The argument by Prime Minister Shinzo Abe that the country needs to buy U.S. corn because pests are eating up its domestic production is looking rather dubious.

"With regard to the potential purchase of American corn, in Japan we are now experiencing pests on some agricultural products," Abe said during a joint news conference with Trump last week, after striking a deal to buy more American agricultural products in return for delaying additional tariffs on Japanese auto exports.

But Japan produces just 1,000 tons of feed corn a year, a tiny amount compared with imports of about 11 million tons. That means that even if every single field was damaged by the moth larvae that Abe was referring to, the country would still only need to import 1,000 tons to fill the gap. The U.S. is already the top supplier of corn, holding a 92 percent share of Japan's total imports last year.