Traders in the small-cap stock ETF IWM might be getting nervous ahead of Thursday's slate of U.S. economic data that includes the latest read on the Federal Reserve's preferred gauge of inflation, the personal consumption expenditures (PCE) price index.

Despite a 40% rally the past year – compared to 27% in the S&P 500 and 39% in the Nasdaq-100 – options traders look the most bearish on the Russell 2000. Put trading activity accounted for more than 70% of all options premium exchanged Wednesday, compared to 60% in QQQ and less than 40% in SPY.

Almost three times more put contracts traded by volume compared to calls, with more than 380,000 puts likely bought compared to under 270,000 sold, according to ThinkOrSwim data. In contrast, calls and puts were mostly even in SPY.

The bearish flow could reflect nerves around the Russell 2000's relationship with interest rates, given the small-cap index has a higher percentage of companies without profits, which can make the group particularly susceptible to spikes in Treasury yields. That said, bonds are on a 6-day bounce-back after yields made multiyear highs as recently as last week.

Thursday's economic data includes weekly jobless claims, durable goods orders, an update to American GDP, and the latest read on personal consumption expenditures.

One representative example of the bearish trading on Wednesday was a trader purchasing $11.4 million of the 277 puts expiring July 17 and selling $3.6 million of the 271-strike puts expiring June 18 for a long put spread trade for roughly $8 million net, betting on a 7% drop in IWM by mid-July.