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Insects and other pests
can badly damage plant roots, stems, leaves, flowers and seeds. Many
insect pests multiply very quickly, so it is sensible to check plants
regularly, diagnose problems accurately and give the appropriate
treatment promptly to prevent a rapid build-up. Eaten, distorted or
discoloured parts of plants are possible signs of insect damage. In some
cases the pest will be clearly visible, but this does not always occur.
Descriptions of some of the types of damage caused to various plant
parts are given below.
Pest & Disease Identifier Link (BBC Gardening Website)
Leaf damage - pests
visible
Pests which can be clearly seen on the leaves and other parts of the
plant include greenfly and blackfly (aphids). and whitefly.
Scale insects and mealy bugs can also be spotted quite readily.
Various species of aphids will attack most garden plants and can be seen
massed on stems and flowers as well as leaves. There are many different
species of aphids - over 500 in the UK and northern Europe.
Although aphids are commonly known as greenfly and blackfly, the colour
range can be anything from white to shades of red, yellow, orange, brown
or dark blue. The more well-known aphids include rose aphids, black bean
aphids, (which affect French and runner beans and some ornamentals);
cabbage aphids (attack cabbages and other brassicas as well as swedes);
and woolly aphids. The latter are covered by a white woolly 'wax' and
attack apples, predominantly, although will affect related ornamentals,
such as ornamental crab apples, hawthorn and cotoneaster.
Aphids are sap feeders
and can take in large amounts of plant sap. Excess sugars are excreted
in the form of a sugary substance, known as honeydew. In some cases,
ants will 'farm' aphids for this, which gives the pests some protection
from natural predators. Aphid attack can be very debilitating, causing
weakening and distortion of the new growth, in particular. A secondary
symptom of aphid attack is the growth of black sooty moulds, encouraged
by the presence of honeydew.
Whitefly are also sap feeders and, as the name suggests, are white in
colour. They look like tiny moths and attack many greenhouse and garden
plants, especially fuchsias, pelargonium, cucumbers and brassicas. Scale
insects are brown, almost stationary 'scale-like' creatures which affect
a wide range of plants. Mealy bugs have a white covering, which often
appears as a gooey, cotton-wool like mass in the leaf axils.Both scale
insects and mealy bugs are sap feeders and, like aphids, they secrete
honeydew and infestation can lead to the presence of sooty mould.
Leaf damage -
holes/leaves eaten
Some pests will either eat holes in the leaves or eat part or all of the
leaf. Pests in this category include slugs and snails, caterpillars,
capsid bugs, flea beetles and weevils such as the pea and bean weevil
and adult vine weevil.
Slugs and snails are a particular problem in mild and/or damp weather.
They eat the leaves of most plants, with a rasping action, leaving
irregular shaped holes or tattered shredded edges. Young plants can be
completely eaten and slime trails can often be seen on the soil surface
around affected plants. Slug pellets are the most common means of slug
control. Sometimes gardeners are concerned about using pellets if they
have pets. There is no need for concern if they are used sparingly and
according to label directions. Some gardeners may prefer to use a liquid
preparation which is less accessible to pets than pellets.
Caterpillars can attack most plants, their biting action causing
relatively large, irregular but clean cut holes in the leaves.
Caterpillar frass (droppings) may be seen on the leaves, which serves as
a further diagnostic feature of attack. Flea beetles cause damage on
turnips, radishes, wallflowers and brassicas, especially at the seedling
stage, where small, almost circular holes can be seen on affected
leaves.
Leaf damage -
discolouration
Leaf discolouration is a diagnostic feature of some pests. Mite damage,
including that of red spider mite, causes a speckling and often a
yellowing or bronzing of the leaves, typically of fruit trees, bushes
and many greenhouse plants. Aphids can cause a general yellowing of the
leaves. This is a secondary diagnostic feature. These pests themselves
are normally present on the leaves in large numbers when a plant is
under attack. Leaf miners cause light curved lines of discolouration to
appear on the leaves, due to the mining action they use when feeding on
plant leaves. Affected plants include chrysanthemum, celery, fruit
trees, primulas, cinerarias and other greenhouse plants. Leaf hoppers
cause a white, relatively large mottling on the upper surface of leaves.
Plants attacked include roses, fruit, potatoes and various other
ornamental and greenhouse plants.
Pea and bean weevils and adult vine weevils cause characteristic
U-shaped notching around the edges of leaves. Pea and bean weevil, in
addition to obviously affecting peas and beans can cause similar damage
on roses, lupins, carnations and other
ornamentals. Adult vine weevils cause notching around leaves of
camellias and rhododendrons in particular, but it is vine weevil larvae
that are particularly troublesome in the garden.
Sawfly larvae can cause severe damage to leaves - gooseberry sawfly and
Solomon's seal sawfly can virtually defoliate plants and they do so very
rapidly. Gooseberry bushes should be checked every week in May,
particularly near the centre of the
bushes, as this is where most of the eggs are laid. The sawfly larvae
are green in colour and look a bit like caterpillars. Where spotted they
should be treated as a matter of urgency because of the potential rapid
nature of the damage. Rose slug sawfly (also known as rose slugworm) has
small slug-like larvae which eat away the leaf tissues, exposing the
veins and causing a skeletonised appearance. In many vegetable and
ornamental crops, rabbits and wood pigeons can eat virtually all the
plant.
Damage to roots and tubers (soil pests)
The common soil pests which can damage roots and tubers include
wireworm, leatherjackets, chafer grubs, millepedes, vine weevil larvae
and some species of slugs and snails. Wireworm are stiff yellow or
orange grubs which can cause extensive damage in potatoes and root
crops, usually, boring holes into the roots or tubers. They are
particularly troublesome on areas which have previously been grass.
Leatherjackets are the offspring of the crane fly or 'Daddy long-legs'.
They are legless, leathery grubs which eat the underground parts of many
plants. They can be a particular nuisance on lawns, eating the roots of
the grass which results in yellowing areas of turf. A sure sign of the
presence of leather jackets is a group of starlings pecking at the grass
in an attempt to eat the grubs. Chafer grubs are curved, off-white
coloured grubs with three pairs of legs. They will feed on the roots of
trees, shrubs and flowers and grass, causing wilting and death. Vine
weevil larvae are whitish coloured grubs with an orange coloured head.
They are usually smaller in size than chafer grubs, are less curved and
do not have legs. They are mainly a problem on strawberries, alpines and
many pot plants.
The first symptoms of attack by vine weevil larvae attack are wilting
and/or collapse of the plant. If the growing medium
is inspected the culprits can be seen around the base of the plants.
There are species of aphids which attack roots of plants such as
lettuces, auriculas, primulas, carnations and pinks. Root aphids are
whitish in colour and often covered by a white powdery wax. They cause
affected plants to wilt where infestation is severe.
Cabbage root fly can be a serious problem on cabbages and other
brassicas. Radishes, swedes, turnips and wallflowers are
also particularly prone to attack. Transplanted brassicas are especially
susceptible. Affected plants wilt and can be tinged
purple/red. Similar symptoms are caused by club root, so it is easy to
confuse symptoms with this or other root damage.
Sometimes, however, the attack of cabbage root fly is limited to one or
two plants along a row, initially at least.
Fruit trees and bushes
A wide variety of insect pests attack fruit trees and bushes. Pests of
top fruit (apples and pears) include aphids, capsids,
caterpillars, suckers, moths, sawfly and blossom weevils. Codling moth
and sawfly are generally the most troublesome
pests of apples. Codling moth larvae tunnel into the fruit, towards the
core. Sawfly larvae eat just under the skin and
cause brown, ribbon-like scarring of the fruit. Pear midge is a common
pest of pears. The fruitlets swell and do not
develop properly, often falling from trees prematurely in May or June.
Stone fruit (peaches, plums, nectarines, almonds and cherries) are
susceptible to attack by aphids, caterpillars, weevils,
sawfly and slugworm. Plum sawfly larvae tunnel into the fruit and dark
sticky frass can be seen around the entry holes in
affected fruit.
The main pests of cane fruit (raspberries, blackberries and
loganberries) are aphids, capsids, leafhoppers and raspberry
beetle and bush fruit pests include aphids, capsids and caterpillars.
In many instances once the damage is seen, it is often too late to do
much about controlling the pest or pests responsible. |
