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By using a non-local and time-dependent convection theory, we have calculatedradial and low-degree non-radial oscillations for stellar evolutionary modelswith $M=1.4$--3.0\,$\mathrm{M}_\odot$. The results of our study predicttheoretical instability strips for $\delta$ Scuti and $\gamma$ Doradus stars,which overlap with each other. The strip of $\gamma$ Doradus is slightly redderin colour than that of $\delta$ Scuti. We have paid great attention to theexcitation and stabilization mechanisms for these two types of oscillations,and we conclude that radiative $\kappa$ mechanism plays a major role in theexcitation of warm $\delta$ Scuti and $\gamma$ Doradus stars, while thecoupling between convection and oscillations is responsible for excitation andstabilization in cool stars. Generally speaking, turbulent pressure is anexcitation of oscillations, especially in cool $\delta$ Scuti and $\gamma$Doradus stars and all cool Cepheid- and Mira-like stars. Turbulent thermalconvection, on the other hand, is a damping mechanism against oscillations thatactually plays the major role in giving rise to the red edge of the instabilitystrip. Our study shows that oscillations of $\delta$ Scuti and $\gamma$ Doradusstars are both due to the combination of $\kappa$ mechanism and the couplingbetween convection and oscillations, and they belong to the same class ofvariables at the low-luminosity part of the Cepheid instability strip. Withinthe $\delta$ Scuti--$\gamma$ Doradus instability strip, most of the pulsatingvariables are very likely hybrids that are excited in both p and g modes.

Turbulent convection and pulsation stability of stars - II. Theoretical instability strip for $\delta$ Scuti and $\gamma$ Doradus stars