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Silphium Weevil

Posted by Kelly Allsup - Bugs

August 30, 2012

News source: Kelly Allsup, 309-663-8306, kallsup@illinois.edu

News writer:

Silphium Weevil

URBANA - The 2012 Master Naturalist class has started, says Kelly Allsup, University of Illinois Extension Educator, Horticulture. A guided tour to study prairie plants highlights the most floriferous season in a prairie: Summer. Purple iron weed, mountain mint, Rattlesnake master, lead plant and four Silphium species were represented.

The compass plant, prairie dock, the cup flower and the whole leaf rosin weed send up a flowering stalk covered in multiple yellow-orange sunflowers held high above the tall grass prairie and are all part of the genus Silphium.

The compass plant (Silphium laciniatum) was in full flower. It gets its name from the north and south orientation for which the leaves are held. Original explorers used the plants to give them directions while walking through the prairie.

The prairie dock (Silphium terebinthinaceum) had sent up flower stalks in preparation of the floral display. The prairie dock boasted very large spade shaped leaves. The cup plant (Silphium perfoliatum) had spent flowers and is beneficial to the prairie because the leaves and stems clasp together like a cup collecting rain water.

The whole leaf rosin weed (Silphium integrifolium) had already flowered earlier in the growing season. Passing by the 5-6' tall flowering stalks, I noticed the flowers had been clipped and left to dangle on the plant or some flower heads had been severed completely from the flowering stalk. Resin was dripping from the cut stems and reflecting the sunlight. This characteristic is indicative to these prairie Silphiums. Of course my venture off the path to explore this phenomenon gave me the answer to the clipped flower heads. I had found a silphium weevil (Haplorhynchites aeneus) probably eating pollen. A weevil is a small beetle with an elongated snout. These are about ΒΌ" long and brown-black. After copulation, the silphium weevil uses her mandible to cut the stem just below the base of the flower so that it hangs on the plant. The adult female then lays her eggs in the ray flowers of the silphium flower head. In about a week the developing larvae hatch and begin to feed on pollen and decaying plant tissue. Around October they fall to the soil to pupate (cocoon) for the winter. The silphiums are not harmed by the silphium weevil's tactics because they are perennial in nature.



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