How They Become a Problem
Trouble often starts in greenhouses when the grower opens up a bag of peat-lite or other sphagnum peat moss mix and adds water. Some growers think this stuff is sterile. Far from it. The truth (and the bagged-dirt people won’t like me saying this) is the manufacturers should add fungus gnat eggs to the list of the medium’s constituents. “Just Add Water” and “Amaze Your Friends.” Dirt-Monkeys! [Intro]
Serious Pests
Fungus gnats are easy to detect — by using yellow sticky traps and potato disks — and their control is pretty straightforward. If you let them get out of hand, though, they can be a serious problem. I’ve seem them take an otherwise healthy plant re-potted in an artificial growing medium, contort and disfigure its leaves from their root-feeding, cause it to develop a serious root rot and die within two months of re-potting. Man, all I did was add water! Shortly before the plant’s death the fungus gnat larvae were actually tunneling up the plant’s main stem as the rot progressed. Then came the humpbacked flies-they love rot.
Fungus gnats can be a problem with nearly any plant, large or small (in addition to landing in your coffee after their misadventures of soaring in your cup’s coffee-steam thermals). Needless to say, the smaller or younger the plants, the more quickly they fall and the more vulnerable they are. Fortunately, since many plants sown by seed are planted in a fine milled sphagnum mix, which tends to be freer of viable fungus gnat eggs, the plants can usually be problem free until they are re-potted into a coarser mix-unless their nursery is in a structure where surrounding plants may already be affected.
The eggs hatch, the larvae feed and pupate in the media, and the adults subsequently come forth — a complete metamorphosis. The adults mate, light on the soil and lay more eggs to repeat the cycle. If there is rotting debris on the medium’s surface, or algae from top-watering a nitrogen-based fertilizer solution, they will have a preference to those plants. If those problems can be better dealt with, you’ll reduce your fungus gnat problem. Algae is directly attributed to many chemical fertilizers being so nitrogen-rich, and is not easily avoided unless you use another methods of fertilization or alter the way it is applied, i.e., use a bottom-irrigated method in lieu of top-watering. The amount of water being used can also contribute to the problem. If the plants will do well if their substrate is allowed to dry between waterings, let them. This will hamper the activity of fungus gnats. Regarding debris on the medium’s surface, clean it up. Run a tight ship. [Intro]
Some Advice
Shore flies, which are larger and fuller-bodied than fungus gnats, are attracted to algae, and can be seen on pots, but their larvae are strictly algae-eaters, so if you can tolerate their presence they will be nonthreatening. Like mom said: “Leave them alone, and they’ll leave you alone.” However, if you’re in a retail environment, you may have to deal with your customers’ objections — it can be bad business — so you’ll have to cure the algae problem and dry areas of standing water. Oh, and hang lots of yellow sticky traps. Humpbacked flies and moth flies, or drain flies as they’re sometimes called, though not real pests in the plant-destructive sense of the word, can be a bad thing to have flying around. They can be reduced if algae, standing water and its associated algae, and rotting plant debris is removed, and sticky traps employed to reduce adult numbers. [Intro]
For the Scout
Fungus gnats are four to five millimeters in size, delicate and mosquito-like, and have a distinctive Y-shaped vein on their wings (the top, open end of the “Y” is towards the wing-tip). They readily get themselves stuck on yellow sticky traps. The traps can be useful in reducing adult numbers in addition to their usefulness as scouting aids. Yellow traps will be most effective if laid sticky side up on the medium’s surface; just peel back one side at a time. Do expect the traps to become littered with debris from watering and other common activities, though, what with being so close to the action. The debris may also make bug-counting more difficult. Potato disks, which have been previously mentioned, are very useful in detecting the gnats’ larvae, isolating problem containers, and at reducing larval numbers — assuming you frequently change the quarter-inch disks. Moreover, potato disks are useful in playing a part of a control program and for population monitoring where sticky traps would be impractical, such as in many interiorscapes. [Intro]
Some Solutions
There are few biological pest control agents available to address the multitude of soil pests which lurk in our growing areas. Fortunately, though, the ones which are available are fairly cost-effective, readily available and are very good at what they do. One control, in particular, entomogenous or parasitic nematodes, control a huge number of soil pests, including all of the pests discussed on this page (they can even control some above-ground, boring and similar pests if applied appropriately).
Need to find control, do be sure to check out our Parasitic Nematodes, Atheta coriaria, and our favorite eight-legged friends Hypoaspis miles. You may also want to check out sticky traps and biorational sprays. [Intro]
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