Cephus pygmeus L Wheat stem sawfly

This widely distributed sawfly is a pest of winter wheat but will also attack barley, rye, other cereals and grasses. Although of major importance in many parts of its range (notably in eastern Europe) in some areas (including the British Isles) the insect is now less important than formerly; possibly, numbers declined following the adoption of post-harvest straw burning. It remains to be seen whether banning of straw burning will lead to an increase in the pest status of this insect. Plants attacked by wheat stem sawfly larvae are undernourished, dwarfed and weakened; typically, the ears fail to develop properly and the seed heads turn white. Such plants often break off a few centimetres above ground level, especially in heavy rain or in a strong wind.

BIOLOGY

Adults occur from late May or June to July. They are often found in the vicinity of cereal fields, where they forage for pollen on Umbelliferae and various other plants, including late-flowering oilseed rape. Eggs are laid singly in the stems of cereal plants, each inserted just below the ear through a slit formed close to the uppermost node. The egg hatches 7-10 days later. The larva then bores downwards within the pith and eventually, typically about a month later, reaches the base of the stem. The larva then bites around the wall of the stem to form a line of weakness, and plugs the hollow stem below this point with a mixture of frass and fragments of tissue removed from the wall; the larva then spins a cocoon within which to overwinter. Pupation occurs in the spring.

DESCRIPTION

Adult female 9-10 mm long, elongate, mainly black and shiny, with bright yellow patches or bands on the abdomen (especially on the fourth and sixth segments); antennae black, filiform and noticeably thickened towards the tip; wings hyaline, with black veins (Fig. 329). Adult male similar in appearance to female but marked more extensively with yellow (Plate 16a). Larva up to 12 mm long; body elongate and mainly yellowish-white; head yellowish-brown (Plate 16b); anal segment with a slightly chitinized dorsal spine; thorax with three pairs of fleshy leg-like tubercles (these are not always obvious) but body otherwise apodous (Fig. 330).

Fig. 329 Female wheat stem sawfly, Cephus pygmeus (x4).
Fig. 330 Larva of wheat stem sawfly, Cephus pygmeus (x9).
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