Last year I sprayed the beneficial nematodes after the fact to get rid of ticks, and it seemed to help. This year I would like to be proactive and put out trichogramma wasps (for our yearly spring invasion of grasshoppers), and spray the beneficial nematodes before the arrival of the ticks. Is it now safe to do both or, with a chance of another freeze possible, is it still too early?

You should wait until the end of the month for the wasps. The nurseries are just now getting them in and you have to make sure there will be no freezes or it would kill them. It may be the same for the nematodes but maybe someone else can answer that one to be sure.