Comida compestre, merienda compestre

Comida compestre, merienda compestre
 

picnic lunch? Any difference between them?

"Campestre" :)

I've seen that in some places one can have a "merienda" at any time, but in Mexico, that's only in the evening. A "merienda campestre" should need a lot of precautions against night bugs and lamps. :D
"Comida" should be some time after midday, but well before sunset.

The usual expression for "picnic lunch" around here is "día de campo". It is assumed that one is going to spend a while in the country and have lunch there. :)

Campest lunch

I don't know if it can be translated of a literal way.

No, it doesn't work - lunch in the country is simply a "picnic".

"Campestre" in this sense translates to "Rustic"...

hmmm can't you have a picnic at a park? That isn't quite "campestre", I'd say.