Ceutorhynchus pallidactylus (Marsham, 1802)

Male
The Cabbage stem weevil, widely referred to as C.quadridens (Panzer).
Although widespread throughout England, Wales and Scotland and often extremely common, albeit very locally so, this species is rarely encountered around Watford due to its preference for agricultural habitats. During May 2008 several roadside verges adjacent to an extensive oilseed rape crop near the M25 roundabout (TQ076997) were swept in bright sunshine and C.pallidactylus were found to be numerous, closer inspection showed the weevils to be confined to the few accidental rape plants growing among the grass. With the knowledge that pallidactylus was both common and active on this date the opportunity was taken to search for them away from the crop. Sweeping yellow flowering brassicas only a hundred metres or so from the crop failed to produce the species. On the same day many sweeping samples were taken from between this locality along the Grand Union canal to Common moor, a walk of about 6 km along which yellow flowered brassicas were common although sporadic, without finding the beetle. From the last four years of sampling we have found only very occasional specimens, all from Common moor, and while there are many local areas yet to be investigated e.g. several extensive areas of allotments, it seems so far the species is more or less confined to agricultural land.

All cultivated cruciferae seem to be attacked (Jones and Jones, 1966) and in southern counties virtually every plant in a field may be infested. The beetles may be overlooked as plants are not killed, although growth may be reduced, and infestations tend to be uniform. The species is an important pest of spring sown rape but may be difficult to detect as adults spend much time on the ground, damage begins in the very early stages of growth, generally as soon as the first green buds have formed (Gratwick, 1992). Adults emerge from hibernation in the spring. The female bores small holes into a seedling stem at the base of the petiole into which eggs are laid in small groups. Larvae tunnel up and down the midribs and, as they get larger, move into the main stem where they destroy the pith. During July or August mature larvae, 4-6mm long, bore small but obvious exit holes in the lower part of the stem although sometimes entry holes around lower leaf scars are enlarged for the purpose, and make their way into the soil where they pupate.

Winter rape, sown in the autumn and currently representing the majority of the UK Oilseed rape crop, is also affected but to a much lesser extent (Kirk, 1992). The predominant weevil species between September and March, and as mature larvae during April and May, is C.picitarsis, the rape winter stem beetle. Although C.picitarsis is of the same elongate shape and has reddish tarsi contrasting against the otherwise black legs, it lacks the overrall pubescent appearance of C.pallidactylus, the interstices being finely pubescent but mostly lacking scales. In general adult picitarsis occur between October and March whereas pallidactylus occur between April and June.

2.4-3.3mm. Entirely black but for red tarsi and red or pitchy red antennae. Head densely punctured, with fine, erect scales and much wider, pale recumbent scales laterally and between eyes. Rostrum very long, more than five times longer than broad, glabrous and finely punctured. Scrobes not visible from above. Antennal scape gradually thickened from middle, funiculus 7 segmented, club small, much shorter than funiculus, and pointed. Pronotum broadest at base, narrowed to raised front margin and with hind margin strongly sinuate. With strong prominence laterally near middle and a central longitudinal furrow although this may be evanescent on disc, strongly punctured throughout and with scattered pale scales, usually dense along centre and in anterior half. Elytra slightly elongate (8:7), with strong humeral prominence and broadest at best, contracted and curved weakly to apex. Entire surface with small, pale semi-erect setae which are clearly visible along lateral margins from above. With scattered white or creamy scales, generally dense behind scutellum and each stria with a single row of scales. Interstices broader than striae, at least X3, surface rugose and shining,without tubercles except for a group subapically on 5-8 and smaller ones on apical margin. Legs long and densely scaled, femora weakly toothed beneath and tibiae without apical spines. Segment three of all tarsi strongly bilobed, all tarsi with two claws.

Description from 5 Watford specimens at X20

References
Gratwick, M. 1992. Crop pests in the UK. Chapman and Hall
Kirk, W.D.J. 1992. Insects on cabbages and Oilseed rape. Richmond Publishing

Female

Male Femoral Tooth

Male

Female

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