Dark streaks, water-soaked spots, and possibly slits develop in new leaves in the spring to early summer. Leaf edges may be chewed and ragged. By midsummer, the foliage is wilting and discolored. Leaf bases are loose and rotted. Rhizomes (elongated underground stems) are often filled with holes and may be soft and rotted. Pink caterpillars, from 1 to 2 inches long, are feeding inside the rhizomes.
To kill the borers in lightly infested rhizomes, poke a wire into borer holes. In May and June, squeeze the leaves in the vicinity of feeding damage to kill feeding borers inside. Destroy heavily infested plants and rhizomes. Kill the larvae before they enter the leaves with an insecticide containing malathion. Spray weekly from the time growth first starts until the beginning of June. Clean up and destroy plant debris by April to eliminate overwintering borer eggs.
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