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Quantum electrodynamical (QED) calculations of ionization energies of the$1snd\,D$ states are performed for the helium atom. We reproduce the previouslyknown relativistic and QED effects up to order $m\alpha^5$ and extend thetheory by calculating the complete $m\alpha^6$ correction. The totalcontribution of the $m\alpha^6$ effects is shown to be much smaller thanpreviously estimated, due to a large cancelation between the radiative andnon-radiative parts of this correction. As a result of our calculations, weconfirm the previously reported deviations between measured transition energiesand theoretical predictions for the $nD$--$2S$ and $nD$--$2P$ transitions.Possible reasons for this discrepancy are analyzed.

QED calculation of ionization energies of $1snd$ states in helium