Natural Sprays to Eliminate Indoor Plant Gnats

Fungus Gnat Lifecycle

• Fungus gnat larvae thrive in the top 2 to 3 inches of very moist soil. Mature fungus gnats lay eggs that hatch into larvae, which feed on the moist growing medium, developing quickly and reaching maturity in 2 to 3 weeks. Once mature, larvae pupate in or on the soil. Fully mature fungus gnats don't fly well. They tend to hover around infested houseplants for their life span, which lasts 7 to 10 days. During that brief time, female fungus gnats lay as many as 200 eggs throughout the growing medium.

Ideal Survival Conditions

• Fungus gnats survive and complete life cycles on a year-round basis in rooms where the temperature range is between 65 and 75 degrees Fahrenheit. A complete life cycle from egg to adult may take 3 to 4 weeks. Adult insects drink water from the growing medium or water on plant leaves. During the winter months, plants' growing cycle typically slows and they enter dormancy. During this time, plants require less water, but as long as the soil mixture remains consistently wet, ideal conditions for fungus gnat reproduction and survival allow infestations to continue and thrive.

Understanding Seasonal Problems

• Houseplants that spend warm summer months outdoors dry out considerably faster than houseplants that remain indoors or plants that come back inside for winter. During the time that plants are outside, they may be colonized by fungus gnats. Once inside, fungus gnat problems are more noticeable, partly because people spend more time inside, but also because the gnats are drawn to moist soil. Growing mixtures degrade over time, retaining more moisture in the process. A combination of decomposing soil and excess water creates the perfect conditions in which fungus gnats survive and continuously reproduce.

Natural Control Sprays

• In addition to adjusting watering habits to accommodate different indoor conditions, natural sprays can help with both extended and short-term control. Pyrethroid-based insecticides are man-made or synthetic insecticides that are similar to the natural pyrethrum that comes from chrysanthemum flowers. These insecticides contain other active ingredients. Shorter-acting insecticides containing insecticidal soaps, pyrethrins, oils and neem don't provide long-term control and require reapplication as often as every two days. Although it is not routinely available at retail stores, the microbial insecticide Bacillus thuringiensis subsp. israelensis (Bti) may effectively kill fungus gnat larvae when applied as a drench to the growing medium.