To cite glmertree in publications use:

  Fokkema M, Smits N, Zeileis A, Hothorn T, Kelderman H (2018).
  “Detecting Treatment-Subgroup Interactions in Clustered Data with
  Generalized Linear Mixed-Effects Model Trees.” _Behavior Research
  Methods_, *50*, 2016-2034. doi:10.3758/s13428-017-0971-x
  <https://doi.org/10.3758/s13428-017-0971-x>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Detecting Treatment-Subgroup Interactions in Clustered Data with Generalized Linear Mixed-Effects Model Trees},
    author = {Marjolein Fokkema and Niels Smits and Achim Zeileis and Torsten Hothorn and Henk Kelderman},
    journal = {Behavior Research Methods},
    year = {2018},
    volume = {50},
    issue = {5},
    pages = {2016-2034},
    doi = {10.3758/s13428-017-0971-x},
  }