Agents of control

Fungus gnats (Bradysia spp.) can be a major insect pest of greenhouse-grown vegetable crops including tomatoes, peppers, cucumbers and eggplants.

They are one of the few insect pests in which the damaging stage (the larva, in this case) is located in the growing medium. Fungus gnats can become problematic under excessively moist conditions during propagation when young transplants, plugs or seedlings are developing root systems. One option for dealing with fungus gnats is the use of biological control agents such as a predatory mite, beetle and entomopathogenic nematode. All three are extensively used to suppress populations of fungus gnat larvae in greenhouse grown vegetable crops worldwide.

The biological control agents commercially available from most suppliers include the predatory mite, Stratiolaelaps scimitus (formerly Hypoaspis miles), the rove beetle, Dalotia (formerly Atheta) coriaria, and the entomopathogenic nematode, Steinernema feltiae.
 

The predatory mite:

• The soil-dwelling predatory mite, S. scimitus is used to suppress fungus gnat larval populations, however, eggs and pupae are typically not attacked.
• The mites are 1/50 inches (0.5 mm) long and light-brown in color. They tend to reside in the top ½ (1.2 cm) inch of the growing medium.
• Females lay eggs in the growing medium that hatch into nymphs after 1 to 2 days. Nymphs develop into adults in 5 to 6 days.
• The life cycle takes about 7 to 11 days to complete.
• Both nymphs and adults feed on the larval stages of fungus gnats. Stratiolaelaps scimitus is well adapted to moist conditions but will not survive in growing medium that is excessively moist or saturated.
• The optimal development and reproduction of S. scimitus occurs when the growing medium temperatures are between 59 and 86ºF (15 and 30ºC).
  
   

The rove beetle:

• Both the larva and adult stages of the rove beetle, D. coriaria attack fungus gnats including eggs, larvae and pupae.
• Adults are approximately ? inch (3.0 mm) in length, and dark-brown to black in color. Larvae are similar in size, and are yellow-brown in color.
• Both life stages are active and fast moving. They can be seen on the surface of the growing medium, but, they tend to reside within the growing medium. Rove beetle adults, in particular, are usually present on the growing medium surface at night, and migrate into the growing medium during the day or when exposed to light (either natural or supplemental)
• The life cycle, egg to adult, takes 11 to 22 days to complete depending on temperature. Rove beetle adults are mobile and may fly long distances within a greenhouse from the original release site. However, they tend to spend most of their life-span in the growing medium. In our research at Kansas State University (Manhattan, Kansas) we have found that rove beetle adults are very effective predators of fungus gnat larvae.
  
   

The entomopathogenic nematode:

• The entomopathogenic nematode, S. feltiae is a microscopic roundworm that attacks fungus gnat larvae by entering through natural openings such as the anus, mouth, or spiracles (breathing pores).
• Once inside a fungus gnat larva, they release a bacterium that consumes the internal contents of the larva. Any infected fungus gnat larvae typically die within 24 to 48 hours.
• The entomopathogenic nematodes must be applied before larval populations’ build-up to damaging levels or when “high” numbers of adults are flying around.
• The ability of entomopathogenic nematodes to suppress populations of fungus gnat larvae is influenced by factors such as application rate, application timing, host plant and nematode strain used. Furthermore, the infectivity of the entomopathogenic nematodes against fungus gnat larvae may differ depending on the growing medium type and moisture content. It's important to irrigate the growing medium before and after applying the entomopathogenic nematodes to ensure survival.
• Also, fungus gnat larvae may exhibit differences in susceptibility to entomopathogenic nematodes based on the strain and larval instars present.
• Temperature is a major factor impacting suppression of fungus gnat larval populations by S. feltiae as the entomopathogenic nematode requires growing medium temperatures between 46 and 86ºF (8 and 30ºC) for infection, and 50 and 77ºF (10 and 25ºC) for reproduction.
• It should be noted that before applying any entomopathogenic nematodes, remove filters that are 50 mesh or finer and maintain pump pressure below 300 psi. Furthermore, keep the solution agitated in order to prevent the entomopathogenic nematodes from settling to the bottom of the container.
   

An issue associated with using biological control agents for suppression of fungus gnat larval populations is “compatibility” between agents. Although there is limited information in regards to assessing intraguild predation (this is when natural enemies attack other natural enemies that share the same pest or host) it appears that S. feltiae may be used with D. coriaria whereas D. coriaria larvae may be fed upon by S. scimitus. It's important to remember that all the biological control agents mentioned above must be released before fungus gnat larval populations reach excessive numbers and are causing damage to plants. Therefore, be sure to establish a reliable scouting program by using yellow sticky cards to monitor for adults and potato wedges to detect the presence of larvae, which will ensure the success of biological control programs used against fungus gnats.

Raymond A. Cloyd is a professor and extension specialist in Horticultural Entomology/IPM at Kansas State University-Dept. of Entomology.