US STOCKS-Wall St drops, S&P on track for worst day since April
* Investors fret over Argentina default fallout, Russia tensions
* Exxon, Whole Foods both fall after results
* Labor costs post largest gain in more than 5-1/2 years
* Indexes down: Dow 1.5 pct, S&P 1.6 pct, Nasdaq 1.8 pct (Updates to afternoon)
By Caroline Valetkevitch
NEW YORK, July 31 (Reuters) - U.S. stocks fell more than 1 percent on Thursday, with the S&P 500 on track for its worst daily decline since April and first monthly drop since January, as concerns mounted over the strength of overseas economies and ongoing tensions with Russia.
The benchmark index moved solidly under its 50-day moving average, a level it has not closed below since April 15. The moving average is viewed as a sign of short-term momentum, and selling accelerated after the level was breached.
All 10 macro S&P 500 sectors were down on the day, with energy among those leading declines with a drop of 1.8 percent.
The CBOE Volatility index jumped 23.3 percent to 16.43, its highest level since April, though well under its historical average of 20. Continuación...