Author statements (Readers) in biological nomenclature
As @C_Z_ mentions, the double author citation is due to a naming revision. In your case, 'Panzer' has revised the name given by 'L' (Linneus). It can for instance be due to an assignment to a new genus or a change in rank status (e.g. going from subspecies to species).

Note that author citations differ between botany and zoology, where changes in authorship are usually not recognized in zoology (as long as there is a proper and complete species description in the first place), while the botanical code allows for multiple author citations. From Wikipedia: Author citation (botany):


  ... in contrast to the situation in zoology, where no authorship change is recognized within family-group, genus-group, and species-group names, thus a change from subspecies to species, or subgenus to genus, is not associated with any change in cited authorship


So under the Zoological code, the old author name is kept when a species is reassinged to a new genus, e.g. when Seriola gracilis (Lowe, 1843) --> Cubiceps gracilis (Lowe, 1843). On the other hand, the Botanical code allows for multiple and sequential revisions, so you can have situations as "Dummy species (L.) Thom. emend. Müll.".