Plagiomimicus spumosum, the frothy moth, is a moth of the family Noctuidae. The species was first described by Augustus Radcliffe Grote in 1874. It is found in North America, where it has a transcontinental range in the United States, north to southern Ontario and southern Alberta.

The wingspan is 33–40 mm. Adults are on wing in mid-summer in one generation per year.

The larvae feed on Helianthus annuus. They burrow into the head of the host plant and feed on the seeds.

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