QuestionI have a Meyer Lemon Tree (2' + tall) that I purchased a few weeks ago from my local Lowes hardware store. I live in Seattle Washington and this tree will be a permanent indoor plant. It is doing really well in my southern window after the transplant into its new pot (so far, no leaf loss and new flowers and lots tiny lemons have appeared since bringing it home), but I fear I now have a Fungus Gnat infestation.
The "flies" are small, black and clumsy. They stay within the vicinity of the plant base and canopy. I am kind of obsessing over this tree and I have done a ton of research to try and find a way to rid it of them without using pesticides. Over the last week I have tried the bowl of soapy water trick. I put two small bowls of soapy water with a little yellow food coloring in them right in on top of the pot and I am catching a bunch of the little buggers every day. But my goal is to eliminate them. I have read that I can use 1 part hydrogen peroxide to 4 parts water to kill the larva on contact and stop the cycle. But I am terrified that I will damage the tree! I don抰 want to risk doing any harm to it. Or should I just keep soldiering on with my little bowls of soapy yellow water until I have out-lived the life cycle of the very last one and make sure that I let the soil pretty much dry out before watering it again? I was also wondering it is true that the gnats possibly came the potting soil I used to repot the plant?
AnswerMore than likely they came from the potting soil--gnats like the high humus and damp potting soil.
They key is prevention, and you can do this two ways: The first is to avoid overwatering your plants. Overwatering, to fungus gnats, is like laying a big steak on the floor in front of a starving dog ?they can抰 resist it. The second way to prevent the problem is to inspect the soil of a plant before bringing one home. Do you see gnats buzzing around it? If so, that抯 not a good sign. Put the plant down and just walk away.
Some good monitoring methods can help cut down fungus gnat issues, too. Yellow stick cards (small, yellow cards with sticky adhesive on both sides) often do the trick. These can be purchased online. Many insects, including flies, are attracted to the color yellow. So upon seeing the yellow stick card, they抣l mindlessly fly right into it, and the adult fly is stuck. These cards are most effective when placed horizontally near the surface of potting soil. You can use popsicle sticks or straws to hold the cards, or some come with sticks in the package. Keep in mind, however, that this control method only traps adults ?not the larval stage of fungus gnats.
Another way to control these plant invaders is with the potato trap method: Cut chunks of potato into 1 ?inch-square pieces. Place them on the surface of your potting soil. This is like a chuck wagon call for fungus gnat larvae! They抣l head straight for it and start munching. Leave the potato for a few days and then lift it up ?you抣l quickly discover if you have larvae in your soil. (And obviously remove the potato once you抮e finished with it ?no one wants rotting potatoes.)
