A USDA study indicates that:
…these data suggest increasing tree density associated with the past century of deforestation and succession in the eastern U.S. may currently be overwhelming the negative impacts of the adelgid at the regional scale, however, the long-term stability of this situation is not known, and data from long-infested counties suggest the landscape may be at a ‘‘tipping point’’.
http://www.nrs.fs.fed.us/pubs/jrnl/2013/nrs_2013_trotter_002.pdf