Hi and thanks for your question about treating cranefly larvae in your lawn. The first thing you want to do is dig up a couple of square 6X6 inches lawn patches about 2 inches deep and count the "leatherjackets" (see picture below) by tearing the sod apart and multiply the number found by 4 to figure out how many would be in a square foot. 

If you have more than 25 per square foot, you have an infestation. If less than that, there is probably something else going on. If you have an infestation and want to apply an insecticide, now or January is fine, since we are having such a mild December. 

Be sure to read and follow both the warnings and application rates very carefully. Good fertilizer programs can also take care of the problem according to several scientific observations, and damage is usually temporary even without treatment according to Washington State University.
https://solvepestproblems.oregonstate.edu/property-guides/prevent-lawn-problems

https://bugguide.net/node/view/12436

Hope this takes care of the problem,