Page 179 - Tài liệu cây cảnh Bonsai4me Bonsai Basics

loss. Evergreen trees will have periods each year where they drop old foliage as

it is replaced by new. If leaves are yellowing and dropping from old inner areas,
this is likely to be the case. However to ensure that this growth is replaced, make
sure that light and energy are given to old, inner areas of the tree by pruning the
apical growth. Similarly, deciduous varieties that are left unpruned will shed
inner growth at the expense of new growth at the ends of the branches.

Visible Pests on Leaves, Bark or Roots

If there are visible pests on the leaves, identify them and take the appropriate
action;
Black Fly and Greenfly are both common forms of aphids. They suck sap

from the tree and in large numbers can cause dieback of new or unripe growth.
On trees in poor health, this can eventually lead to death if not dealt with.
Aphids can also carry virus diseases from one plant to another.
Trees are normally attacked by a few aphids, which within a few days can
multiply to very large numbers. Fortunately, once detected, aphids are easily
dealt with. Small numbers of aphids and their eggs can be dealt with by rubbing
them off with fingers. Larger infestations can be quickly killed off by using one
of any number of insecticide sprays. Soapy water can also be used if sprayed
onto infested areas.
Some trees such as Acers are particularly susceptible to aphid infestation and
systemic insecticide use might be worth considering during periods of repeat
attack. It should be noted that the presence of ants should be looked for on trees,
which are repeatedly infested by aphids. Ants will commonly carry aphid eggs

into trees, protect them from predators and milk them of their sticky, sweet
excretion called honeydew. If ants are spotted, they should also be dealt with!
Caterpillars are very destructive to leaves and young growth leaving holes in
leaves and in some cases completely stripping them altogether. Often very
difficult to spot through excellent camouflage, close inspection of leaves, stems
and in particular the underside of foliage is required to find and remove them by
hand. Immature caterpillars will often be spotted in leaves that are folded over to
protect them from predators. Contact insecticides are rarely affective though
repeated problems can be reduced by using systemic insecticides.
Slugs and snails are also very destructive and quickly cause large areas of
defoliation. During periods of warm, damp evenings they are particularly
prevalent causing holes around the edges of leaves, this can be so extensive as to
completely strip all leaf from its stem. The most common tell-tale sign of slugs
or snails is the silvery trail that they leave behind them. Slugs and snails are only