The mercenary camp is a feature of bug heroes 2 where you can hire bugs to help you. These include:
Flea squad - 250 Gold -5 fleas - a bunch of fleas which are fast moving but have low health
Armor ant - 350 Gold - a durable bug with high defence but slow attacking
Packer ant - 250 Gold - an ant which collects food and junk material and brings them back to your base
Firepower - 750 Gold- 1 flea gunner team + 3 army ants - low health and are easily killed, but provide good assistance in time of need (especially good for siege bug missions).
Sluggy -2000 Gold - Giant slug which has high health and high damage. Slow moving
War snail- 5000 Gold - Giant snail with huge firepower and high hitpoints. Primary weakness are the spinning armor ants which can take down the war snail quickly during their period of invincibility.
Depending on the mission type, you may or may not choose to utilise the mercenary camp.
Mercenaries are generally not worth buying in ranged or armoured missions that go for 10 rounds due to the fact that they do not last very long in these circumstances due to the high attack capacity of enemy bugs.
If you have the junk and gold gems then they may be a worthwhile investment: buy out the mercenary camp and combine it with 1 armory fully upgraded + 1 slime quarters fully upgraded and 2 fire cracker turrets. You could also use 3 or 4 bug banners (since their buffs multiply) to buff your army beyond reason.
In assassination missions mercenaries are very useful as they last a significantly longer time due to the decreased amount of bugs present on the field. In siege bug missions mercenaries are also a worthwhile investment due to the decreased attack capacity of enemy bugs.
An issue arising from mercenary/allied bugs is that the slower ones tend to congregate in a large group and move together. This may be a problem as many of them could me wiped out with a single AOE attack. (e.g slug puddle) On certain fields your slower allied bugs will congregate in certain areas, generally the areas where the enemy bugs tend to spawn most.