Wis. Stat. § 34.07: Security.

A surety bond or other security, including an irrevocable letter of credit issued by a federal home loan bank, state bank, national bank, federal or state savings bank, federal or state credit union, or federal or state savings and loan association, may be required of or given by any public depository for any public deposits that exceed the amount of deposit insurance provided by an agency of the United States and the coverage provided under s. 34.08 (2).