Adult fungus gnats are tiny black flies which spend time running around on the surfaces and sucking sap from the leaves. After the adults lay their eggs on the soil (medium), the young worm-like larvae hatch out and begin to feed upon the roots. The larvae initially eat the fine root hairs before moving on to burrow throughout the larger roots and even up into the stems. After feasting on the roots the larvae will pupate before hatching out as adults and repeating the life cycle by laying their eggs. The larvae resemble little worms. They have clear or white bodies with black heads.
Life cycle: Adults live about 7 to 10 days and deposit eggs on the moist soil surface or in soil cracks. Females lay up to 100 to 300 eggs in batches of 2 to 30 each in decaying organic matter. Eggs hatch in 4 to 6 days; larvae feed for 12 to 14 days. The pupal stage is about 5 to 6 days.
Damage from the fungus gnat larvae feeding on the roots shows up in the leaves as overfertilization and nutrient deficiency symptoms: yellowing, browning, burning and curling leaves.
Three fungus gnat larvae biocontrol agents:
Bacillus thuringiensis var. israelensis (Bti) bacteria which can be purchased as mosquito dunks or gnatrol. Bti is the most appropriate anti gnat biological control agent for DWC systems.
Hypoaspis mites.
Steinernema feltiae nematodes.
Occasionally a dry layer of sand is recommended as a deterrent to fungus gnats. This technique should be discouraged as the close packing nature of the sand particles reduces the quantity of oxygen which penetrates down into the root zone.
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Watering with neem oil at 1tsp per gallon every three days should clear up infestations. The trick is to make sure the roots are coated when new larva hatch. Within a week of two you shouldn’t see any at all.
Remember to also do the same routine on any other soil you may have laying around as well.
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The ideal time to target FG is during the larva stage. The adults do not eat.
So far after dealing with them for years and trying different control methods chemical and biological I’ve found Mosquito Bits in the granular form works the best. The granules after they are wet put out spores that are toxic to the diptera class of insects.
There are a few strains of Bacillus Thurigenesis (aka BT) and some products have different strains. Gnatrol IMHO is not as Effective as the Mosquito Bits brand.
They seem not to build up resistance to BT toxins, and followed up with sticky cards, sanitation and adjustment of irrigation practices and weekly application of the bits you can almost eliminate FG problems as long as contaminated soil or media is kept out of the area, and all media is inoculated with BT.
Suggested application rates, using Mosquito Bits granular for canna FG control
Cover the top surface of soil or media with the bits and in the watering dish, a very thin layer. A visual of how dense I put the grains on is 5 grains over the size of your thumbnail is the density of how I use BT. Wet the surface like you are watering, over the next few days you will see mycellium from the grains, that is normal.
You can use it denser, I just found that the rule of thumb is the minimum for effective control.
I’ve found that a layer of the bits does not hurt plants at all, I have a Dracaena Marginata that caught FG from some contaminated soil and put a layer of bits on the top of the soil with no problems.
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