Fungus gnats are a major greenhouse pest in many crop production systems. The two most common species of fungus gnats found in greenhouses are Bradysia coprophila and Bradysia impatiens. Fungus gnats are one of the few greenhouse pests who go through their damaging stage within the growing medium. They are generally a problem under moist conditions, especially during propagation, and before plants develop well-established root systems.
Fungus gnats are a problem in crop production systems for the following reasons. Large populations of adults flying around are a nuisance, which may affect salability. Both the adult and larval stages are capable of disseminating and transmitting diseases. The larvae cause direct plant injury to roots and create wounds that may allow secondary soilborne pathogens such as Pythium to enter. Finally, larvae tunnel into cuttings and stems, which makes management with insecticides or biological controls very difficult. Greenhouse managers can better implement pest management strategies by understanding the life cycle of fungus gnats and the conditions that promote them.
Fungus gnat adults are winged insects with long legs and antennae. They tend to fly around the surface of the growing medium, and they live for seven to ten days. Females deposit 100 to 200 eggs in the cracks and crevices of the growing medium. They are highly attracted to growing medium that contains peat moss and pine bark.
The eggs hatch into white, transparent or slightly translucent, and legless larvae that grow to approximately 1/4 inch long. Fungus gnat larvae have a black head capsule, a characteristic that distinguishes them from shore fly larvae. Larvae are generally located within the top 1 to 2 inches of the growing medium surface, but they have also been found in the bottom of containers near drainage holes. They are highly attracted to cuttings before callous formation. Fungus gnat larvae undergo four larval instars before entering a pupae stage. Adults emerge from the pupae after approximately four to seven days. A generation from egg to adult can be completed in 20 to 28 days, depending on temperature.
The best approach to minimizing fungus gnat problems is to understand the conditions that favor these insects. Several conditions in the greenhouse may lead to fungus gnat problems. These include overwatering plants and/or irrigation leaks so that there is excessive moisture; fertilizing plants with high levels of nitrogen, promoting algae growth, which provides breeding sites for fungus gnats; using fresh (new) growing medium, which is very attractive to fungus gnat adults for egg laying; spacing plants tightly together, which retains moist conditions that favor fungus gnat development; and, finally, using gravel or dirt floors, which may accumulate or retain moisture for long periods of time allowing algae to build up.
Greenhouse managers who understand the conditions that favor fungus gnats can implement the necessary cultural management strategies. First, avoid overfertilizing plants. This prevents the growth of algae in containers and underneath benches, and it minimizes root damage from excessive fertilizer salts. Next, reduce excess moisture by locating the sources of moisture and fixing irrigation leaks. Check to be sure that cool-pad distribution tubes are not leaking. Third, allow the top 1 to 2 inches of the growing medium to dry out. This may reduce egg hatch, and it may force the fungus gnat larvae to migrate deeper into the growing medium where they are easier to control. Fourth, remove old plant and medium debris. Maintain clean areas underneath greenhouse benches and floors. The last step in fungus gnat management is to control weeds in containers and underneath benches. This practice helps reduce moist conditions, which may lead to lower fungus gnat populations. I will discuss chemical and biological management of fungus gnats in a later issue of this newsletter.