Looks like Whole Foods new lower prices are paying off, but not for Trader Joes .

Trader Joes and Sprouts Farmers Market have been hit hardest by customer defections since Amazon cut prices at Whole Foods, a data analytics firm said on Tuesday.

Amazon slashed prices on select items at Whole Foods on Aug. 28, the day the worlds largest online retailer closed its $13.7 billion purchas e of the premium grocer.

On that day, Whole Foods customer traffic spiked 31 percent from a year earlier, according to a Thasos Group analysis of location data from millions of mobile phone users.

Traffic was up 17 percent during the full week after the price reductions and remained up 4 percent for the following week, ended Sept. 16.

According to the Thasos data, 10 percent of Trader Joes regular customers visited Whole Foods between Aug. 28 and Sept. 3, more than any other competing chain, followed by Sprouts, with 8 percent. Target saw 3 percent of customers visit Whole Foods while Costco and Safeway both saw 2 percent.