Wis. Stat. § 114.095

No operator of an aircraft and no passenger therein shall drop any object therefrom except loose water or loose sand ballast; provided, however, that this section shall not prohibit the dusting or spraying of vegetation with insecticides dropped from airplanes, or the sowing of seeds, or the depositing of fish in lakes or streams, or the delivery of packages or mail by dropping from airplanes, or other similar practices, when such is done in accordance with the federal regulations applicable thereto.