I have a herb planter in the kitchen which currently has mint. There seems to be some pest that comes in only at night and eats the leaves. The result is holes in and browning of leaves. I have attached pictures here. There are also black specks/dots which indicate feaces. I am not sure what critter / insect this could be and what remedy I could use. Would anyone be able to help?
The mint that I have outside keep getting some holes in it from pill-bugs and slugs. At first this was a problem because the mint seemed to be struggling but now that my plants have been in the ground for over a year they are out growing any damage the critters can do.
Since your plants are inside I would recommend trying some food grade diatomaceous earth sprinkled around the base of the plants. Only issue is that it needs to remain dry so make sure to give the plants some good water, let the surface dry out and then apply the diatomaceous earth. Wait a few days and see if the damage stops - once you water the plants again you would need to add fresh diatomaceous earth. Its safe and the food grade stuff is eaten by people as a supplement.
Assuming the critter is some sort of invertebrate I think using diatomaceous earth would work. It kills bugs but does not harm people and the food grade type is safe to eat.
Cultivate abundance for people, plants and animals - Wild Homesteading
That first picture (holes in the leaves) could be any one of a number of plant eating critters. Do you have chickens secretly sneaking in at night? That's what they do to my cabbage, the little vandals.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf
The beetles are living in the soil in that container and only come up to feed after it is dark, as Marco mentioned, FGDE is a good start and you should sprinkle it on the soil and on the mint plant too.
DE works by slicing the joints of the insects and that causes moisture loss (dehydration = death).
Also watch for white flies, they will eventually find the indoor plants, DE to the rescue again.
I'll second the caterpillar hypothesis. Some of our mints get annihilated by loopers (inchworms) every summer. Hand picking works if you can find them (they may be going into the litter to hide during the day). Otherwise, Bt is a pretty sure bet. Praying mantises love to eat caterpillars, too.