  In this paper we study the properties of diquarks (composed of $u$ and/or $d$
quarks) in the Bethe-Salpeter formalism under the covariant instantaneous
approximation. We calculate their BS wave functions and study their effective
interaction with the pion. Using the effective coupling constant among the
diquarks and the pion, in the heavy quark limit $m_Q\to\infty$, we calculate
the decay widths of $\Sigma_Q^{(*)}$ ($Q=c,b$) in the BS formalism under the
covariant instantaneous approximation and then give predictions of the decay
widths $\Gamma(\Sigma_b^{(*)}\to\Lambda_b+\pi)$.
