  MWC158 is a star with the B[e] phenomenon that shows strong
spectrophotometric variability (in lines and in UV and visible continuum)
attributed to phases of shell ejection. The evolutionary stage of this star was
never clearly determined. Previous interferometric, spectropolarimetric and
spectro-interferometric studies suggest a disk morphology for its environment.
We investigate the origin of the variability within the inner astronomical unit
of the central star using near-infrared interferometric observations with
PIONIER at the VLTI over a two-year period. We performed an image
reconstruction of the circumstellar environment using the SPARCO method. We
discovered that the morphology of the circumstellar environment could vary on
timescales of weeks or days. We carried out a parametric fit of the data with a
model consisting of a star, a disk and a bright spot that represents a brighter
emission in the disk. We detect strong morphological changes in the first
astronomical unit around the star, that happen on a timescale of few months. We
cannot account for such variability well with a binary model. Our parametric
model fits the data well and allows us to extract the location of the asymmetry
for different epochs. For the first time, we detect a morphological variability
in the environment of MWC158. This variability is reproduced by a model of a
disk and a bright spot. The locations of the bright spot suggest that it is
located in the disk, but its precise motion is not determined. The origin of
the asymmetry in the disk is complex and may be related to asymmetric shell
ejections.
