Pershing Square’s returns have been mostly negative since late August The news just keeps on getting worse for activist investor Bill Ackman. Ackman’s hedge fund Pershing Square Holdings is down 21.2% year to date, battered by the plunge in Valeant Pharmaceuticals International Inc. VRX, -6.50%one of its biggest holdings. Valeant shares have fallen more than 50% over the past month after Citron Research’s Andrew Left accused the specialty pharmaceutical company of issuing fraudulent invoices to pad its revenue. But that has not deterred Ackman from putting more money in the troubled company. The billionaire hedge fund manager bought another 2 million shares on Oct. 21, adding to his existing 5.7% stake in the company. Until Left’s allegations, Valeant had been among Pershing’s best investments. Between January and June, Valeant contributed 2.8% to the gross performance of the hedge fund’s portfolio, second after Allergan Inc.’s 3.9%. Pershing’s reported 3.2% return at the end of June. Meanwhile, Herbalife Ltd. HLF, -1.17% in which Ackman holds a sizeable short position, has defied its critics to rally 45% this year. Ackman has been on a campaign to out Herbalife as a scam since 2012 when he went public with his $1-billion bet against the nutrition-products company. “Despite the substantial increase in HLF’s share price year to date, we remain confident that HLF is an unlawful pyramid scheme that will eventually be shut down by regulators or collapse on its own,” said Ackman in Pershing’s quarterly report to investors in June. Pershing posted positive returns this year until late August, which coincided with a sharp selloff in the stock market. Except for one week in September, Pershing’s returns have been in the negative since then. The hedge fund closed out 2014 with returns of 40.4%. More from MarketWatch