Black Ants

Of late, I have been finding black ants crawling on my work station at work! I am really perplexed as to where the ants are coming from! I have been at my work station since 2008 and I have never had any ant problems. Today was the last straw for me when I spotted no less than a dozen black ants crawling around on my table in under an hour and there were also ants on the floor as they crawled up my legs! I CANNOT work like this!!
I spent a good half an hour verifying where the ants are coming from. Ruled out two colleagues as their work stations are clean and cleared of food. Only left with one colleague and true enough, I managed to trace the ant trail back to a potted plant on her work station shelf. As I moved the plant, black ants swarmed out of the pot onto my hands. It was a nightmare! I could only go as far as the entrance of the research centre (I couldn't even make it out of the research centre!) without the ants crawling onto my upper arms. There is definitely a nest in the pot. I had to call in the relevant department to help get rid of the ants. Only when the pest control person came, I realised someone else had also complained about the ants coming from this particular colleague's work station.
I just don't buy the colleague's excuse that the ants weren't around when she last took her plant down from the shelf to water. Ants don't form a colony overnight! It must have been weeks since she last watered it?! Ants also don't hang around if there is no food for them to forage. There was an unwashed mug (with the remains of her last drink forming a layer in it) on her table (and I don't know how often she leaves unwashed stuff on her table). This is a work environment, we each have to do our part to keep it clean, otherwise it is just irresponsible.