An insurer shall not invest or have invested at any one time more than 65 percent of its assets in investments in real estate, real estate contracts, and notes, bonds and other evidences of debt, secured by mortgage on real estate, as described in sections 561 and 566 of this title. Any insurer which, on the effective date of this title, has in excess of 65 percent of its assets so invested shall not make any further such investments while such excess exists.