Critters Down Under: Fungus Gnats
Tuesday, September 29, 2020 | Troy Buechel
One of the most common insects seen in the greenhouse are fungus gnats (Bradysis and Orfelia sp.). While fungus gnats rarely cause significant damage to larger plants, they can cause root damage and may stunt young and weak plants.
Adult fungus gnats do not damage plants and primarily consume water and flower nectar. However, the larvae feed on fungus, organic matter and sometimes plant roots. If larval populations are high enough, they have even been found to travel up inside the crown and stems of the plants. By feeding on plant roots, the larvae create openings for disease organisms to enter the plant while also spreading disease between plants. They have been found to transmit Fusarium, Pythium and Thielaviopsis.
Adult fungus gnats are often confused with shore flies, but more closely resemble mosquitoes. They have long antennae, thin bodies and do not have any white spots on their wings. They are more often seen on the surface of growing media and foliage than in the air since they are very weak fliers. The larvae are white/translucent with a dark head, whereas shore fly larvae have no discernable head and the entire larva is tan to light brown in color.
“Fungus Gnat Larvae.
“Adult Fungus Gnat
Source: Gardening Know How”
The life cycle of fungus gnats is approximately 28 days but can be faster if temperatures are warm or slower if temperatures are cold. The female will lay her eggs on damp organic matter where the eggs hatch in 3 days. The larvae will then feed mostly on fungi (their preferred food source), organic matter and possibly plant roots for 12-14 days before entering the pupal stage, with adults emerging 3-6 days later.
“Life cycle of a fungus gnat
Source: Repot me”
Since much of its life is spent in the larval stage, in or on the surface of growing media, the most effective controls should focus on that stage by minimizing moisture and fungi growth near the surface and under benches, and by eliminating any areas where water tends to accumulate. Clean up spilled growing medium especially if it stays moist to wet, such as under benches.
Research has shown that all growing media are equally attractive to fungus gnats, but growing media that hold more water and organic growing media tend to have higher populations of adults emerge from them. The assumption is that growing media that hold more water encourage fungal and microbial growth, increasing the food source, and organic media naturally have higher populations of fungi and microorganisms.
As with all insect and disease problems, keeping the greenhouse clean must be a top priority. Yellow sticky cards are an effective tool for monitoring fungus gnat populations and will indicate when a chemical control may be necessary. Chemical or biological control, however, is seldom necessary since minimizing fungus and other microbial growth in the greenhouse will minimize the food source necessary for fungus gnats to survive.
For more information about plant nutrition and selecting the PRO-MIX product for your growing needs, please contact your Premier Tech Horticulture Grower Services Representative.
Jose Chen Lopez
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