Question
QUESTION: Truthfully I'm not sure whether this infestation is the fault of the dwarf banana tree and three children thereof in my office or just my habit of eating while I work, but would appreciate any suggestions on how to get rid of these teeny and obviously very prolific monsters that won't hurt the dwarf banana tree and its kids, already suffering mightily as a result of a recent separation and transplant.  Many thanks for your help, and please let me know if you need any further information!

ANSWER: Hi Holly,

These small flies are most likely fungus gnats.  They usually do no harm to the plants, but can be annoying.  These gnats thrive in a moist environment in soils rich in organic matter.  Thus, they thrive in most peat-based potting mixes.  The very, very best control is to limit the amount of watering.  Allow the soil to dry out to a depth of 1 inch before watering again.  Fungus gnats reproduce just beneath the soil-line, but require a moist environment to do so.  There are chemical treatments, but for the most part they are effective/useful only in greenhouse settings.

To confirm fungus gnat infestation, scrape just beneath the soil line with a pencil and you should see the adults fly away from the soil, but you can uncover the larvae.  Sometimes they are visible with the naked eye, but can be seen more easily with a magnifying glass.

Good Luck.

---------- FOLLOW-UP ----------

QUESTION: My compliments on an excellent diagnosis since further research led me to the same conclusion (fungus gnats) and I now observe how hordes of them hover just above the soil.  Only problem is that it can be said of dwarf banana trees that it's not possible to over water them such that I'm really loath to allow the soil to dry out given its current very-close-to-death condition (only two brown-edged leaves and an unfurled-for-a-long-time new leaf now remain).  My current weapon of choice was to lie strips of fly paper on the soil today, but I would love to hear any further suggestions for doing these critters in.  Many thanks again,  Holly

Answer
Hi Holly,

Regarding the fungus gnat infestation, fly paper can reduce the adult population, and that is a great strategy.  Contrary to popular belief, banana trees are quite tolerant of moderately dry conditions for short periods of time.  Many of the banana plantations are located in parts of the world where there are distinct wet and seasons. Allowing the surface of the rootball to dry out between waterings would not significantly harm the banana tree, especially if you were to water the tree from the bottom (saucer, etc.).  There are chemical drenches that are marketed for fungus gnats, but that largely ineffective because these chemicals/biocontrols do not kill all of the pupal and larval stages.  There are strains of the Bacillus thuringiensis (variety israelensis) bacterium that have been used effectively on fungus gnats and other flies.  The bacteria produced toxins are available in the product Gnatrol.

The banana tree shoots that are removed from the adult plant can take a while to develop their own root systems, but the fleshy stems provide more than enough water to allow the plant to survive.  It is quite normal in this process to have the leaves brown around the edges and even to turn yellow and die.  I would not worry about these short term symptoms.  If they persist for longer than approximately 4 to 6 weeks, then there is something else going on.  Once the root system is well established, you should begin to see the emergence of new leaves.  This is the time to begin providing additional nutrients--water soluble is fine for potted specimens.  Banana plants are high nitrogen users, so nitrogen and iron supplements may be necessary to promote vigorous green growth.

Good Luck!