The answer is: probably zero. ISU entomologist Donald Lewis says fears and myths about spiders are overblown.
"Most of us can live our entire lives in Iowa and never see a dangerous spider."
He goes on to explain that the critters are largely harmless, but they get unfairly blamed for human pain.
"'Spider bite' is a euphemism for 'I have a skin irritation and I don’t know what caused it.' Spider bites are incredibly rare. [...] Unless you see a spider in the act of inserting its teeth into your skin, there's no reason to believe that whatever red marks you have on your skin are a spider bite."
Host Charity Nebbe talks with Lewis about arachnids and insects and ISU extension specialist Richard Jauron joins the conversation to answer horticulture questions on this episode of Talk of Iowa.