Silo is an Italian cognomen, here in its (Greek) genitive case. See e.g. Plutarch, Marius 33.4 (and Moralia 202D) on Q. Poppaedius Silo, leader of the Marsi against Rome in the Social War (OCD4 s.v. Poppaedius). A later (40s BCE) Silo, mentioned several times in Josephus, was a colleague of the Roman general Ventidius Bassus.