Never before have America’s banks been so wary of risking their cash deposits on U.S. government debt.

After holdings of U.S. debt surged to a record $1.89 trillion in 2012, lenders from Citigroup Inc. to Bank of America Corp. and Wells Fargo & Co. are culling for the first time in six years and amassing dollars. Banks’ $1.8 trillion of the bonds now equal less than 70 percent of their cash, the least since the Federal Reserve began compiling the data in 1973.