Question
Dear Tracy

I recently re-potted a house plant with compost bought from my local nursery. Within in a few weeks, out of the compost, came an infestation of what appear to be fruit flies. I have sprayed the plant pot and top layer of soil with insect killer which has kept the problem under control. The flies are no longer leaving the confines of the plant pot itself. However, they are continuing to breed and hatch. Is the only solution to try to re-pot again? Will this be just transferring the same problem onto the new compost?

Kind regards

Sarah (from England)


Answer
Hello Sarah,
     What you have if Fungus Gnats and it is probably too late now to solve the problem by changing the soil. The Fungus Gnats have probably already moved to other plants, even if you haven't seen them yet.

Fungus Gnats are little flying insects often seen flying around plants. They usually stay near the surface of the soil or under the leaves of plants unless disturbed. The larvae are tiny and worm like and live in the soil. The larvae can barely be seen with the naked eye.
Fungus Gnats are more an annoyance than a threat to the health of the plants infested with them. Fungus Gnats are hard to get rid of. The key to controlling Fungus Gnats is to kill the larvae since adult Fungus Gnats have a short life span (2-3 days). The absolute best way to get rid of them is to release beneficial nematodes into the soil that will seek out and kill the Fungus Gnat larvae. Another, more practical way of controlling Fungus Gnat larvae is to drench the soil with a 2% solution of malathion. If you don't like using such a toxic chemical pesticide in the soil there are a couple non toxic ways to control the larvae. One way is to place a slice of raw potato into the soil. The larvae will be attracted to the potato slice and will crawl inside of it. Simply throw the potato slice away and replace it with a fresh slice every day until there is no more sign of Fungus Gnat presence. A good way to get rid of the adult Fungus Gnats is to place a bowl or cup of water with a few drops of dishwashing liquid mixed into it next to your plants. The adults will be attracted to it. Change the soapy water everyday.


I would also recommend that you repot the plant into commercial potting soil. Compost that is good for plants outside in the ground is NOT good for plants inside that live their lives in the confines of a planter. For one thing compost is too heavy, it does not drain well enough so it will stay too wet for too long. It also doesn't have much aeration, which could suffocate the roots. Commercial potting soil is also sterilized. If the compost came with Fungus Gnats, think of what else it may have come with that you can not see or don't know about yet.
If you like you can use the compost to make a good potting soil. Bake a tray of the compost in the oven at 250 degrees for 25 minutes. After it cools completely, add peat moss, perlite and vermiculite at a ratio of 3 parts compost, 3 parts peat moss, 1 part perlite and 1 part vermiculite. To make a lighter potting soil (recommended) mix it at 2 parts compost, 3 parts peat moss and 1 part each perlite and vermiculite.

I hope this helps. If you have any questions or need additional information please don't hesitate to ask.
          Thanks
              Tracy