Wheat futures on the Chicago Board of Trade plunged at midday to a 2-1/2 week low as neighbouring corn futures stayed locked down their daily limit, traders said. As of 12:05 pm CST (1805 GMT), CBOT March wheat was down 51-1/2 cents at $5.21 per bushel after falling to $5.20-1/2, the contract's lowest level since December 23.