Black Flies & Fungus Gnats on Plants

Black Flies

• High levels of humidity can promote black fly infestations because it provides the optimal conditions for breeding. Many types of flies, including black flies, prefer to lay their eggs on the surface of standing water. Other types of flies include shore flies, moth flies and March flies, although these are unlikely to appear around your houseplants.

Fungus Gnats

• One of the most common types of pests in houseplants, fungus gnats grow to a mature size around 1/8 inches. These small, flying insects have long legs and tiny heads. They possess visible antennae and a single set of clear wings. Although they are weak flyers that generally hover near the soil, their wings allow them to fly from plant to plant, increasing the risk of these pests spreading throughout your entire collection of houseplants or flowerbeds. Mature, female gnats lay tiny eggs in the damp soil. The eggs hatch into clear larvae within four days.

Damage

• The larvae of fungus gnats feed on the lower stem tissues and roots of plants, causing rot and decay of previously healthy vegetation. Common symptoms of pest damage from fly infestations include wilting and poor growth. The damage tends to be worse in young plants, including new seedlings and rooted cuttings.

Treatment

• Reducing the moisture in the planting medium and removing standing water in the drip trays is the first step in minimizing the risk of plant damage due to fungus gnats. Avoid wet, soggy soil by watering only when necessary, according to each plant’s particular requirements. Removing mulch, such as bark nuggets, from the surface of the soil can help prevent these plant pests. Treating the soil with a fungicide drench as well as spraying the affected plants with a foliar insecticide spray, such as Bacillus thuringiensis, can help reduce fungus gnat infestations around your plants.