The tariff back-and-forth between the US and Canada has tanked air travel between the two countries. According to aviation analytics firm OAG, cross-border passenger bookings for US-Canada flights between April and September are down between 71% and 76% from last year.

Peak travel season, July through August, is seeing the biggest drop. In all, airlines have shed about 320,000 ticketed US-Canada seats through October. Shares of the big four airlines — Delta Air Lines, American Airlines, United Airlines, and Southwest Airlines — are all down Thursday.

More than 20 million Canadians visited the US last year, the top source of international visitors. Tariffs could hurt the Canadian dollar and make travel to the US more expensive for Canadians — and a 10% decline in Canadian travel could result in $2.1 billion in lost spending, according to the US Travel Association.