THE WITCHER 3: WILD HUNT
78 Achievements
Pest Control
Destroy all monster nests in one area (Velen, the Novigrad region or Skellige).
How to unlock the Pest Control achievement in THE WITCHER 3: WILD HUNT - Definitive Guide
(more on the witchermap: https://witcher3map.com/s/#3/-10.83/-0.79)
here is a great interactive map you can filter and compare with your in game one to find any you need http://witcher3map.com
It's currently missing one in velen. If you go south on the map from Novigrad there is a wide area of water cutting the map in 2 with 3 /4 small islands linked to both sides by bridges, the missing nest is on the right most island and it has a signpost called Hindhold on it. Hope that helps those missing the last Velen one.
Word of warning if doing these in Velen , there are 2 in the east very close together , side by side in fact that are guarded by some lvl 21 enemies.
There's also another caveat to potentially consider - there's 12 unmarked "monster nests" during the 'Contract: Missing Brother' Witcher contract. An optional objective tasks you with destroying Arachas eggs - after examining the first one you come across, all twelve get marked with a Monster Nest logo. It's entirely possible these also count, however I did that mission at level 20-something, before Hearts of Stone felt feasible - so I didn't test as to whether you need those.
Probably a fairly pointless solution since the real solution will always just be "Skellige", but ever in the name of science, I'm posting this anyways. So for the three of you that want to or tried to do this in Velen / Novigrad... it'll be helpful.
There are a total of 5 nests in Skellige:
There are 4 nests on the main island (Ard Skellige) and one on the northern island of An Skellige. Having that the quest to get to Skellige is level 16 its recommended that you are around that level.
Refer to this map: http://witcher3map.com/s/#3/24.13/-4.31
Clear all markers on the left and just enable the "Monster Nest" one. You could enable the "Sign post" one as well to see where the closest travel point is.
If you are dead set on doing this trophy in Velen/Novigrad refer to this map insted: http://witcher3map.com/v/#3/60.00/-5.00
It's recommended you do this on Skellige as it has the fewest monster nests, making it the quickest way to earn this trophy (Velen/Novigrad has over 20).
4 of the Skellige Monster Nests are on the main island (Ard Skellig) and 1 on the northern island (An Skellig). You must have destructive bombs in your inventory to destroy the nests (e.g. Grapeshot, Dancing Star Bomb). Bombs can be crafted using alchemy and replenished using strong alcohol while meditating. Monster nests will be shown as question marks on the map after interacting with all quest boards of that region. For a quick overview of their locations see the video below:
Though you will most likely be destroying all the monster nests you encounter across the Northern Realms for XP, this achievement can be knocked out of the way quickly by focusing your attention on the nests in Skellige once you arrive on the island. This is mainly due to the reason that Skellige has the fewest monster nests, while Velen/Novigrad have well over twenty nests. Monster nests appear on the mini-map and world map as a "dragon head" icon and must be destroyed via direct interaction, where Geralt will then place a bomb on the nest to eliminate it (only Grapeshot, Samum, or Dancing Star bombs work).
Four of the Skellige monster nests are located on the main island (Ard Skellig) and the fifth monster nest is located on the northern island (An Skellig). Make sure you're stocked with the appropriate bombs when working on this; it'll even net progress towards the "Fire in the Hole!" achievement. Go ahead and refer to the video guide below for the location of all five nests in Skellige.
Video to help.