The flying insects living in pot soils

My Romaine lettuce seedlings were growing well in my sunny front porch. They were planted in moist composted soil which has fed their growth. One day I noticed the occasional insect flying out and around or walking across the soil surface. I wondered whether these are a problem for the plants, whether they signify problems in the soil, and whether I should remove them somehow.

Gardening Australia presents a video titled ‘Unwelcome guests’ which answers all my questions. Jane Edmondson calls these insects the ‘fungus gnat’. Through this video I learnt the flying adults are no problem, except that the female will continue to lay eggs in the soil. The real problem is the larvae which are underneath chomping away at the plant roots and even getting into the stems. She explained that a slice of potato left resting on the soil surface overnight would attract the larvae because they love starch. I tried that. The first photo below shows the fresh potato on the soil.

At 24 hours and then 48 hours I couldn’t see any activity under the potato slices.  By the end of day three there was movement. Fast speedy running. By tiny pale brown insects about the size of two small pinheads. I understand Fungus Gnats develop through four stages: egg, larvae (four larval stages or instars), pupa, and adult and all photos show a typical larvae and pupa and then the adult fly – none of those stages are the runners under the potato. All soil has great numbers of critters of all sizes and natures coexisting – I wonder what these runners are (and it’s too small to photograph even if it didn’t want to run from the light).

One solution is to leave the top 10 cm of soil to dry out because the larvae cannot survive without moisture. I wonder whether my fledgling lettuce plants will cope – I had noticed they didn’t seem so secure in the soil so maybe the larvae have been having a feast down under. I will work on this solution.

Another solution to the problem is to drench the pots in neem oil and repeat once a week for three weeks. Adding in predatory nematodes who will enjoy eating the larvae, may also help. If I had some carnivorous plants they would trap and eat the flies and therefore future breeding of the fungus gnat would cease. None of these three solutions appeals to me.

The final solution is to layer tiny pebbles across the soil surface which would prevent the female getting access to the soil and laying her eggs to perpetuate the cycle. This was an easy one for me to try but all I had was large pebbles. Oh well. Giving it a try.

I suspect I need to buy some diatomaceous earth – something tiny and fine and pointed which doesn’t soak up moisture.

Another website suggests layering sand over the soil with a similar intention to that of the fine pebbles/gravel offered by Gardening Australia.

Since applying a couple of  methods, the situation seems calmer. No longer am I seeing adult fungus gnats on the surface of the pot or flying. In three weeks’ time when the life cycle of the gnats could be over, maybe I will be pest free. And naturally.

Days later the lettuces which were looking and feeling very limp (assuming their roots were being nibbled below) were upstanding and firm. Fingers crossed my problem is solved.