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Home BYGL Bug Bytes July 3, 2008 HOLEY THISTLE - HOPE SPRINGS ETERNAL
HOLEY THISTLE - HOPE SPRINGS ETERNAL PDF Print E-mail
ver the past few weeks, several BYGLers have been reporting observing significant injury to CANADA THISTLE (Cirsium arvense); however, the weed-whacker responsible for the heavy leaf-feeding damage remained elusive. High numbers of FOUR-LINED PLANT BUG (Poecilocapus lineatus) adults were found, and the circular, sunken leaf-spots produced by the nymphs and adults were observed. While this bug is a well known pest of annuals and herbaceous perennials, the extent of the damage to thistle seemed beyond the bug's capabilities. Curtis Young solved the mystery this week by reporting that the main culprit (hero?) is the THISTLE TORTOISE BEETLE (Cassida rubiginosa).

The tortoise beetle is native to Europe and northern Asia and it was imported into North America specifically as a biological control for thistle. The beetle is also known as the "thistle defoliating beetle" and feeds on other non-native thistle nasties including musk (Carduus nutans) and plumeless (C. acanthoides). Both the adults and larvae feed on the leaves of thistle as skeletonizers. Feeding scars are irregularly shaped ovals with one leaf epidermis still in tact producing what looks like a window pane. Most feeding occurs on the upper leaf surface. Feeding can be heavy enough to reduce or completely prevent the host plant from blooming and producing seed. Curtis and Joe Boggs observed the beetle on thistles in northwest, north central, and southwest Ohio. It is an intriguing beetle with some bizarre behaviors.

Like other tortoise beetles, the thistle tortoise beetle adult has a body shaped like a pith helmet or WWI army helmet. The body color of the adult is pale green which allows it to blend in with the leaves upon which it is feeding. The head and legs of the adult are typically hidden under the flares of the helmet and the antennae can be hidden as well or extend out from underneath. The larvae are oval shaped and have a ring of spiny spikes sticking out the rim of the oval like a crown. The larvae carry their excreta (feces) and exuviae (shed exoskeletons) around on their backs. These items are attached to the larvae by twin slim appendages (cerci) at the posterior tip of the abdomen. They hold the accumulated waste over their backs on the upwards curving abdomen and cerci. Most likely this is a defense against predation.

Curtis observed two other insects feeding on the Canada thistle: the THISTLE CATERPILLAR (Vanessa cardui) (the adult is the PAINTED LADY BUTTERFLY) and the CANADA THISTLE BUD WEEVIL (Larinus planus), a native of Eurasia that was also imported to control non-native thistles. BYGLers were thrilled with the anti-thistle observations!

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Last Updated ( Friday, 04 July 2008 10:58 )
 

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