  We prove a surgery formula and an excision formula for the Furuta-Ohta
invariant $\lambda_{FO}$ defined on homology $S^1 \times S^3$, which provides
more evidence on its equivalence with the Casson-Seiberg-Witten invariant
$\lambda_{SW}$. These formulae are applied to compute $\lambda_{FO}$ of certain
families of manifolds obtained as mapping tori under diffeomorphisms of
$3$-manifolds. In the course of the proof, we give a complete description of
the degree-zero moduli space of ASD instantons on $4$-manifolds of homology
$H_*(D^2 \times T^2; \mathbb{Z})$ with a cylindrical end modeled on $[0,
\infty) \times T^3$.
