  Electrostatic gyrokinetic instabilities and turbulence in the Wendelstein 7-X
stellarator are studied. Particular attention is paid to the
ion-temperature-gradient (ITG) instability and its character close to marginal
stability [Floquet-type turbulence (Zocco et al. 2022) with no electron
temperature gradient]. The flux-tube version of the $\delta$f code stella
(Barnes et al. 2019) is used to run linear and nonlinear gyrokinetic
simulations with kinetic electrons. The nature of the dominant instability
depends on the wavelength perpendicular to the magnetic field, and the results
are conveniently displayed in stability diagrams that take this dependence into
account. This approach highlights the presence of universal instabilities,
which are less unstable but have longer wavelengths than other modes. A
quasi-linear estimate of the heat flux suggests they are relevant for
transport. Close to the stability threshold, the linear eigenmodes and
turbulence form highly extended structures along the computational domain if
the magnetic shear is small. Numerical experiments and diagnostics are
undertaken to assess the resulting radial localisation of the turbulence, which
affects the interaction of the latter with zonal flows. Increasing the
amplitude of the magnetic shear (e.g. through current drive) has a stabilising
effect on the turbulence and thus reduces the nonlinear energy transport.
