Bloomberg yesterday reported Automatic Data Processing Inc. (NASD: ADP) plans to file a complaint with the US Securities and Exchange Commission over an 18-page letter activist investor Bill Ackman sent to Institutional Shareholder Services Inc. in an escalating proxy fight. In Monday’s letter, Ackman claimed ADP had provided “non-public, inaccurate and misleading statements” of its own to the proxy advisory firm, according to a report by Bloomberg. In a separate letter to ADP’s board included in a filing with the SEC today, however, Ackman’s Pershing Square Capital Management dismissed the complaint. “You know, of course, that in a proxy contest both sides write letters to the SEC complaining that the other side has put out false and misleading information,” according to the letter. “This practice is sufficiently common that there is a name for these letters. They are called ‘bed bug letters,’ and are entirely ordinary course in every proxy contest.”