1) Wolf spider AI causes them to go to brood chambers first, and the queen second, but this entire behavior can be prevented if the spiders are kept outside, making it a decent strategy to intercept them on the way to the entrance of the nest.
2) In the 2.X missions, the warriors are not worth upgrading. You get a lot more health from having 9 lvl 1 warriors vs 3 lvl 3 warriors. The damage does get a pretty substantial increase at lvl 3, as this gives them increased attack speed, but since wood ants get spitters, you will always get the most damage out of your food by investing it into the ranged attackers, and putting anything that goes into warriors into having the highest health pool possible. You want your warriors to be about stalling the enemy so that your spitters can do all the dirty work. Having multiple is also quite efficient when fighting wolf spiders, as they will generally oneshot anything they come across anyway, so having numbers will enable a longer stall time while the spider swaps between 15 targets instead of 5.
3) Rapid fire is quite efficient for casual play, especially 2.2 for fighting ladybugs, but otherwise mortars are always going to be better if you get as many as possible to lvl 3, as they will be able to wipe out most enemies quite quickly if they are grouped up, while their damage is substantial enough that killing things one at a time is still extremely effective.
4) Workers can be used in situations with limited time (1.2, 1.1 and 1.2 challenge mode), or distant food (2.1 and 2.2), as they can collect food while you continue to do other work with more combative units, although make sure to turn combat off.
5) Any workers that you would normally put in your nest group, move to group 1 and set to no combat. This will make them far more capable of clutch victories, as they will now focus on bringing back a dead army instead of fighting and dying if something reaches the queen (although they can still get attacked, and thus this isn't a perfect solution).
6) Count up how much food it will take to restore all of your combat-oriented ants (max soldiers * 5). Make sure to keep your food at a level that is at least twice this, as having multiple of a smaller army is far better than wasting it on a large army that can't be restored after death. Also keep close track of what food you expect to be incoming, and how long you need it to last.
7) Devil's coach horse larvae and adults are fairly threatening in the 1.X missions, but in the 2.X missions you can expect them to be far easier to deal with, as you are not limited to only lower damage melee units.
8) A food tile stores 10, the first upgrade adds 30, and the second upgrade adds 40, totaling at 100 per tile when maxed. It is thus better to upgrade existing food storage than to make more of it.