There has been a bit of a fungus gnat invasion going on in our green house over the last few weeks. Now working at a garden center I talk to customers all the time who seem in a panic over these seemingly, thousands of little bugs that fly around when ever they water their plants and I laugh and tell them not to worry because the gnats don’t harm their plants and they need to cut back on watering for a bit and they will go away, the plant is probably being over watered. Well, I never thought I would be in a panic when it happened to me, but I was and it caused me to learn a bit more about the effects of fungus gnats or worse, their larva that is living in the soil and eating the roots of my little seedlings causing them to just fall over like they do when they get damping off. I am not sure where these little guys came from, but they sure do multiply fast. I did try a new seedling mix this year and was really loving it, but I am thinking that it might be the home of these cute little creatures wreaking havoc in the green house. The soil I am using is coir, it came in a nice light compact block that you add water too and it swells up making a nice fluffy light soil that seems perfect for starting seeds in. Could it also be a wonderful home for fungus gnat larva? What’s a gardener to do? I turned to Google for the answer(s) and found a few…
First I found the advice that I give out to customers which was to let the soil dry out and I was not willing to do that with my little seedlings. You can read all about these cute little bugs on the website Learn 2 Grow. That is where I got the great picture of this little guy. I learned that while the adult gnats don’t harm your plants they can carry diseases from one plant to another, but the larva do the greatest damage especially to little seedlings.
Our solution has been to put up lots of yellow sticky traps to catch the adults and keep them from reproducing. We also treated the soil with a dose of nematodes. They use nematodes at the nursery when they plant new seedlings for this very reason. I was hesitant to spend the $20 it costs for a little container claiming to have something in it that I could not see, but the results seem to be have been worthwhile. The gnats did not seem to go away over night, but slowly we are seeing less of them and the seedlings are standing tall and looking healthy. I have even started to transplant many into larger containers.
Several websites also recommended placing a slice of potato on the soil surface to attract the feeding larva. The potato slices can be used to collect and dispose of larva and to gauge when the larva are actively feeding. I didn’t try this since it didn’t seem to be a solution when using 6 pack plant trays, but it would be a good monitor to see if they were present.