Gnats On Cacti

The Victim

• Cacti are primarily warm-weather plants with little tolerance of freezing temperatures. They are succulents, which means they have thickened stems and leaves for storing water during times of drought. Cacti require water just as any plant does, but it is best to allow their soil to dry out before irrigation. One of the most common cacti-care mistakes is over watering, which can cause rot and fungal diseases. Cacti that are properly watered, kept in full sun and grow in gritty planting medium rarely have problems with fungus gnats. Cacti seedlings are the most susceptible to the gnats because they need to be kept moist, which attracts the pests. Cacti that are grown in greenhouses also commonly are infected; one plant develops gnats and infests all the rest.

The Villian

• Soil gnats, fungus gnats or sciarid flies are 1/8 inch long. They look like tiny flies but have longer legs and antennae. The adults lay eggs in the soil and the larvae hatch and begin feeding. Once you see adults, the larvae are already in the soil. It takes 24 to 32 days from egg to egg layer, which means many gnat generations can be born in a season. The gnats feed on fungus in the soil or on the plant and may damage living tissue in the course of their feeding. Fungus gnats require moist soils to complete their life cycle. Gnats in the group Bradysia are the most common pests on cacti.

Damage

• The larvae do the most damage to a cacti. They are found in the top inch of soil. Cacti roots are shallow and spreading, usually with a deep taproot. If gnat larvae are present, they will eat the surface roots. The adults will attack any tissue that has been colonized by fungus or enter the plant if it gets wounded. Usually they don't cause serious harm, but large infestations can reduce the vigor of a cacti. Any further stress can cause plant death.

Controls

• Fungus gnats are hard to get rid of, so prevention is the best approach. Use a soil meter to avoid over watering the soil. When potting a cacti, use a sterilized soil or sterilize it yourself in the microwave or oven to kill any fungal spores. Bacillus thuringiensis is a naturally occurring soil bacteria that can effect some control over the gnats. It is safe and non-toxic. Re-potting the infested cacti can remove the population lodged in the soil and perhaps prevent further outbreak. If all else fails there are chemical sprays that have good control over the pests.