Agriculture Reference
In-Depth Information
economically more important but non-European
pest.
host plants to seek sheltered situations on nearby
trees, walls or other structures, as do mature fe-
males when they are about to lay eggs.
Family TETRANYCHIDAE
(spider mites)
Bryobia praetiosa Koch
Clover bryobia mite
DESCRIPTION
Adult female 0.7 mm long, blackish-red or dark
reddish-brown; legs pinkish, the first pair
very long; body oval and flattened; dorsal
setae spatulate (cf. stone mite, Petrobia latens,
p. 267). Egg 0.2 mm in diameter, dark red,
roughly spherical. Larva reddish-orange, 6-
legged. Nymph dark green to brown or dark red.
This generally abundant species (along with the
other species of Bryobia cited below) was once
considered part of a complex of closely related
biological races. Former members of this com-
plex are structurally very similar, but there are
noticeable differences in their habits and life-
cycles. The clover bryobia mite infests clover
(and many other legumes), grasses and various
herbaceous plants; in glasshouses, damage is
often caused to the leaves of cucumber plants.
Especially in the early spring, the mites fre-
quently invade buildings, where eggs may be
laid; the mites also aggregate during the spring
and summer on sunny window-sills and on re-
cently constructed brick walls, where they also
shelter in cracks and crevices. Infestations are
particularly common in weedy, overgrown sites.
Leaf injury on host plants varies from speckling
to an overall bronzing or silvering and may result
in premature leaf-fall; bryobia mites do not
produce webbing (cf. fruit-tree red spider mite,
Panonychus ulmi, p. 266; two-spotted spider
mite, Tetranychus urticae, p. 267).
Bryobia cristata (Duges)
Grass/pear bryobia mite
This polyphagous bryobia mite occurs through-
out the year on grasses and herbaceous plants,
including various ornamentals. They feed on the
upper surface of the leaves and cause speckling
and silvering. There are several generations
annually, and breeding continues throughout
the year. Although there is no period of winter
diapause, development is greatly protracted
during cold weather. In late spring, infestations
sometimes extend to trees and shrubs, including
fruit trees and ornamentals, before a return
migration to more normal hosts; such attacks,
however, are of little or no importance. In com-
mon with Bryobia praetiosa (above) (from which
it is distinguishable by the slightly narrower
dorsal setae and other microscopic features),
this mite often aggregates on walls and may
enter buildings to lay eggs, especially in May and
June.
BIOLOGY
Mites produced from eggs deposited in the late
summer or autumn develop slowly throughout
the winter, and eventually reach the adult stage
in the following February or March. Eggs depos-
ited by the overwintered females produce a
summer generation of mites, members of which
feed on a wide variety of host plants and reach
maturity by the autumn. Males are unknown and
reproduction is entirely parthenogenetic. Migra-
tion to and from host plants is a common feature
of bryobia mites and continues throughout the
spring, summer and autumn, whenever the mites
are active. Also, when about to moult from one
growth-stage to another, nymphs usually vacate
Bryobia ribis Thomas
Gooseberry bryobia mite
This species was formerly an important pest of
gooseberry but is now relatively uncommon.
Heavy infestations have a pronounced effect on
host bushes, the young foliage turning pale and
then brown; damaged leaves often shrivel and
fall prematurely. Attacks also have a detrimental
effect on cropping, reducing both fruit size and
quality. The mites are similar in appearance to
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