  Quadratic curvature Gauss-Bonnet gravity may be the solution to the dark
energy problem, but a large coupling strength is required. This can lead to
conflict with laboratory and planetary tests of Newton's law, as well as light
bending. The corresponding constraints are derived. If applied directly to
cosmological scales, the resulting bound on the density fraction is |Omega_GB|
< 3.6 x 10^-32.
