OK, so now Mr. Rewrite is picking on a church. Well, that isn't true in the least. Churches aren't bound by the AP Stylebook (Mr. Rewrite's other good book), and the two days of posts Mr. Rewrite will cheap from this sign involve words that can be spelled different ways.
Anyway, while it may be a sin for Mr. Rewrite to feature a church here, it's no sin the church to write mosquitos on its marquee. It's listed as an alternate spelling in Webster's New World College Dictionary, for example. AP style, however, calls for mosquitoes.