I need advice on how to rid my house of fungus gnats which were introduced in a bag of potting soil I used when repotting my houseplants. One plant is difficult to repot because it has long branches that cascade down the side of the pot in an intertwined mass.
I always let the soil dry out completely before potting, and also in between waterings. Recently I added a bunch of sand to the top of the soil. Would repotting again help? Is there a no-pest strip that is safe for use indoors for this insect? (I’m chemically sensitive and also concerned about the soil’s fungus).
I’m sorry to hear of your struggle with fungus gnats. I consulted
University of California, Davis Integrated Pest Management online, and
here is a link to their page on this insect and methods of controlling it. Here are excerpts which may be relevant to your situation:
Purchase and use only pasteurized container mix or treat potting soil
with heat or steam before using it; this will kill flies as well as the
algae and microorganisms they feed on. Store pasteurized potting soil in
closed containers to prevent it from becoming infested before use.
Commercially available Steinernema nematodes, Hypoaspis mites, or the
biological insecticide Bacillus thuringiensis subspecies israelensis
(Bti) can be applied to control fungus gnat larvae in container media.
North Carolina State University Extension also has suggestions on indoor control of this pest. Excerpt:
Potted plants and other types of interiorscaping are often the culprits.
Check plants to see if the soil is excessively wet. Drain any excess
water from the dish below the pot. If the weather permits, move the
plants outdoors or allow the soil to dry down (not to the point where
plants wilt). You can also drench the soil as mentioned previously. Then,
increase the interval between regular watering and the problem should
abate.
If you can possibly repot the plant(s) which had the infested soil, and
use sterile potting soil, that should help. If this doesn’t work, the
Bacillus thuringiensis (Bt) or parasitic nematodes might be an option. I
think the Bt might present problems for your chemical sensitivity, as you
would need to avoid breathing it in, and prevent it from getting on your
skin and clothing. However, the Steinernema feltiae nematodes should not
be a problem at all. One example of a source for these is Peaceful Valley Farm Supply.
I don’t think adding sand on top of the potting soil will be effective.
It might actually create a kind of crust over the top of the soil,
causing a drainage problem. If you are concerned about fungus in the
soil, using sterilized or pasteurized potting soil is a good idea. You
can try using yellow sticky traps to catch the gnats; it can’t hurt,
although it won’t completely solve the problem unless you are willing to
repot with new soil. Most garden centers sell these traps, or you can
make your own as described by New Mexico State University Extension.
You can also employ trapping techniques using yellow sticky traps. These
may be purchased, or you can make them from yellow surveyors tape or
yellow plastic butter tubs, etc., coated with vegetable oil, Vaseline, or
other sticky material. Put these traps in a window or other well-lighted
location. The adult gnats are attracted to the yellow color and get stuck
on the trap. This removes them from the home environment and reduces
their ability to reproduce. (They die on the trap.) After you catch a lot
of gnats, just discard the whole trap or wipe the insects off and reapply
the sticky material mentioned above, and you are ready to catch more.