RIO RANCHO, N.M. — Q: All of a sudden I have quite a few, lots actually, of small black flying bugs in my house. Other than repotting a hibiscus that I got last year I’ve done nothing different with all of my houseplants. I haven’t brought in any new houseplants and I am taking care of all of my treasures, as you call them, as I usually do. Where do you think these pesky bugs came from, are they causing harm to my plants and how can I get rid of them? – H.H., West Side
A: From your description I think the pesky bugs you’ve been invaded by are called fungus gnats. It seems they are best suited to doing the backstroke on your morning coffee or aiming to fly up one’s nose. Truly pesky!
In small concentrations, no, they aren’t causing your plants any trouble other that being a bother. To a certain extent, fungus gnats could be considered semi-beneficial. While living in the soil, they eat organic matter and poop making fertilization that the plants can use. Now if you had a jillion of them living on one plant, then, yes, they could be a trouble. The only clue I have from you is the newly potted hibiscus being the only change in your world. I’m hedging my bets that they came in with the potting soil you used to repot. It doesn’t happen often but these wee pests can sometimes infiltrate bagged soils. To keep that from happening again, you can bake soil to kill any adults or immature fungus gnats. It’s sort of stinky, baking soil, but it works.
Set the oven thermometer between 175 and 200 degrees (no hotter than that), spread your soil in a thin layer on a cookie sheet and bake for about an hour. Promise that you’ll never use a microwave oven to quick cook your soil, OK? When it comes out of the oven, allow it to cool and store it in an airtight container until you’re ready to use. The baking will kill any pests that might be harboring in the soil, preventing an infestation later.
Or another way to get rid of the pests that won’t stink up the house is to pour the soil in a black trash bag and seal it tightly shut. Then lay the bag out as flatly as you can in a hot very sunny spot for a day or two. Getting the soil heated up or solarized in the black bag has the same effect as baking soil. You’re just using the sun’s rays to do the work.
Now getting rid of them isn’t difficult, you just need to be really consistent about treatment for the next several weeks. The safest way to eradicate the gnats is to water your plants with a soap solution. Mix two tablespoons of dish soap in a quart of water. Slowly and thoroughly drench the top of the soil with the soapy solution. Do this the next three times you water your houseplant collection. The soap should kill the young fungus gnats. After the third watering, if it’s possible, take your plants to the sink and give each one a slow watering, getting the soap solution washed through (so to speak) the soil. That’ll keep the roots healthier in turn keeping the plants healthier. If the soap solution isn’t effective, step up to an insecticide labeled for indoor use. They are easily found and usually contain insecticidal soap (although not soap as we know it) and the safest in ranking order of concern. Next step up after that would be an indoor labeled insecticide containing pyrethrum or the mad-made equivalent, permethrin. Just please be sure any pesticide you’d choose to apply is labeled for “indoor use.” Be sure to dust or vacuum the space where your plants live, too. It’s time to tidy up. It is a smidgen early yet to take on the big chore of re-potting houseplants but be prepared by getting your potting soil “cooked and stored.” That way you are ready when the time is right. Happy Digging In.
Need tips on growing your garden? Tracey Fitzgibbon is a certified nurseryman. Send your questions to Digging In, Rio West, P.O. Drawer J, Albuquerque, NM 87103.