Hadroplontus trimaculatus (Fabricius, 1775)
This species is widespread and locally common throughout Europe from Spain to western Russia, Ukraine, Turkey, Northern Iran, Iraq, Israel and Syria, extending north into the U.K. but absent from Scandinavia; it is present around the northern Mediterranean and extends south to Morocco, Algeria and Tunisia. In the U.K. it occurs locally through England north to Yorkshire, and there are coastal records from the West Country and Wales. The usual habitats are undeveloped grassland, wooded margins and beside pathways etc. in open situations, often among lowland calcareous grassland communities. Host plants include a wide range of Asteraceae but in the U.K. it is mostly musk thistle, Carduus nutans L. and to a much lesser extent other species of Carduus and Cirsium. On the continent also on sheep thistle, Carduus tenuiflorus Curtis, and Carduus pycnocephalus L., Italian thistle, as well as other species of Cirsium, Carduus, Onopordum, Silybum and Galactites; in Mediterranean areas on G. tomentosa Moench, Bear thistle, and in North Africa on Carduus myriacanthus Salz. In some southern areas e.g. Italy the weevil has been an economic pest of artichoke thistles, Cynara Spp. In the U.K. adults become active in the spring when the host is beginning to grow. Eggs are laid into new growing tips of the host and young larvae feed on the foliage, often damaging the meristem, before boring into the stem, and then the roots where they will feed gregariously, causing cankering and promoting diseases etc. Fully grown larvae emerge from the stem and enter the soil to pupate. New generation adults appear in early summer and these will go on to overwinter.
3.0-4.0mm. This is a strikingly chequered species that is easily identified in the field with a little experience. Head dark with scattered pale scales; eyes convex and weakly protruding, the rostrum parallel and evenly curved with the antennal insertions in the distal half. The antennae are mostly pale with the apical thickening of the scape and the club dark. Pronotum densely and strongly punctured; broad and parallel in the basal half then narrowed to a raised anterior margin; the lateral scales pale and generally not surrounding the dorsal tubercles although this is very variable, the median scales contrasting creamy or yellow and extending from the base to about half way, and often present behind the anterior margin as well. Elytra dark with pale scales across the middle and at the apex giving a characteristic chequered pattern, those behind the scutellum creamy-yellow or brown, as those at the base of the pronotum. The striae are narrow and punctured, the interstices flat and rugose, lacking tubercles. Femora and tibiae dark with generally well-defined groups of pale scales. Tarsi pale orange or brown.
HADROPLONTUS Thomson, C.G., 1859
This is a small genus of very typical ceutorhynchine weevils, it includes three species; two are widespread across the Palaearctic, including the U.K, while H. ancora Roelofs, 1875 occurs through eastern Asia to Korea. All are associated with various species of thistles, Asteraceae, Carduae. H. litura (Fabricius) has been used as a biological control agent to help contain invasive thistles in various countries. They are characterized among the Ceutorhynchinae by the following combination of characters:
Small dark species; black to brown with distinct a pattern of pale scales to the pronotum and elytra.
Elytral interstices not tuberculate.
Antennal funiculus seven segmented.
Anterior pronotal margin doubled, appearing channelled when viewed from directly in front; dorsal surface with a strongly raised tubercle either side of the disc.
Prosternal rostral groove short; at most extending to the anterior margin of the pro-coxae.
Pro-coxae close together, meso-coxae a little more widely separated but by less than the width of a coxa.
Pro-femora with a strong tooth along the ventral margin.
In males the meso- and metatibiae have a terminal spur, the first abdominal ventrite is impressed and the second has a pair of small tubercles. Females lack these characters.
The U.K. species are readily separated:
On average a little smaller, 2.8-3.8mm. Pale scales on the dorsal surface the same colour throughout, those on the lateral margin of the pronotum usually extend around the dorsal tubercles. Scales on the legs variable, they may be dense but are generally uniform or irregularly distributed.
On average a little larger, 3.0-4.0mm. Scales at the base of the pronotum and behind the scutellum creamy or yellow, contrasting with the other dorsal pattern, usually not extending around the inner margin of the dorsal pronotal tubercles. Scales on the femora and tibiae generally in well-defined bands.
In the field both may be swept from various thistles, trimaculatus is striking and will soon become obvious from the much more strongly contrasting and chequered pattern of the dorsal surface and legs.