What kind of fertilizer? Has it rained since it was applied? How much?

Wouldn't worry much about the fly landing on your snake, but depending upon where you live, more about what parasites were waiting in the grass to get onto
or into your snake. Nice little things like hookworms & ticks...& pathogens carried by wild creatures. Personally, when I take a snake outside for fresh air & a
little natural sun, it doesn't leave my hands...much as they'd like to.