Direct damage can occur to young and/or weak plants in a moist, organic environment, when larvae chew the plants' roots. This reduces the uptake of water and nutrients, causing the plants to die. Strong plants are only affected at very high levels of infestation.
Indirect damage is caused when larvae transmit mites, nematodes, viruses and fungal spores. Adult sciarid flies can also transmit various fungal spores. The spots where larvae have chewed the plant are also potential spots where fungi can take hold. The damage can be lethal to the plant.