Agriculture Reference
In-Depth Information
Panonychus ulmi (Koch)
Fruit tree red spider mite
This widely distributed species is an important
pest of fruit trees, especially apple, damson and
plum. Infestations also occur on other hosts, in-
cluding cherry, pear, various ornamentals and,
occasionally, bush or cane fruits; spider mite
damage on these hosts, however, is usually
attributable to the two-spotted spider mite,
Tetranychus urticae (p. 267). The mites feed
mainly on the underside of leaves, withdrawing
the cell contents to produce a fine speckling,
visible from above. As pest numbers increase,
affected leaves become generally dull and pallid,
and finally silvery or bronzed. Severely damaged
leaves are brittle and may drop prematurely.
Heavy infestations reduce plant vigour and will
affect fruit yields and fruit bud formation for the
following year. Damage is usually most evident
from July onwards, and attacks are especially
severe in hot, dry summers.
Fig. 343 Female apple & pear bryobia mite. Bryobia
nibrioculus (x60).
BIOLOGY
Mite eggs overwinter on the bark of host trees,
often clustered in dense red masses on the spurs
and small branches. Less frequently, winter eggs
may occur on the young shoots. Eggs hatch from
late April or May to about mid-June. However,
the precise timing of egg hatch varies consider-
ably from orchard to orchard, as there are both
early- and late-hatching 'strains' of the mite.
There are several, usually about five, overlap-
ping generations during the summer, and devel-
opment from egg (through larval, protonymphal
and deutonymphal stages) to adult takes about a
month: individuals developing into males often
omit the deutonymphal stage. Colonies develop
on the underside of the expanded leaves, where
white cast-off nymphal skins accumulate and the
paler-coloured summer eggs are laid (cf. colonies
of Bryobia spp. on fruit trees, p. 265 et seq.). The
mites do not shelter under webbing; however,
individuals may produce strands of silk which
will then allow them to be carried by the wind to
adjacent trees. Winter eggs are laid in Septem-
ber, in response to shorter days and declining
temperatures, but may be deposited earlier if
Bryobia praetiosa but biologically distinct, as it
overwinters in the egg stage. There is just one
generation each year.
Bryobia rubrioculus (Scheuten)
Apple & pear bryobia mite
This widely distributed mite (Fig. 343) is asso-
ciated with fruit trees, including apple, cherry,
pear and plum; infestations, which may also
occur on related ornamentals, rarely occur on
sprayed trees. The mites feed mainly on the
upper surface of the leaves. They also congregate
in masses on the bark of shoots and branches
whilst moulting from one stage to the next,
especially in late May and June, August and
September. The greyish-white cast-off skins
remain in situ and are a useful clue to the pres-
ence of this pest. There are usually two or
three generations each year, and this species
overwinters in the egg stage. Aggregations of
harmless oribatid mites on the bark of trees are
sometimes mistaken for bryobia mites (see
p. 270).
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