Why did the Racnoss use a long term plan involving Donna?

Question

In the 2006 Doctor Who Christmas Special, The Runaway Bride, over a period of months Lance, who is working for the Racnoss doses Donna with Huon energy in order to attract the Children of the Racnoss out from the centre of the Earth. The Doctor helps Donna escape. During this time, as Wikipedia succinctly describes;

Lance reveals he was working for the Empress and purposely fed huon particles to Donna to help free the Empress' children. Donna and the Doctor escape, and the Empress decides to use Lance as a substitute, force-feeding him huon particles and then throwing him into the pit.

Is there a reason given for the elaborate dosing of Donna over a period of months instead? If the substitute can be force fed a batch and chucked into the pit in a matter of seconds why concoct this long running scheme that culminates in going along with a wedding before being discarded at the last moment? Why not just pick a random off the street and activate the plan in seconds?

Answer 1

While the show does not explicitly state this, one of the lines the Empress says right before she force-feeds Lance paraphrases to "Now we know how much huon energy is needed."

Now I have measured the bride's catalysis, I can force feed it.

While it's not logical, it implies that Lance was using this long-term dosing method because they weren't sure how much they needed to be effective and dosing too much at once could have had disastrous consequences. She only resorted to the force-feeding process because she had been found out and needed to move quickly instead of quietly.