We have investigated quadrupole effects in tetragonal crystals of PrCu2Si2 and DyCu2Si2 by means of low-temperature ultrasonic measurements. The elastic constant C-44 of PrCu2Si2 exhibits pronounced softening below 70 K down to a Neel temperature T-N = 20 K, which is described in terms of a quadrupole susceptibility for a Gamma(5) doublet ground state and a Gamma(3) singlet first excited state located at 15.6 K in the crystalline electric field scheme. The C-44 and C-66 of DyCu2Si2 also show softening below 70 K down to T-N1 = 9.7 K. A low-lying pseudo-sextet state consisting of three Kramers doublets of Gamma(6) circle plus 2 Gamma(7) brings about softening of C-44 and C-66 in DyCu2Si2.