NEW YORK — Falling oil prices are testing investors’ commitment to the Wall Street-funded shale boom.Energy stocks led the plunge earlier this month in US equities and the cost of borrowing rose. The Energy Select Sector index is down 14% since the end of August, compared with 3.8% for the Standard & Poor’s S&P 500 index. The yield for 190 bonds issued by US shale firms rose by an average of 1.16 percentage points.Investors’ sentiment toward the oil and gas industry has “certainly changed in the last 30 days,” said Ron Ormand, managing director of investment banking for New York-based MLV & Company. “I don’t think the boom is over but I do think we’re in a period where people are going to start evaluating their budgets.”What distinguishes this US energy boom from the way the industry operated in the past is the involvement of outside investors. In 1994, drillers funded 42% of their own capital spending, according to an Independent Petroleum Association of America IPAA member survey.Today, shale companies are outspending their cash flow by 50% thanks to borrowed money, according to the IPAA. They are selling more than twice as much equity to the public as they did 10 years ago, according to Tudor Pickering Holt & Company.”After the tech bubble and then the real estate bubble, Wall Street had to put its money somewhere, and it looks like they put a lot of it into domestic onshore oil and gas production,” said Michael Webber, the deputy director of the Energy Institute at the University of Texas.West Texas Intermediate, the benchmark US oil price, has fallen 25% since its recent peak on June 20. Between the S&P 500’s record high on September 18 and its five-month low on October 15, energy firms led the index down 14%, more than any other industry, Bloomberg data showed. When the market rebounded on October 16, energy again took the lead, gaining 1.7%.The drop wiped $158.6bn off the market value of 75 shale producers since the end of August, according to Bloomberg data.Investors are demanding 170 basis points, or 1.7 percentage points, in additional yield to own top-rated energy-company bonds compared with US government debt, up from 148 basis points a month ago, according to a Bank of America index.For junk bonds, of which energy firms are a fast-growing component, the additional yield is 507 basis points, up from 407 and close to the highest since 2012, Bank of America data show.Issuance of high-yield energy bonds increased 148% since 2009 to $211bn, according to Fitch Ratings. The bonds account for 16% of the market, Fitch said.The yield on the $350m bond due in 2018 issued by Whiting Petroleum, the company that is pursuing a transaction to become the largest producer in North Dakota’s Bakken shale, jumped to as much as 6.1% on September 30, the highest in almost three years, according to data from Trace, the bond-price reporting system of the Financial Industry Regulatory Authority. The yield returned to 5.566% on Wednesday, 67 basis points higher than a year earlier.Whiting Petroleum has low debt compared with earnings and enough cash on hand, vice-president of investor relations Eric Hagen said.