Larry Hodgson has published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings available to the public. This text was originally published in the newspaper Le soleil on March 23, 1995.
You’re sitting quietly reading your newspaper (or this blog) when, all of a sudden, a tiny black insect starts flying a few inches from your face. Where can it come from in the middle of winter? From your houseplants, vegetable seedlings or annuals, no doubt, because this is one of the most common insects in container gardening: the fungus gnat.
Black fly, midge, fungus gnat, biting midge, fruit fly—these are just some of the nicknames given to this small insect, barely larger than a pinhead. However, it is not a real fruit fly, because it is not interested in decaying fruit, but in soil rich in organic matter.
But no matter what name you give it and what genus it belongs to (Sciara or Bradysia), it always has the same annoying habit: when it reaches maturity, it leaves its native soil in search of a new place to lay its eggs… and in doing so, disturbs everyone in your home. This is a big mistake on his part, because if we can forgive him the minimal damage he causes to plants, we hardly accept to see insects flying everywhere in the house!
The adult fungus gnat, let’s say it right away, does not touch plants. Living only a few days, its only purpose is to join a gnat of the other sex, to procreate and then to lay its eggs. If it approaches us, it is because we give off heat… and heat is often an indicator of decaying organic matter. He’ll soon forget about us, because we do not offer any interesting egg-laying ground. This is not the case with the soil of your houseplants!
The female lays between 75 and 200 eggs in compost or potting soil rich in organic matter. The eggs hatch quickly to release small white maggots that will consume the organic matter in the soil. In small numbers, they do not harm the plants, on the contrary, because they are part of the natural decomposition process that releases organic elements from the soil, thus helping to feed our plants.
When many larvae are present, however, they may eat the young roots and rootlets of the plants, but there is no need to worry too much. Plants produce much more roots than they actually need to grow properly, so a few less rootlets isn’t a big deal. In addition, by “pruning” the roots, the maggots encourage them to branch out, resulting in more useful short roots than long roots that, in pots at least, are useless.
Just a second, will say the inveterate horticulturists, don’t maggots carry diseases? Yes, it’s possible… but it’s rather rare in our homes because these diseases are very specific to certain plants. A strain of Phytophthora that attacks geraniums, for example, will not affect African violets, and vice versa. Greenhouse growers who cultivate thousands of plants of the same species are, and rightly so, very afraid of the fungus gnat… but the amateur who grows several kinds of plants has very little to fear. They remain more of a nuisance than a real problem.
It would be absurd to use powerful pesticides to eliminate an insect that causes so little damage, especially when these products can be toxic to plants and human beings, if badly applied. It is better to choose soft methods to control them. The simplest method is to let the soil dry out more between waterings. This is because maggots require moist soil to survive. Dry soil is fatal to them.
As for the adults, it is easy to catch them. It is a matter of placing sticky traps in the soil. These are, in fact, sheets of paper or yellow plastic, covered with glue. Such traps are available in stores. Gnats have a weakness for yellow and, when they get too close, quickly stick to it. When the trap is full—or loses its sticky effect—simply replace it with another one.
Gnats: a very minor inconvenience. It is your degree of tolerance that will determine whether you let them live or whether you chase them away.