Have you tried talking to the 'lone wolf' players out of game? If you didn't make it clear to the group that you would be expecting them to work together as a group (especially if, as you say, you've stuck to a policy of encouraging inter-party strife in previous games), they might not be aware of your expectations. It's generally a good idea before starting a game that the GM informs the players "Right, the premise of this scenario is that you'll be volunteering to help save some people out of a sense of civic obligation - so, no Chaotic Evil characters, alright?", or "This scenario assumes that you're the crew of a merchant ship, so make sure your characters know how to swim, or have some sort of suitable skill set." etc.
If you're running a game that sets specific expectations to the player characters, it's always better to make it clear up front, so the players can make their characters accordingly.
Failing that, take the players in question aside and explain the situation. Chances are that they'll be willing to either adjust their characters' attitude, or roll up new ones.
Or, if they insist on playing loners, then suggest that A) their odds of survival in-game will be tragically low, and B) the game will suffer if you're forced to split your attention between two or more permanent fractions. Suggest that they could stay with the main party, but play up their loner personalities, and let it influence their interactions with the other characters.
Perhaps they could lead scouting parties into the jungle, or go on their own. Assuming that these characters have suitable wilderness survival skills (hunting, tracking, foraging, stealth etc.), they could spend a lot of time skirmishing through the undergrowth, going on hunting trips and bringing back food and supplies that'll keep the group alive.
Unless they're playing city-dwelling urbanite wizards, who've never stepped foot outside their ivory tower before. In that case, they're just screwed.
On a related note: What alignment are the characters in question? If they're any kind of Lawful or Good, and their wilderness survival skills are vastly to those of the members of the main group - this is presumably the case, since they feel confident enough to go off on their own - you should gently point out to them the moral implications of leaving the 'big city folk' on their own, without anyone to help them survive in the jungle.
Maybe you could suggest that, if they're really keen on trying to survive on their own, that you could play a different game at a later occasion, something that is designed to allow the players a chance to survive independently. There are several story-telling indie RPGs that would be much better suited for this sort of endeavour, or perhaps a board game or a tactical miniatures wargame.