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Carnivorous Plants/Sarracenia Seedlings Soil full of Fungus Gnat Larvae


QUESTION: I have a fungus gnat infestation in the soil of my Sarracenia seedlings.  I went and bought Mosquito Dunk.  I put that crumbled on the top of the soil, and then a chunk in the water tray.  I then bought some Bayer Advanced Dual Action Rose and Flower Insecticide spray and sprayed the plants and the top of the soil.  I did this October 2nd.  I went and looked at the soil again today and saw a few larvae crawling around.  I decided to respray the insecticide again.  How long does the spray take to kill the bugs once they are contacted with it?  How long does this spray last on the soil, and does it seem down?  I have noticed a reduction in the flying adults in my house, seeing dead ones everywhere a day or two after my first spray.  Any advice would be greatly appreciated.

ANSWER: Hi David,

Mosquito dunks, the round floats, take about 3 days to kill mosquito larvae, so it's about the same with fungus gnat larvae.  Since it's needing to permeate the soil, it's going to take a little longer.  You can also top-water your seedlings with water that has had dunks soaking to get more of the bacterial toxin into the growing media.  Summit Chemical that makes those has another product, granules, that are much stronger for quick knockdown.  You might consider using those.  Here's what they look like:

Since Fungus Gnats are such a ubiquitous pest, it's quite possible they have developed some resistance to the chemicals in the Bayer product.  We found this problem happening with thrips and scale.  The main active ingredients, imidacloprid, has been very over-used in recent years.

Some other things that will help slow them down is having sundews, butterworts or sticky traps hanging in the area of your seedlings.  This will catch more adults and slow down their reproduction.

Let me know if this helps.

Good Growing!

Jeff Dallas
Sarracenia Northwest

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

QUESTION: Okay. I just got some sundews actually for that very reason. I put the dunk in the tray on Saturday so I guess I just have to wait. Is it possible the Bayer spray could work on the adult, not the larva? Is there anything else I can do? I was told to take my plants out if their dirt, microwave it, cool, then replant them. But that took hours for just three tiny pots.

ANSWER: Hi David,

Yes, you'll need to give the dunks a bit longer.  The spray should work on adults.  The other ingredient in the Rose and Flower spray is Cyfluthrin, a pyrethroid, which should kill them fairly fast.  Again, I don't know if you have resistant bugs or not.

Microwaving the soil is heading the right direction, but here's a better, more effective way.  When you have an infestation like that, don't even try and re-use the soil.  Microwaving also leaves cool spots frequently, so eggs may survive.  Instead, get fresh peat, bring it to a boil on the stove, then cool and drain it using that for your media.  If your seedlings are super small this may not be practical, but you would be starting with clean media.  With that then make sure you're using dunks, and you'll prevent the problem.  Also, instead of pots, consider cell-plug trays.  We use these all the time:

Good Growing!

Jeff Dallas
Sarracenia Northwest

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

QUESTION: I know it seemed to be taxing on my seedlings that I did earlier with the microwaving.  Will just the dunk eventually solve the issue then?  Will the larvae harm my seedlings too much?

Hi David,

From my experience, they do work, but you have to give them some time.  If you haven't already, double the dose of them in your trays since it seems like your infestation is bad.  The directions on the package says to use more if lots of organic matter is present in the water.  Pot of soil would qualify.

The fungus gnats can damage the seedlings, but they tend to be worse with smaller plants like sundews.  

Good Growing!

Jeff Dallas
Sarracenia Northwest

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If your carnivorous plant is showing poor growth, discoloration, abnormal leaves or possible infestation, the expert growers at Sarracenia Northwest can help! They have a great depth of experience dealing with diseases, pathogens, and abnormal growth in carnivorous plants.

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