PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Twospotted mite, Red Spider
An example of a spider mite
This is a serious pest of a wide range of plants
during warm weather both indoors and outdoors.
Plant materials such as fruit carrying more than a
certain number of mites may be refused entry to
some countries. The most important pest of
ornamental plants and cut flowers in Australia.
Most common spider mite and a severe problem
wherever it occurs.
Scientific name
Tetranychus urticae (Class Arachnida, Order
Acarina). Twospotted mite is a spider mite (Family
Tetranychidae). Other spider mites include:
Banana spider mite (T. lambi)
Bean spider mite (T. ludeni)
Hydrangea spider mite (T. hydrangea)
See also page 201.
Host range
This is a world wide pest which feeds on a
variety of plants, eg
Ornamentals, eg indoor plants, eg umbrella tree,
palms; carnation, fuchsia, orchids, roses, violets.
Fruit, eg deciduous fruit trees, especially apple
and pear, trailing berries, strawberry.
Vegetables, eg bean, cucumber, tomato.
Field crops, eg cotton.
Weeds, eg various.
Description & damage
Plant damage is caused by the nymphs and adults
sucking plant sap from the leaves.
Adult mites are just large enough to be seen
without a hand lens (about 0.5 m long). They are
pale greenish or yellowish, the colour varying
somewhat on the different host plants, and have
8 legs. The mites have distinctive dark markings
on either side of the body. These markings are
particularly large and prominent in the adult
female. The females are rather pear-shaped, can
move actively and spin fine webbing over the
surface on which they are feeding. The males are
smaller and narrower. Nymphs initially have
6 legs but later nymphal stages have 8 legs.
Leaves. Infestation usually starts on the more
mature leaves and moves upwards. Mites feed
mostly on leaf undersurfaces and in heavy
infestations they also feed on the upper surfaces.
Feeding mites cause leaf mottling or speckling.
Often quantities of webbing are seen and adult
mites are easily seen with a hand lens. Yellowing
of leaves may occur; leaves may die and finally
fall. On some hosts, eg apple and beans there may
be bronzing of leaves and an uprolling of leaf
margins. Mites can crawl all over the plant and
envelope it in a mass of webbing. This is used as
an aid to wind dispersal.
Fruit. Apples may be undersized and red
varieties may fail to colour evenly and fully. Fruit
may also become sunburned due to excessive
exposure to sun caused by defoliation. Common
pest of strawberry.
Limbs may become sunburnt due to defoliation.
Green twigs of citrus may have yellow spots.
General. Repeated severe infestations year after
year can result in weakening of trees and affect
root growth. Herbaceous plants may die.
Diagnostics.
Twospotted mites are identified by the
distinctive markings on each side of the body
(pages 199 and 203, Fig.126).
Do not confuse twospotted injury to leaves with
that caused by some sap sucking insect pests
(see Table 40
below).
If in doubt contact a diagnostic service.
Diagnostic tools for mite identification are
continually being developed including for
DNA-based technologies.
Lucid keys, eg Mites of Quarantine Importance,
Mites in Soil, Invasive Mite Identification are
available at www.lucidcentral.org/
Pest cycle
There is a gradual metamorphosis - egg,
nymph (several stages) and adult with as many as
9 generations during warmer months (Fig. 126).
Each female lays 70-100 eggs, the life cycle takes
7-14 days in summer. In coastal areas continues
throughout the year but in colder areas females
become inactive. Females change colour from
greenish to orange in winter and become inactive.
Table 40. Comparison of damage caused by twospotted mites and some sap sucking insects.
LEAVES
UPPER
SURFACE
TWOSPOTTED
MITE
Sandy speckling
(leaf stippling)
VARIOUS
LEAFHOPPERS
Speckled feeding
patterns
GREENHOUSE
WHITEFLY
LACE BUGS
Azalea, olive
GREENHOUSE
THRIPS
Sandy speckling Sandy speckling Silvering
UNDER
SURFACE
Mites, webbing
Insects if present,
fly if disturbed,
possibly a few
cast skins, but
surface may be
Whiteflies, white
stationary nymphs,
honeydew, sooty
mould
Lace bugs, spiny
nymphs, black tarry
drops of excreta
Adults and
nymphs often
dark coloured,
black tarry
drops of excreta
202 Insects and allied pests – Mites (Arachnida, Acarina)