Sitona lineatus L Pea bean weevil

This generally abundant weevil is a pest of various members of the Fabaceae, including broad bean, field bean and pea. Adults form character-

Fig. 236 Hop root weevil, Plinthus caliginosus (xlO).
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 unimportant but can be of some significance if growing points of backward bean or pea plants are attacked at the establishment phase, especially when seedling growth is retarded by poor growing conditions. Larvae feeding on root nodules can cause yield reductions of beans, owing to the premature shedding of pods. The adult weevils 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 (sometimes 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.

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 agricultural significance, mainly as pests of clover, lucerne or other forage legumes. These include: S. hispidulus (F.) - a generally common, dark-scaled species with particularly prominent eyes (Fig. 239), that commences egg laying in the autumn and is associated mainly with clover; S. humeralis Stephens - a common but southerly distributed species associated with clovers, trefoils 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. 238 Pea & bean weevil, Sitona lineatus (xl2).

ported in eastern Europe than in the British Isles; attacks tend to be of greatest significance Tanymecus palliatus (F.) under dry conditions. The adults (8-12mm long)

Beet leaf weevil are reddish-brown, with a pale margin to the thorax and hind-most parts of the elytra. In This weevil is a minor pest of sugar beet. The outline, they are elongate with a relatively nar-adults notch the edges of the cotyledons and row thorax; also, the elytra taper noticeably young leaves but damage is more often re- posteriorly.

Fig. 238 Pea & bean weevil, Sitona lineatus (xl2).

ported in eastern Europe than in the British Isles; attacks tend to be of greatest significance Tanymecus palliatus (F.) under dry conditions. The adults (8-12mm long)

Beet leaf weevil are reddish-brown, with a pale margin to the thorax and hind-most parts of the elytra. In This weevil is a minor pest of sugar beet. The outline, they are elongate with a relatively nar-adults notch the edges of the cotyledons and row thorax; also, the elytra taper noticeably young leaves but damage is more often re- posteriorly.

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