|Aspidapion radiolus (Marsham, 1802)|
|A locally common and often abundant species throughout England and Wales,
less common and local across southern Scotland to Stirling (Morris,1990). Associated with various species of
Malvaceae, excluding Althea officinalis, but including some cultivated varieties eg Lavatera and
Althea rosa which are common garden ornamentals throughout our area. The predominant hosts are Common
Mallow (Malva sylvestris) and Dwarf Mallow (M.neglecta) which are common locally although both are
absent, or virtually so, from the Cassiobury park and Whippendell wood area. In most domestic and municipal
situations both plants are treated as weeds and are rarely allowed to develop to any size but both are quick to colonize,
especially on disturbed or waste ground, and grow quickly; the prostrate Dwarf Mallow often produces extensive growth in
lawns that are not intensively maintained and close sweeping in such situations may produce the weevil anywhere in our
area. Common Mallow occurs throughout Watford town east to Bushey and along the Colne valley north towards Radlett, and
the weevil has been found, albeit sporadically and in small numbers, throughout but it is along the canal going south
from the A412 that the species becomes really abundant. Here, and onto the north western part of Common moor, Common
Mallow develops into large plants and these are usually infested with A.radiolus, beating individual plants over
a sheet from June onwards may produce dozens of adults though the greatest abundance usually occurs in early or mid September
when more than a hundred adults were found on a single plant (2007). Adults occur from late April and by mid or
late May are abundant and remain so until the cold weather in late September or October. Mallow plants usually
persist through the winter but sampling at this time has so far failed to produce the weevils. Larvae develop
within the stems.
2.5-3.3mm (Morris,1990). Elongate and very convex, entirely black with a dull leaden reflection, female more shining. Head strongly and closely punctured, flat or slightly convex between eyes and without any depressions although the puncturation may be longitudinally confluent. Eyes moderately strongly convex and fringed with long grey setae, view from side. Rostrum cylindrical, shining and sparsely punctured, as long as head and pronotum in female, shorter, less shining and more strongly punctured in male. Thickened by antennal insertions in both sexes, female rostrum narrower, more strongly curved and antennal insertions closer to base. First antennal segment pitchy red at best towards base, as long as 2-4, club sharply pointed. Pronotum curved laterally, narrowest at front and broadest across base, with a short longitudinal groove in centre by hind margin. Puncturation strong and shallow, on disc separated by a little less than their diameter, microsculpture strong, just visible at X20. Scale-like pubescence stronger and more prominent in female. Scutellum distinct ¹ ; elongate and pointed, apical part weakly raised and with a prominent ridge either side near base. Surface as elytra; shining in female, less so in male. Elytra very convex, outer interstices under reflexed part is not visible from above, usually with a distinct blue or violet metallic reflection. Interstices wide, at middle about three times wider than striae, and convex especially towards apex and base. Each with two rows of very fine punctures, these are less obvious on the duller male elytra. Elytral pubescence stronger in female. Legs entirely black with sparse grey pubescence, male pro-tibiae curved inwards towards apex and with an internal apical tooth. Claws appendiculate. Penis distinctive, rather feably rounded at apex and without projection on dorsal surface before apex.
¹ Our only other species with this scutellary sculpture is Aspidapion soror Rey, a very local weevil with a few records from Kent and East Sussex although according to Morris (1990) likely to be more widespread. Males are readily separated by the form of the penis; in A.soror there is a small apical swelling and in the side view it is more strongly curved with small projections visible before the apex. Externally A.soror is on average a little smaller than A.radiolus, 2.5-2.9mm. Less shining and more strongly shagreened, the puncturation is deeper and closer and the head is weakly depressed between the eyes. In females the rostrum is more strongly curved and shorter in A.soror, not as long as head and pronotum combined and almost parallel sided throughout. The upper surface is less shining and the scutellum broader, the tip rounded and more strongly raised than in A.radiolus. With the exception of Althea officinalis, A.radiolus occurs on a range of Malvaceae, A.soror is said to occur only on Althea officinalis in the UK.
Description from 4 Watford specimens at X40