Beetle (Thistle tortoise) Cassida rubiginosa
Species: C. rubiginosa
Binomial name: Cassida rubiginosa
Common name: Thistle tortoise beetle, Green thistle beetle, Tortoise beetle
Cassida rubiginosa is a herbivorous tortoise beetle native to Eurasia which was released in 2007 in New Zealand as a biological control agent against the weed Californian thistle (Cirsium arvense).
Adults Cassida rubiginosa are 6–7.5 mm long with a green dorsum. The elyton (hardened forewing) is covered with small pits. The adults are long-lived, surviving for around 80 weeks.
The female lay groups of eggs in a brownish case on the undersides of the leaves at the base of plants. The female lays in 6 week stretches with 7-week breaks in between throughout the warmer months. Each female can lay as many as 1000 eggs each during their lifetime. Development of an egg to an adult takes around 6 weeks.
The larva hatch after an about a week. They a brownish or greenish, oval, and has spines around the perimeter. It has a forked tail spine on which it accumulates moult skins and excrement; it is held as a protective parasol over the insects back. Larvae and adults may be present from spring to late autumn. There is one generation per year with adults over wintering in soil litter. While the adults make some holes in the leaves the main damage is caused by the larvae which can defoliate plants.
The Californian thistle (Cirsium arvense) the host plant.
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