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Carnivorous Plants/Fungus gnat larve



Several days ago, I've found some fungus gnat larve on my north american pitcher plants. Today, I figured out how to treat that. I watered the soil with a pyrethrum (diluted with water). Should I do that over again within a few days or is it sufficient?

Thank you in advance. Greetings from Belgium.

Yours faithfully


p.s.: I like the monthly podcast! Keep on!

ANSWER: Hi Joachim,

Pyrethrins works fine, but here's a better approach for fungus gnats that will not harm non-target insects.  Use BTi dunks.  They are usually sold as rings or granules that you put in the water to kill mosquito larvae.  They release a bacterial spore into the water.  It just happens that fungus gnats and mosquitos are closely related, so it kills fungus gnat larvae too.  It's very safe for the plants, people, animals, and only harms larvae of Dipteran insects.  I'm not sure what's available in Belgium, but here's an example:

Glad you like the podcasts!

Good Growing!

Jeff Dallas
Sarracenia Northwest

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

QUESTION: Hello Sarracenia Northwest,

I couldn't find a BTi product in Belgium, so I've ordered it from Germany ( I've dissolved a piece of a pellet in rain water and watered the plants, but it seems that it isn't working 100%. Now, I'm considering to add a top layer of fine lava stone to each plant to solve the issue. Should I rinse the lava before using it? Hopefully this will do the trick.

Thank you in advance!

Greetings, Joachim

It takes time for the bacteria to work.  It doesn't work all of a sudden, like an insecticide.  Typically what you'll see is a gradual decline in the fungus gnat population, from several weeks to a couple months.

With pyrethrin, it should also be reapplied every 5-10 days until you're satisfied that the infestation is eradicated.  As always, use the pyrethrin at the strength recommended by the manufacturer.  If you dilute it, you weaken it's ability to kill the insect AND you risk creating gnats that are immune to the insecticide.  

Lava rocks may work temporarily.  I just haven't seen it to be a long term solution.  

There is no single magic bullet that gets rid of pests.  Typically it's something that you have to monitor and treat as they occur.  Insecticides offer quicker results.  BTi offers a reasonable long-term solution that can be incorporated with your water system.  

Good growing!
Jacob Farin

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