workers. Given this fact, it is surprising how
frequently cuckoo-bees of some species may
be found. This is particularly true of the males
in mid to late summer. The females arc more
commonly seen in spring as they seek out the
nests of Bombus species.
Bumblebee Decline
A research monograph by Steven Falk,
produced by the Nature Conservancy Council
in 1991, listed no less than seven species of
Bumblebee as scarce or threatened. But
distribution maps published by the Institute of
Terrestrial Ecology in 1980 (ITE 1980)
already indicated cause for concern. Out of
twenty five species mapped, twelve showed
significant declines in distribution (comparing
pre- and post- 1960 records), including two
declared 'probably extinct'. It is also
reasonable to guess that even those species
showing no evident decline in distribution
may have been declining in population size.
More recently five species have been included
in the U.K. Biodiversity Action Plan, and a
national Bumblebee Working Group has been
established with the involvement of four
conservation agencies: the Countryside
Council for Wales, English Nature, Scottish
Natural Heritage, and the World Wide Fund
for Nature. National and county-wide 'Action
Plans' have been devised for selected species
at risk.
Against this background, field work on the
Essex species of Bumblebees has intensified in
recent years, and we now have fairly
comprehensive distribution maps covering the
whole county (see Harvey 1996 and 1998. and
Benton 1995). These make possible some
informed guesswork about changes in the
Essex Bumblebee fauna since the ITE maps
published in 1980 (in fact, the most recent
data used for those was 1974 -see Edwards
1997). Those maps record the presence, in
10k. squares wholly or partly in Essex, of
seventeen species during the post-1960 period.
Field work during the last two decades has
revealed the continuing presence in the county
of sixteen of these. One, Bombus subterraneus
has declined drastically throughout its
Essex Naturalist (New Series) 16 (1999)