Sound like a classic Bonsai beginner syndrome. The problem seems to be that you are over watering your Bonsai.

When you said that there were larva (which were probably little white worms) in the old soil it sounds like Fungus Gnat larva. They are present in soil that is kept too moist for too long as Fungus begins to build in the soil which they feed on. They also feed on the roots, especially the feeder roots which are most important (The finer smaller roots) which can lead to the bonsai's death.

The good news is if you ever have this pest problem they are fairly easy to kill. just simply make sure you allow the soil to dry adequately before the next watering and once the overly moist soil condition is removed they die off.

As far as the fact at hand like I said it sounds like you are over watering. If moist conditions persist for a longer period of time the tree will begin to get "Root Rot" which is pretty much self explanatory, and that will result in the plants imminent death. Ficus are in my opinion the best Bonsai for beginners as they are very forgiving with over and under watering (to an extent). Some beginners water every day and aren't aware that they need to allow the soil to become for the most part dry before watering again.

The method I use for watering is taking the tip of my finger and gently sticking in the soil about one or two inches around the edge of the pot. Ideally you want the soil to feel almost completely dry with just the very slightest indication of moisture, and if that's the case its time for watering. After you get the feel for this it would probably be the best method. Its also okay for some beginners to stick a chopstick in the soil near the pots edge. You can then tell by pulling out the chopstick periodically until it appears dry.

If you are interested in beginning the Art of bonsai there are many great learning tools online and in books. One of the best bets is to go to your local library and read up, as they should have a good selection of Bonsai books. That's how I learned most of my fundamentals as a beginner.

Hope this helps clarify things for you.

-Mike

Hello Mike

Thank you for your answer. I was very careful with not watering the bonsai very much, I always use a long stick to check if the soil is still wet. I insert it into the soil and wait 5 minutes. If it comes out wet then I do not water.

The problem with the wet soil started when I tried to get rid of something that I thought it was a sort of a "root lice" and I had to use some sort of pest with water and I guess that was a moment when the soil became to dry. But I changed the soil and the pot and now I watered only when it became dry but still there are only 4 leaves left and all the twigs are drying. There are a few buds but they become brown after a day or two. I still see a few of the extremely small white worms which I also believed to be fungus gnat. When I re potted I noticed that the roots were green and strong, I did not see any sign of rotting but there were very few thin roots (the feeder roots as you called them)...

This sounds like a tricky case. When did you repot? The Bonsai should have been kept in the shade or out of the sun for at least a week after repotting (generally waiting until vigorous new growth is beginning).

Have you noticed small black Gnats flying around the room the tree is in or little balls of sap on the tree (which sometimes are unseen unless you look closely)?

When you say the buds brown in a day or two, is the tree in strong direct light for longer periods during the day? I am unfamiliar with the weather at this time of year in Romania if you could describe what sort of light exposure it receives and where the plant is located.

The pot is near the window, but it is an interior window facing a balcony. The room is pointing south so the balcony receives loads of sun all day long. You can see some pictures here: http://imgur.com/TiNQr.jpg (in march) and http://imgur.com/espiZ.jpg (now).

It is late autumn here with temperatures between 10C and 16 C (50 - 60 F) and the light is not very strong. I even considered using some fluorescent light as the light might not be strong enough for him.

There are no adult Gnats in the room and I never saw any around the bonsai or in the room. Nor did I see any balls of sap but a few days ago I've noticed now that a white cottony "stuff" started appearing on some parts of the trunk. Here are some photos of it: http://i.imgur.com/mRT0E.jpg, http://i.imgur.com/EPA3u.jpg, http://i.imgur.com/uP71a.jpg. Someone suggested Mealy bugs trace but it doesn't look like that...

Also I decided to change the type of water used. I read a lot about which is best. I usually used tap water but I saw that there are many small white things on the soil that look very much like salt and I read this could be from the tap water so I managed to get some rain water from the mountains where it is not so very polluted. Hopefully it will be better...

I'm not familiar with what is in the picture but it definitely looks to be some sort of fungus or pest. I would recommend going to your local nursery and buying some Neem to spray the Tree with which should rectify the problem. You might even want to print the second picture out to show some one at the nursery that might be more experienced.

I would also recommend when safely possible to repot the Ficus in "soil less" Bonsai Mix since it seems like you have the plant in a high Peat mix. Prolong soil moisture can lead to a large array of pest sickness and diseases.

If there is a high salt/mineral content in the water it is usually apparent by white rings on the soil line and bottom drainage holes. This can be both from its content in the actual water used and by not properly watering as to allow the soil to "take out the trash" and flush the unneeded chemicals broke down by the plant to pass through. Here is my watering proceduce I use for all my healthy Bonsai.

1)Water once to get the soil semi wet.

2)I then wait five minutes and water again as the soil does not full except and absorb the water the first time.

3) After another five minutes I water a third time with any Fertilizer that I am using.

4) Then after waiting ten minutes (allowing time for the Fertilizer to settle) I waster a final fourth time to allow the soil to wash out all the Trees waste. This can take up a lot of time but is the most beatifically for the tree.