Received from: Bryan Wilks [Grayditch / Capitol Wasteland] or Doctor Lesko [Marigold station]
Solution: There are two ways of activating this quest. The less obvious scenario assumes that you've decided to visit Marigold station without talking to Bryan Wilks. In that case you'll have to get to Doctor Lesko's hideout. If that happens, Lesko will send you to Grayditch in order to find Bryan Wilks.
The more likely scenario is that you're going to come across Bryan Wilks. He lives in Grayditch, but he may approach you when you're close enough to that town (for example - when visiting a Super-Duper Mart). Either way, you'll have to talk to him and listen to his problem. Bryan will ask you to find his father and he will also tell you that the entire Grayditch area has been overrun with killer ants. If you steer the conversation in the right direction (speech, Child at heart perk - screen) Bryan will give you a shack key and he'll inform that it opens a stash which can be found at the back of the diner. Head over to Grayditch when you're ready to take care of the ant problem.
Once you've reached Grayditch you'll have to be ready to get rid of a lot of fire ants. They're hard to kill, especially if you visit this location early during the course of the game. You may try using various assault rifles against them, however I would recommend switching to frag grenades and landmines. If you're persistent and want to use guns aim for the antennae, but only if you're dealing with groups and not single ants. Find Wilk's house and enter it to find out what has happened to Bryan's father. It won't take you too long to find body of Fred Wilks. Examine the body, as well as the rest of the house. You will find a chinese assault rifle amongst other things. Go outside. Wilks is taking cover inside a tall shelter near the diner (screen). Talk to Bryan and you'll be given a new objective - you'll have to find and destroy the source of these mutations.
Make sure that you've checked the container behind the diner. I would only like to remind you that you can obtain a key for this stash from Bryan. There are a lot of frag grenades inside the dumpster and obviously you should start using them against the ants. You can also visit a small shack which is located near the shelter. In order to open it you would have to use a key obtained from Fred's body. You'll soon notice that doctor Lesko lives here. You can access his terminal if you want to (screen). Hack into the system or listen to a holotape found on the table. You'll find out about experiments carried out by Lesko. Go back to Grayfield and proceed south. Find the entrance to the Marigold station and use the stairs to get to the subway.
Other dangers are waiting for you in the tunnel, because you'll be going against fire ants. You can fight with them or you can focus only on reaching southern area of the map. Look at my screen - this is the room you'll have to reach in continue with this quest. Try opening the door and you'll be greeted by Doctor Lesko (assuming you don't know him). Talk to the scientist and listen to his ideas on how to solve the problem with the ants. If you're an intelligent person you can share a few ideas with him and he'll be impressed. Lesko will ask you to fight your way to the nest of an ant queen. You won't have to refuse him right away if you don't like his idea to keep the queen alive, because you'll make your final decision once you're close to the nest. If you decide to kill Lesko, you must use his terminal to open a door to the hatchery. I wouldn't recommend getting rid of the scientist, because you'll miss out on a chance of receiving a very cool reward. Proceed to the southern door and travel to Marigold station - hatchery.
There's only one passageway leading to the queen's hatchery, so you can't get lost. Obviously you'll be dealing with fire ants along the way. I have to warn you, because you'll come across stronger variations of these creatures. You will probably have to kill at least two out of five guardians mentioned by Lesko. Once you've reached the nest (screen) you'll have to decide how to act next.
You can kill the queen using your most powerful guns. Even then it's nothing to be an easy assignment. You must watch out for queen's attack and you can't get to close to the monster or you'll receive large doses of radiation. It's important that you keep moving. You can use the terminal which is located to the right of the queen. In this case you can approve Lesko's decision or you can destroy the mutagen which will cancel his plans. If you want to follow Lesko's instructions ignore the queen entirely and eliminate all the remaining guardians. Remember that there's a total of five of them and you can find their locations on your personal map.
Once you've dealt with the problems in the hatchery, go back to the Marigold station using the same passageway as before. Inform Lesko about your progress. Thankfully, even if you killed the queen or (not AND!!) destroyed the mutagen you can still convince him that it was the right thing to do. Lesko will hand over his lab coat and you'll also be given a choice between two perks (screen). If you choose Ant Might your strength will increase by one point and if you choose Ant Sight your perception will increase by 1 point. You will also become more resistant to fire which is a good thing, especially if you'll encounter fire ants in the future (or raiders equipped with flamethrowers :-)).
You may go back to Grayditch. Find Bryan Wilks again and talk to him. You can convince Bryan to stay in Grayditch or you can promise that you'll find him a new home. If you ask him about relatives, he will mention that his cousin Vera lives in Rivet City. This isn't the only solution, but the most obvious one. Vera can be found in Rivet City - upper deck. Go to hotel Weatherly. Vera will probably be standing in the main room, however you may have to find her in her quarters (a small room in the back). Talk to Vera and inform her about Bryan's situation (screen). She will gladly agree to having him over. Return to Grayfield and inform the boy about your progress.
If you don't want to go to Vera you can head over to Little Lamplight. Talk to Mayor MacCready about Bryan's unfortunate situation. The mayor won't be thrilled to having a new tenant, but you can convince him to change his mind if you win a speech challenge or by using Child at heart perk. If you plan on receiving a negative outcome, you may also sell Bryan to the slavers. In this case you'd have to speak to Eulogy in Paradise Falls. Make him an offer and he'll pay you 100 or 300 bottle caps for delivering Bryan to him.