PESTS -- soil gnats + recommended organic insecticide

Hi Dr. Myers

Thanks for taking my question as always. This time its about some pests I have observed in my indoor garden. I have noticed lots of small flying insects that I suppose are either gnats or fruit flies. I think the culprit must be the soil mix I used as the gnats seem to congregate on the soil.
Second, I noticed some of the leaves on my plants are being eaten up, like by a
leaf cutter. I did find a caterpillar in the dirt but he was so big I didn't think he could be the culprit. ( I took him outside)..
I used good soil ( Miracle grow Organic + perlite). Any suggestions on
soil mix ?
For insecticide, I currently am using Vinegar in a glass with a funnel. This is working but not completely. I was thinking about "Safer Soap" or getting some LadyBugs. What is your opinion? Do you have any other suggestions for a "safe" insecticide?

Hello, thanks for your question.
It sounds like you have soil/fungal gnats. I have bought dirt from the big box stores that had gnats more than a few times. They eat organic matter out of the soil, and should not bother your plants too much. However, if you see small yellow dots on the leaves, you have some other leaf sucking insect.

The gnats can be a real pain, even if they don’t directly feed on your plants. For me they get stuck all over my HPS bulb, and they can get stuck in plant parts too. If they get stuck to your plants they can be a place for mold/fungus to grow which can harm/kill your plants.

If you are growing plants that live years, you may have to use some chemical insecticides. HOWEVER, I would NEVER use a sythetic insecticide in a plant I was going to eat, drink, or otherwise consume. If it is in the plant, it will then be in you. There are a few products under pests at HTGSUpply.com you could use and I have read that soil gnats are susceptible to pyrethrum.

If you want my recomendation on an organic insecticide, try nicotine. Yep, that’s right nicotine is made by tobacco as an insecticide (think of that if you use tobacco products). Nicotine is organic and will break down into non-insecticide molecules in time. Soak the contents from a couple cheap cigars in a gallon of water, I use a five pack of Philly Blunts but any kind will do. I add a couple drops of dish soap to the gallon of water as this will help the tobacco-water coat all soil particles when you water the plants with it. After letting the cigars soak for a day or two I water the plants trying to get the entire surface wet. The gnats live and eat in the top layer so you are not trying to do more than saturate the top few inches/cm of your soil.  Let the soil dry out completly before watering again and water from the bottom if you can.

Another non- insecticidal way to get rid of them, is to let the top layer of your containers dry out. It would also help to water the containers from the bottom if you can. If you water from the top, let the top REALLY dry out to where the plants almost wilt. While doing this, I have also taken perlite and put a 1-3 inch layer of perlite on top of my soil, so that the gnats can't get to their food source (the soil) easily. The perlite also dries out quickly which makes it harder for the gnats to feed and breed. If you are growing an annual, where you can start completely over maybe you can just keep the vinegar/funnel going and keep your soil dry and get through this cycle. You then need to clean everything with hot soapy water and start over with new soil. You could put the soil in an oven set over 400 F for an hour if you want to sterilize it, but if you are using a lot of soil this might be too much of a hassle.

Last, how did that caterpillar get in? If you don't have screens, get them. Caterpillars start out as tiny little eggs layed by moths or butterflies. They can grow at a tremendous rate so there is a chance a butter fly laid an egg on your plants and you just missed seeing the caterpillar until it was large and you happened to be looking around for the gnats…

I hope this helps, let me know if you find any other bugs and what they look like and I may be able to help more.

Good Growing,

Dr. E.R.Myers

Hi Dr. Myers

I had been wanting to let you know that your suggestions completely
eliminated my (gnat/fruit fly?) problems.
I took perlite and covered the top dirt in all my indoor pots. After about 3 to 4 weeks, the little guys just disappeared. No Food !!
Thanks again...... you're the best !!!

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