Biology Reference
In-Depth Information
980
981
980 Larva of elm leaf-mining sawfly ( Fenusa ulmi ).
981 Larva of Heterarthrus vagans .
982
DAMAGE
The mines cause slight distortion of the leaf blade but,
even when numerous, have little or no effect on growth.
Hemichroa crocea (Geoffroy in Fourcroy)
A sporadically abundant pest of alder ( Alnus ) and birch
( Betula ). Widely distributed in Europe; also occurs in
North America.
DESCRIPTION
Adult female: 5-8 mm long; reddish yellow, with
antennae, part of thorax and legs black. Larva: up to
20 mm long; head light brown to black; body mainly
greyish green, with the first thoracic segment yellowish
orange, and with a black longitudinal line above the
spiracles and two lines of black spots above the legs.
982 Mines of Heterarthrus vagans in leaf of Alnus .
LIFE HISTORY
Adults of this mainly parthenogenetic species appear
from May onwards. Eggs are laid in alder or birch
leaves, along either side of a leaf stalk. After egg hatch,
the larvae move onto the leaf blade to feed. Older larvae
feed in company along the leaf edges but they become
less gregarious as they mature. When fully grown, the
larvae enter the soil to pupate, each in a dark brown,
single-walled cocoon. This species is usually double
brooded but, in favourable conditions, there may be a
third generation.
Heterarthrus vagans (Fallén) ( 981-982 )
An often abundant pest of alder ( Alnus ), including
ornamentals and trees planted as windbreaks.
Widespread in central and northern Europe.
DESCRIPTION
Adult: 3-5 mm long; mainly blackish, the abdomen
orange-yellow below; legs yellowish; wings smoky.
Larva: up to 9 mm long; head small, brown and
pointed anteriorly; body flattened and tapering back
from a relatively broad thoracic region; whitish and
translucent, with the green gut contents clearly visible;
distinctive black markings present between the thoracic
legs (except in final instar); legs very small, the anal
pair of abdominal prolegs fused.
DAMAGE
Young larvae form distinctive sigmoid-shaped holes in
the leaf blades. Older larvae devour most of the leaf
tissue, apart form the major veins, and are capable of
causing considerable defoliation; greatest damage is
caused to alder, especially red alder ( Alnus rubra ).
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