Hadroplontus litura (Fabricius, 1775)
This is the most widespread member of the genus, occurring throughout Europe north to southern Scandinavia and the U.K., and through the eastern Palaearctic, The Near East and North Africa. Populations from France, Germany and Switzerland were introduced into North America and Canada in the 1960’s and 1970’s to help control invasive weeds and the weevil is now widespread across Canada and the northern United States. In the U.K. it is locally common across England and Wales, and there are a few records from Southern Scotland. In cool climates the adults overwinter among litter and tussocks etc. but in warmer areas they may remain active throughout the year. In the U.K. the adults become active in the spring when the hosts begin to grow, these include various thistles but here it is mostly Cirsium arvense (L.), creeping thistle, and only occasionally other species. Typical habitats are wherever the host thrives; open woodland, unmanaged grassland, roadsides and wasteland etc., adults may be sampled by sweeping or beating likely host plants but they rarely occur in any numbers. Once active the adults begin feeding on fresh foliage and then mating, the females bore cavities into the leaves where they lay a single egg, or a small group of two to five, and each female will lay up to 120 over her lifetime. In general only smaller plants with tender foliage are chosen for oviposition. Larvae emerge between five and ten days later and mine into the surrounding leaf surface, second and third instar larvae mine the stems, occasionally moving down into the roots, and often cause characteristic callus growth when the plant reacts to the damage; in fast growing plants these can spread rapidly and so surround and kill the larva but if several larvae are mining a stem the callus may engulf the stem and kill it, or even kill the entire plant. Fully grown third instar larvae move into the soil and construct a cell for pupation; the pupal stage lasts two or three weeks and new generation adults will go on to overwinter. Adults generally peak in abundance between June and August. Their use as a biological agent relies on larvae attacking stems before the plant has ‘bolted’, and if attack occurs early enough the plants will be destroyed. Attack by secondary pests or pathogens caused by feeding damage or larval emergence will often cause more damage than the developing weevil.
2.8-3.8mm. Entire insect brown to black, with a characteristic pattern of pale scales. Head slightly transverse with large convex eyes; the vertex weakly convex and covered with fine erect or semi-erect pale scales, the rostrum long and parallel with the antennae inserted towards the apex. The antennae are long and slender; the scape only gradually and weakly thickened apically, the club narrow and pointed. Pronotum with dense pale scales laterally, these extend to surround, or almost surround, a pair of large tubercles, one either side of the disc, and there is a longitudinal patch of pale scales in front of the scutellum; in extreme cases this may extend, weakly defined, to the anterior margin. The dorsal punctation is coarse and dense. Elytra with pale scales coloured as those on the pronotum; generally an arrow-shaped macula behind the scutellum, an oblique macula from the lateral margin to the fifth or sixth interstice, and sometimes another parallel to this towards the apex, and the apex is variously clothed with scales. The striae are narrow, distinctly punctured, and complete to the apex although usually obscured laterally by dense pale scales, the interstices flat and strongly rugose. Legs long and robust, the pro-femora strongly toothed on the inner margin; dark but variously clothed with pale scales, the femora may be entirely covered and so appear light grey, the tibiae and tarsi are generally only sparsely scales and so appear mostly dark.
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.