Thistle Tortoise Beetle Cassida rubiginosa
On the 15th of June 2020 I found what looked like some moving frass on a Creeping Thistle leaf. On closer examination I discovered that there was something underneath the frass. I found another specimen and took the two home. I photographed them and sent the image to Bob Marsh who confirmed that they were Thistle Tortoise Beetle larva. I subsequently examined them under a microscope and observed their behaviour and in the fullness of time I was lucky enough to witness the pupation of one of them and part of the emergence of the adult beetle.
Morphology of Tortoise Beetles in General
Thistle Tortoise beetles belong to the leaf beetle family Chrysomelidae and sub family Cassidini of which there are over 3000 species. They get their name from the fact that they resemble a turtle because of the forward and sideways extensions of the body. The specific name rubiginosa refers to the beetle’s ability to produce a reddish liquid from the head. The elytra are matt green and the beetle often sits with the edges flush to the underside of the leaf where it grazes on the surface. The shell covers the whole of the beetle’s soft body and it is even slightly curved to cover it even more. Its undersurface is therefore protected from ants and because it melts seamlessly with the leaf it’s less likely to cast a shadow for predators to see. The head is hidden beneath a rounded pronotum and is flattened with the outhparts ventrally orientated. The eyes are large and occupy most of the margin when looked at from below. The antennae are eleven segmented and placed close together between the eyes.
The adults and larvae feed on various Asteraceae including thistles and many others. The adults may also feed on the pollen of buttercups and ox-eye daisies. The consumption of these plants is facilitated by a symbiotic relationship with the bacterial symbiont Stammera which is housed in specialised structures within the foregut and allows the beetle to digest pectin which is indigestible to most animals. In New Zealand it has been used as a biological control agent against the thistle Cirsium arvense where the plant is competing with crops because both adult beetles and their larvae remove the photosynthetic tissue reducing the vigor of the plant.
The larvae are oval shaped with spiny lateral projections called scoli on the thoracic and abdominal segments that poke out from the edge of the body. These are sensory organs that tell the larva when something brushes against it so that it can either hunker down on the leaf surface and cover its body with a shield -like structure which it carries around attached to two prong -like caudal appendages called cerci or it can shove it in the face of an attacker. It is most effective against predators with chewing jaws like ladybirds but not so much against predators with piercing mouthparts. The oval body is domed in the centre and down the middle is a yellow narrow passage in which there is movement of particles as the larvae are feeding so I assume that is the alimentary canal. The structure held in place by the cerci or anal fork is composed of several cast skins and an accumulation of the larva’s own faecal material. Apparently the beetles have 5 active instars. At the end of each of the first 4 instars moulting takes place and the old larval skin, the exuvium, is pushed back and becomes attached to the anal fork. Each succeeding exuvium is distinct and attached to the proceeding one. You can see this in photograph No 1.
The main body of the pupa was pale orange but the spines on the sides of the body and the shield-like part over the head were transparent. Still huge waves of energy could be observed causing the pupa to arch in the middle of its body. Each time the front end lifted up it seemed as if it was trying to pull back the remains of the larval skin and I kept expecting to see more than 5 of the side spines but that number didn’t change. Photographs 10 and 11 show the pupa from the side and above shortly after the head split.
UK beetles .co.uk