170 THE ESSEX NATURALIST.
parasites for the suppression of insect pests have been successfully
applied in a few cases, as, for example, the introduction of the
Chalcid wasp, Encarsia formosa, into greenhouses to control
Greenhouse White Fly. With outdoor crops, however, so many
other factors are at work that these biological methods of control
do not always give the results hoped for. Nevertheless, the value
of these parasites and of other predators should not be overlooked,
for were it not for such checks it is doubtful if man would be
able to suppress the damage at all which aphids cause to crops.
Another very useful enemy of the aphid is the Ladybird
beetle (Coccinella) and its larva—both very much in evidence
on beet plants in the summer time, feeding voraciously on the
aphids and thus rendering valuable service to man, whilst
exercising no detrimental effect upon the host-plant itself. The
larva of the Lace-wing Fly (Chrysopa perla) and of members of
the Hover-fly group (Syrphidae) are also worthy of mention,
feeding in the same way on clusters of aphids and being thus
responsible for the destruction of considerable numbers. Adverse
weather conditions during the migratory periods of the aphis
from host to host also doubtless reduce their numbers, whilst
temperature influences the development and consequent spread
of aphids to a great extent. Here again, however our fickle
climate cannot count as a factor to be relied upon to give con-
sistent results.
These natural enemies and controlling influences, valuable as
they are, are not, however, collectively sufficient to balance the
extraordinary reproductive powers of the aphids and keep them
from doing serious injury. If, then, this balance is to continue
to be disturbed by man's encouragement and development of
the summer host plant, it would consequently seem as if man
must be responsible in the main for the control of the aphids
which benefit from such increase. In the meantime the fate of
the Spindle tree as an active participant in this complex cycle is
being held in the balance.
THE ESSEX FIELD CLUB—REPORTS OF MEETINGS.
SPRING RAMBLE IN EPPING FOREST (845th Meeting).
SATURDAY, 29TH APRIL, 1944.
This was arranged to be a general natural history ramble, to include various
interests.
The party of 30 members and visitors assembled at the Loughton bus-garage
at 11 o'clock and proceeded to Baldwins Hill, which affords a magnificent view
over the woodlands : here the Forest was entered, the route pursued being by