Cassida rubiginosa Müller, O.F., 1776
This is a generally common Holarctic species; native from Morocco and Portugal to Japan and Taiwan, and introduced to the Nearctic, accidentally to North America at the turn of the 20th century and later into Canada as a biocontrol agent, also recently introduced into New Zealand as a control agent against Cirsium arvense, creeping thistle. In the U.K. it is our most common tortoise beetle, occurring throughout although less common in the north and absent from the Scottish Islands. Both adults and larvae are polyphagous on a wide range of Asteraceae and so occur in just about any habitat where the hosts thrive e.g. wasteland, parks and gardens etc. In the Pyrenees they occur up to 2100m and in Tibet above 3000m. Host plants include various Carduus and Cirsium as well as other thistles: Sonchus, Silybum, and Onopordum, and various other common members of the family e.g. Centaurea (knapweed), Arctium (burdock), Cynara (artichoke) and Tanacetum (tansy) etc. Adults occur year-round; they overwinter in tussocks, moss or among litter and become active from March or April when they begin feeding upon host foliage, they may also feed on pollen as they are often seen on flowers of Ranunculus and Leucanthemum. They fly well and so may be quick to colonize new habitats. Following a period of feeding, generally a month or so, they mate and oviposition then occurs over a long period, from April to July with each female ovipositing for about 6 weeks; eggs are laid on the underside of leaves in oöthecae, each holding from 1 to 3 eggs, these are closed and covered with frass to conceal them, and the larvae emerge after about 2 weeks. Larval development is rapid, 3 or 4 weeks, and they pass through 5 instars, they are not very mobile and tend to remain on the same part of the plant, young larvae feed mostly on the under surface of leaves while older ones feed above, and while adults make holes in leaves it is the larvae that cause most damage, in severe cases the leaves or even whole plants may be skeletonised. Larvae are very distinctive and easily observed; broadly oval and brown or green, with branched dark spines around the lateral margins and robust urogomphi which hold accumulated moulted skins and frass over the dorsal surface for disguise; these are retained in the pupae which can move them rapidly when disturbed. Pupation occurs from June and continues into the autumn; fully grown larvae discard their accumulated frass and attach themselves to the underside of petioles and midribs with a sticky secretion. Adults eclose after about 10 days and begin feeding but there is only a single generation each year; even the earliest new generation adults from June will not reproduce until the following year. The entire cycle from egg to adult may be as brief as 5 or 6 weeks and the adult lifespan is generally about 20 months although in Japan they have been recorded living for up to 3 years. In the Nearctic females have been recorded ovipositing over several 6 week periods with gaps of 6 or 7 weeks, and also continuously over a 15 week period. Fecundity is high with females recorded producing more than 800 eggs.
6-7.5mm. Elongate, broadly oval and more or less continuous in outline, in life entirely bright green or, often, with a dark mark below the scutellum and one or two along the base of the elytra, this colour fades to yellow in preserved specimens. The pronotum and elytra are widely explanate away from a moderately convex discal area. Head strongly deflexed and hidden beneath the pronotum; black with a longitudinal furrow between the eyes. Antennae inserted between the eyes; dark with pale basal segments. Pronotum rounded or weakly angled anteriorly, the posterior angles perpendicular or slightly acute, basal margin strongly sinuate, and excavate in front of the scutellum. The surface has granular microsculpture and fine punctures on the disc which become larger and confluent laterally, sometimes the cuticle is thin and translucent and so the head is visible. Scutellum distinct and strongly microsculptured. Elytra glabrous (see C. vibex), with the humeral area and two interstices raised, only weakly so behind the middle; with two puncture rows and a scutellary row, or confused, between interstices 3 and 5. The punctures fuse along the explanate margin to form transverse impressions which vary in strength and become weaker towards the apex. The legs are pale with the femoral bases darkened. Claws simple; without a basal tooth.