Fungus Gnats in Office Buildings

Fungus gnats are known to be pests of greenhouses and nurseries, but also they can infest office buildings with interior plantscapes. Although they do not bite or transmit disease-causing organisms, their presence is usually not tolerated by the occupants, especially when they show up in large numbers.

To properly manage any pest, you should properly identify it. It is not unusual for more than one species of small flies to be present inside the same location, which may require different management approaches. The following are brief descriptions of the fungus gnat life stages.

Adults are usually the most recognizable life stage of this pest, while larvae, pupae and eggs are located in the soil and hard to find. Adults are mosquito-like, small (1/8-inch), fragile, grayish to black flies with long, slender legs and thread-like antennae. They have one pair of clear or smoky-colored wings with six veins reaching the edge (the common Bradysia species have a Y-shaped wing vein). Larvae are clear to creamy-white with shiny black head capsules and can grow to about 1/4-inch long. Pupae are initially white and become dark shortly before the adult emerges. Eggs are whitish and tiny (0.2 mm long).

BREEDING & FEEDING. In order to interrupt the life cycle of fungus gnats, it is critical to understand their breeding and feeding habits in the infested area. Like other flies, fungus gnats go through a complete metamorphosis, consisting of egg, larva, pupa and adult stages. Depending on the temperature and humidity levels of the breeding site, eggs can hatch in four to six days, larvae last 12 to 14 days and pupae develop in about five to six days. The life cycle takes about four weeks, with continuous reproduction and overlapping generations where moist and warmth conditions are available. The lifespan of adult flies is short and ranges from 7 to 10 days.

Adults are attracted to lights and are often noticed away from their breeding sources flying around light fixtures or windows. Each female can produce 100 to 300 eggs. These eggs are normally deposited in decaying organic matter of moist soil in 2 to 30 batches. Larvae primarily feed on fungi, algae and decaying plant matter. They also will feed on plant roots and leaves resting on the growing medium surface. Indoors, fungus gnats are normally linked to interior plants with damp soil containing fungi and decaying matter. Rarely, they may survive in damp new construction materials and in areas near leaky roofs or water pipes.

Moisture is key to a fungus gnat’s survival, as eggs and larvae cannot last in dry environments. Indoor plants with over-watering or poor drainage systems create ideal breeding sites for fungus gnats. Thus, it is crucial that the interior plantscape specialist let the growing medium of the potted plants to dry between watering, especially the top 1 to 2 inches. A significant declining of the fungus gnat problem can be achieved by avoiding excessive watering and by increasing the interval between regular watering. Before watering, plants should be checked for moisture level. To do this, you (or your customers) can place your fingertip into the soil. If it feels wet, do not water the plant and drain any excess water from the dish/container below flowerpots. It is also recommended that the interior plantscape specialist re-pot every so often, particularly when the growing medium is retaining too much moisture. In addition, regularly remove decayed bulbs and roots, which provide an excellent food source for fungus gnat larvae.

CONTROL TIPS. Pay attention to critical areas. Try to determine what areas of the building have the highest numbers of gnats. Check window ledges, light fixtures, etc. As mentioned, adult fungus gnats are attracted to light and are usually more likely to move towards a nearby source of light. It is recommended to utilize insect light traps (ILTs) in problem areas. These traps are excellent monitoring tools that the PMP should check regularly to determine population trends so corrective measures are implemented in a proper timeframe. They also can help in reducing the numbers of flying insects in the building.

Chemical remediation options should target the larval stage. If the number of adult fungus gnats cannot be tolerated indoors, these flies can be knocked down easily with liquid pyrethrins or aerosols labeled for use against “gnats” or “flying insects.” Removing the adults is a temporary solution. Unless you can access and treat the breeding source then, once the chemical dissipates, more flies are likely to appear. Indoor plants or interiorscaping that is difficult to remove can be treated with a number of registered residual pesticides including formulations containing bifenthrin, insect growth regulators (IGRs), as well as Bacillus thuringiensis (BT)-based products. Strictly follow all directions and precautions on the pesticide label. The key thing is to eliminate algae, molds and decaying organic matter wherever they are found. There are some pesticides registered for algae and fungi control under and around container-grown plant benches/containers.

Fungus gnat management should be included in the overall pest management program for office buildings. These flies frequently can be brought in with infested plants/soil or they can fly in from the exterior. However, treating outdoor areas may produce mixed results, particularly if the PMP cannot identify or address key breeding areas, such as mulch, grass clippings and moist lawns, which can provide ideal and persistent habitats for gnats. However, if treating outdoor breeding sources is impractical, applications of approved pesticides on foliage cover around the exterior can help. These applications should be at three- to four-week intervals. Good watering practices of interior plants along with regular monitoring for a proper larviciding intervention can keep fungus gnats under tolerable levels.

The author is the technical and training director at Adam’s Pest Control, Medina, Minn.