Dracaena fragrans "Massangeana," more commonly called the corn plant, thrives as an outdoor shrub in U.S. Department of Agriculture plant hardiness zones 10 and 11, but gardeners in cooler climates often grow it as a flowering houseplant. This corn plant cultivar generally ranges from 5 to 15 feet in height and features fragrant, white blossoms and scented, variegated leaves. The rugged plant rarely suffers from serious problems, but occasionally experiences fungus gnat infestations if grown in excessively moist environments. Cultural, biological and chemical treatments easily control most gnat outbreaks.
Fungus gnats belong to the Diptera fly family, making them related to houseflies and mosquitoes. These tiny, flying insects thrive in areas full of damp, decaying organic materials and often infest corn plants growing in containers. Adult gnats are dark, mosquito-like insects with long antennae. Although adults only live about 8 days, the prolific females each lay 30 to 200 tiny, yellow-white eggs on moist soils or plant debris. The larvae reach about 1/4-inch long with translucent, white bodies and black heads. They live in the soil and prefer feeding on algae, fungi and other decaying plant material.
Adult fungus gnats are minor nuisance insects that don't pose any threat to corn plants, people or pets, but the larvae are an entirely different story. Living in the top 2 or 3 inches of growing media, fungus gnat larvae feed on corn plant roots and any leaves that touch the soil's surface. Although the larvae's dietary preference causes little damage to outdoor plants, it can quickly cause wilted or yellow foliage, leaf loss, stunted plant growth and loss of vigor on houseplants.
Because fungus gnats are attracted to moisture and decaying plant material, you can often control populations simply by providing the proper cultural conditions. Corn plants tolerate moderate drought, so allowing the top 2 inches of growing medium to dry completely between watering sessions helps kill off any larvae living in the soil. Avoid overwatering your plant and make sure the soil has good drainage. Removing all decaying or dead plant matter near your corn plant makes the area unattractive to adult females looking for a place to lay their eggs.
Bacillus thuringiensis (Bt) is a soil-borne bacterium that naturally controls fungus gnat larvae. Following the instructions on the product's label, mix 1 to 8 teaspoons of Bt product into 1 gallon of water and drench the top inch of soil. The bacterium only remains toxic for 2 days, so repeat applications when you notice more pests.
Adult gnats are attracted to yellow sticky traps, so laying a few of those across the soil can help reduce populations. Check the traps three times a week and replace as needed. Purchasing and releasing predatory mites directly onto your corn plant can control fungus gnat larvae. Mites belonging to the Hypoaspis family typically offer the best results, according to the Missouri Botanical Garden.
Insecticides aren't usually recommended for fungus gnat problems in home settings, but a ready-to-use, permethrin-based pesticide can help treat very severe or persistent infestations. Following the safety precautions, and wear any safety gear recommended. Keep all insecticides away from children and under lock and key. Follow manufacturer's instructions, and thoroughly spray corn plant foliage and the surrounding soil. Repeat applications every 7 to 10 days until you achieve control. Take indoor corn plants outdoors before treatment to avoid releasing chemicals into your home environment.