Agriculture Reference
In-Depth Information
are also implicated in the transmission of viruses,
including broad bean stain virus (BBSV) and
broad bean true mosaic virus (BBTMV); these
viruses are also transmitted by the pea flower
weevil,
Apion vorax
(p. 152).
BIOLOGY
Adults emerge from hibernation in the spring,
from late March onwards. They often occur in
large groups, attracted by an aggregation
pheromone produced by the males. Mated
females deposit large numbers of eggs (some-
times over 1000), either on the leaves of host
plants or in the soil. The eggs hatch 2-3 weeks
later. The larvae feed on the root nodules of
Fabaceae. They pass through five instars and
complete their development in about 6 weeks.
They then pupate, each in an earthen cell formed
a few centimetres below the soil surface. New
adults emerge in the late summer, 2-3 weeks
later. The young adults are very active and
migrate from host fields, usually by walking.
They may then be found feeding on many kinds
of plant well into October before eventually
hibernating.
Fig. 236
Hop root weevil,
Plinthus caliginosus
(xlO).
DESCRIPTION
Adult
4-5 mm long, mainly black-bodied,
clothed with greyish-brown scales;
prothorax
with three pale longitudinal bands;
elytra
often
with pale longitudinal bands;
eyes
moderately
prominent;
antennae
red and relatively narrow
(Fig. 238). Egg
0.4mm long, oval, whitish but
later becoming black.
Larva
up to 6 mm long;
body
creamish-white and distinctly wrinkled;
anal segment
with a fleshy pseudopod;
head
small, pale brown.
NOTE Various other species of
Sitona are
of agricul-
tural significance, mainly as pests of clover, lucerne or
other forage legumes. These include:
S. hispidulus
(F.)
- a generally common, dark-scaled species with par-
ticularly prominent eyes
(Fig. 239),
that commences
egg laying in the autumn and is associated mainly with
clover;
S. humeralis
Stephens - a common but south-
erly distributed species associated with clovers, tre-
foils and vetches; and
S. puncticollis
Stephens - a
relative large (4.5-6.0 mm long) black-bodied species
clothed with brownish or yellowish-brown scales, also
associated with clovers, trefoils and vetches.
Fig. 237
Head of larva of hop root weevil,
Plinthus
caliginosus.
istic U-shaped notches in the leaf margins; young
adults in autumn can also attack a wide variety
of other plants, including strawberry and many
ornamentals. Damage by adults is usually unim-
portant but can be of some significance if grow-
ing points of backward bean or pea plants are
attacked at the establishment phase, especially
when seedling growth is retarded by poor grow-
ing conditions. Larvae feeding on root nodules
can cause yield reductions of beans, owing to the
premature shedding of pods. The adult weevils
