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Pests And Diseases |
They hatch in about a week and
the young larvae at first feed by scarifying the surface tissues.
Later, they eat irregular holes in the leaves and petals -these
holes may be large and numerous, since all the larvae usually
remain on the same leaf. These bettles are not aquatic so that
the eggs and larvae are found only on the upper surface of the
floating leaves although in the case of erect growing plants
they may appear on both sides. Pupation takes place after
three weeks, also on the upper leaf surface, and the adult beetles
emerge in about ten days. The first batch usually appears
towards the end of June, the second brood of adults appearing
about the middle of August. There is however a considerable
amount of overlapping so that with different broods, the vary-
ing stages may occur together throughout the summer. The
very latest hatched adults seem to hibernate in a sheltered place
during the winter and emerge the following spring to lay their
eggs.
The exact control measures to adopt, largely depend on
whether or not there are fish in the pond. Some fish such as
carp or goldfish, will eat the larvae and beetles if the affected
leaves are carefully pressed down into the water. It is often
advisable to keep the leaves submerged for a day or two so
that they should be held down by some means. This will
make it easy for the fish to remove both the insects and pupae.
It is also possible to syringe infested leaves with water so as
to dislodge the pests.
If there are no fish in the water other methods must be
adopted, for the insects do not drown easily and may make
their way up the stems and on to other leaves. A thin coating
of paraffin oil put on the surface of the water has proved effec-
tive, while if fish are not to occupy the pond for some time,
the leaves may be sprayed with derris or even D.D.T. Both of
these are most harmful to fish, and must definitely not be used
where fish occupy the water.
There are several moths which sometimes attack the foliage
of various water plants. Some of these eat part of the leaf
tissues, causing the remainder of the leaves to decay and present
an unsightly appearance. Eggs are laid on the undersides of
water-lily and other large-leaved plants. These eggs produce
small grubs which devour portions of the leaves.
After pupation the moths emerge from the cocoons which
are suspended between the leaf stalks of the taller growing
water plants.
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