Cushion
Scale
Symptoms
Elliptical disks can be seen stuck onto the underside of leaves
especially along the main veins. They look like nasty yellow-brown
waxy ovals. They are often accompanied by a black soot on the upper
surfaces of the leaves.
Cause
Scale are insects which suck sap from plants. The part that you
can see is the outer shell of mature insects. Small scale (nymphs)
are minute and they run around the plant looking for a good place
to stop. When they do find a spot, they down tools, grow an impenetrable
shell and feed and reproduce with voracity. Scale feed by tapping
straight into the plants' flow of sap (so they don't even need to
bother to suck) which means that quite a lot of excess sap oozes
out. It is this excess which attracts the black sooty mould you
often notice first.
Harm
Scale in itself is unlikely to kill a plant, but it will leave it
very weak and prone to other pests and disease. It also looks very
unsightly.
Treatment
Organic - mature scale can be picked off or wiped off with cotton
wool or tissues. The young nymphs are too small to see, but as they
mature and settle down they too can be picked off. You will have
to be patient because you will have to clean off the visible scale,
and wait for more to mature and then cleaning it off again, before
the plant is clean.
Chemical - the mature scale's outer shell is impenetrable and it
is only the young that can be killed by spraying with chemicals.
This must be done every fortnight so each new batch that hatches
is treated.
ps feeding within hours and will die a few days later.
Source:
Crocus
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