THE SPINDLE TREE. 169
To return now to the economic question—at what points in
this complex cycle is the aphis, by either natural or artificial
means, subject to control ? Fortunately for the botanists it
would seem that the eradication of the Spindle tree, though it
might minimise the aphis attack, would not remove the possibility
of attack altogether. The problem is not so simple as all that.
Besides the Spindle, another winter host plant exists, the Guelder
Rose—Viburnum opulus—which would appear to serve equally
well for the egg-laying females when the Spindle tree is not
present, but which is usually avoided owing to the frequent occur-
rence of eggs of other aphids on this host. Moreover, colonies
have also been found utilising the evergreen garden shrub E.
Japonica as a winter host in the South of England. Therefore
there seems hope that the Spindle tree may after all be spared,
thanks to the adaptability of the aphis to more than one winter
host.
The yearly cutting back of all wild Spindle shrubs during the
winter months (after the eggs have been laid) has also been
suggested as a means of at least reducing the degree of aphis
attack sufficiently for all practical purposes, and this may indeed
have to be resorted to ; but even this disfigurement of such a
beautiful shrub would be deplorable.
Another control measure which has been introduced with
considerable success is the use of fumigating and dusting machines
in the beet and bean fields in the summer time, whereby nicotine
vapour or dust is directed on to the infested plants in such a
strength as to kill the aphids without harming their host plants.
This method appears to be a fairly economic means of control on
a field scale, but crops must be treated before the plants have
been badly checked by the sucking of the aphids, and our English
climate does not always permit favourable conditions for treat-
ment at the right time, which is a serious drawback when it is
remembered that the aphids can multiply at the rate of three or
more families a day.
From the biological view-point the study of the natural enemies
of the aphis is of considerable interest, and under normal con-
ditions these do undoubtedly exercise a marked control over
aphid infestations. Principal among these are certain small
Ichneumon wasps of the genus Aphidius, whose developing young
parasitise the aphids. The eggs of the wasp are deposited upon
the living aphids, and the subsequent young live parasitically
on their bodies until nothing but the hollow skins remain. These
brown, bead-like, parasitised skins are a fairly common sight upon
beet leaves in summer, testifying to the presence of this natural
form of aphid control. Biological methods utilising such