Shares of insurer Argus Group Holdings Ltd plunged 12.8 percent in Bermuda Stock Exchange trading yesterday.The company, whose shares have tumbled by around 24 percent this year fell 64 cents to $4.35, a six-month low. The volume of shares traded was 1,423.The only other domestic issue to trade yesterday was Butterfield Bank’s preference shares, which remained unchanged on $1,235, as 61 shares changed hands.The Royal Gazette/BSX Index fell 11.65 points, or 0.94 percent, to close on 1,226.12. Volume was 1,424 shares and turnover topped $81,500.The BSX Insurance Index of the Island’s leading international insurers rose one point, or 0.11 percent, to close on 893.