Abstract
A key is given to nine species of Macrosiphum (Sitobion) on ferns in North America, including four new species: M. (S.) lambi new species from Athyrium filix-femina (L.) Roth, in Algonquin Park, Ontario; M. (S.) woodsiae new species from Woodsia ilvensis (L.) R. Br., Manitoba and Northwest Territories; M. (S.) clydesmithi new species from "bracken fern" (Pteridium aquilinum (L.) Kuhn), Utah and Oregon; and M. (S.) walkeri new species from Nephrolepis exaltata (L.) Schott., Arcadia, California.

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