Answer: It's most likely a lily stalk borer, Papaipema nebris, says Bob Gilman, executive secretary of the North American Lily Society and co-owner of Hartle-Gilman Gardens, growers of more than three acres of lilies in Owatonna, Minnesota. These borers attack a number of different perennials, including lilies, hollyhocks and asters, but rarely cause extensive damage. The gray-brown, one-inch-long adult moth lays eggs in fall on plant debris around the garden and on weeds such as ragweed, thistle and burdock. The eggs hatch in spring and the inch-long, brown larvae bore into stalks of newly emerging perennials. The larvae tunnel and feed on the stem pith and emerge in midsummer, leaving a hole in the stalk. They then feed on other stems or pupate in the soil. The flower bud above the feeding area usually dies, but the plant remains alive. If only a few flowers are affected, I'd just live with the borer. Cut off and destroy any flower stalks as soon as you see the damage, says Gilman. Then, in fall, cut off and destroy all the old foliage from lilies and other perennials to lessen the problem next year, he says. You may also want to remove the old foliage of burdock, ragweed and thistle around your garden to reduce the overall borer population.
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