Introduction - Pests of Ornamental Trees, Shrubs and Flowers - page 10

punctured, ragged, skeletonized, speckled, thickened,
webbed, wilted or withered, and they may fall off
prematurely. Symptoms are sometimes in themselves
sufficiently characteristic to enable (at least with
experience) the causal organism to be identified; leaf
mines formed by certain insect larvae are good
examples. However, in many cases the cause of plant
damage cannot be determined with confidence unless
the pest itself is found and identified.
Some pests (termed polyphagous) are indiscriminate
feeders, and attack a wide range of plants. Others are
more specific, and often feed on only a restricted group
of hosts - perhaps those from a single group (family or
genus) of plants, or even a single species; such pests are
termed oligophagous or monophagous, respectively.
Plant susceptibility to pests may also vary at the specific
level, and sometimes differs markedly from cultivar to
cultivar. Unlike native plants, exotics introduced from
abroad may prove largely if not entirely immune to pest
attack. However, they may be damaged by pests that
arrived along with them (or subsequently) from the
country of origin; in Europe, Australasian pests on
plants such as Eucalyptus and Pittosporum are
examples.
In some instances (e.g. aphids, leaf beetles, mites and

slugs), all active feeding stages (adults and juveniles) of
a pest cause similar damage. In others the type of
damage caused by phytophagous adults and juveniles
may be different: chafer adults, for example, attack the
leaves and other aerial parts of plants whereas their
grubs are root-feeders. In many cases (e.g. true flies,
butterflies and moths), damage is usually caused only
by the larvae.
Several pests (e.g. certain aphids, midges, wasps,
mites and nematodes) produce characteristic galls on
host plants. Such gall-formers inject a toxin into the
plant cells, thereby stimulating abnormal development
of the plant tissue. In other cases injection of toxins
merely causes distortion of the plant tissue, affected
shoots, leaves or flowers becoming malformed and
often discoloured.
Many pests of ornamental plants have little or no
direct effect on growth but their depredations might be
disfiguring; such damage is often of little or no
consequence on established plants but on young ones
(particularly in commercial nurseries) may have a
significant impact on plant quality. The mere presence
of certain pests (e.g. wax-secreting or honeydew-
excreting aphids, scale insects and other bugs) can be
unacceptable, even when infestations are slight.

Control of pests

Good husbandry will reduce the likelihood of pest
problems developing on ornamentals but, in some
instances, specific control measures may be necessary to
protect plants from attack or to keep pests and their
damage within acceptable bounds. Pest attacks can be
lessened by using traps or physical barriers (e.g. grease
tree-bands for pests such as winter moth, and netting for
birds or mammals) but such methods are not always
practical and are certainly not available for combating
the majority of pests.
Attention to hygiene is important for lessening the
impact of pests, especially in greenhouses - plant debris
should be cleared as soon as cropping is completed, and
buildings, pots and other equipment disinfected before
new plants are introduced. Efficient weed control, both
within greenhouses and outdoors, will reduce the range
of places where pests can find shelter and will also limit
the number of possible alternative host plants upon
which certain pests might survive or breed. Regular
cultivation of soils will help to control weeds and will
also keep soil pests in check, either destroying them

directly or exposing them to desiccation or to the
attention of birds and other predators.
Wherever practical, plants should be examined
regularly for signs of pests, so that appropriate action
can be taken at the earliest possible stage. Newly
acquired plants, including the roots and adhering soil or
compost, should always be inspected to prevent the
accidental introduction of pests into clean sites; this is of
particular importance for combating insidious pests such
a nematodes.
On a small scale, some pests may be controlled by
hand, any egg clumps, larvae or other stages found on
plants being squashed or picked off and destroyed; in
some cases affected parts of plants, such a shoots
containing galls or webbed by caterpillars, may be
removed and destroyed. Prunings and other plant debris,
whether thought to be harbouring pests or not, should
never be left lying around but should be gathered up
immediately and burnt.
Various pesticides are available for use against pests
of ornamental plants. Some are broad-spectrum materials